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Atmos. Chem. Phys., 10, 6551–6567, 2010
www.atmos-chem-phys.net/10/6551/2010/
doi:10.5194/acp-10-6551-2010
© Author(s) 2010. CC Attribution 3.0 License.
Atmospheric
Chemistry
and Physics
Impacts of HONO sources on the photochemistry in Mexico City
during the MCMA-2006/MILAGO Campaign
G. Li1,2, W. Lei1, M. Zavala1, R. Volkamer3,4, S. Dusanter5, P. Stevens5, and L. T. Molina1,2
1Molina Center for the Energy and the Environment, La Jolla, CA 92037, USA
2Massachusetts Institute of Technology, Cambridge, MA 02139, USA
3Department of Chemistry, University of Colorado at Boulder, Boulder, CO 80309, USA
4Cooperative Institute for Research in the Environmental Sciences (CIRES), University of Colorado at Boulder, Boulder,
CO 80309, USA
5Center for Research in Environmental Science, School of Public and Environmental Affairs, and Department of Chemistry,
Indiana University, Bloomington, IN 47405, USA
Received: 14 January 2010 – Published in Atmos. Chem. Phys. Discuss.: 11 February 2010
Revised: 9 June 2010 – Accepted: 6 July 2010 – Published: 16 July 2010
Abstract. The contribution of HONO sources to the photo-
chemistry in Mexico City is investigated during the MCMA-
2006/MILAGO Campaign using the WRF-CHEM model.
Besides the homogeneous reaction of NO with OH, four ad-
ditional HONO sources are considered in the WRF-CHEM
model: secondary HONO formation from NO2heteroge-
neous reaction with semivolatile organics, NO2reaction with
freshly emitted soot, NO2heterogeneous reaction on aerosol
and ground surfaces. The WRF-CHEM model with the five
HONO sources performs reasonably well in tracking the
observed diurnal variation of HONO concentrations. The
HONO sources included are found to significantly improve
the HOx(OH+HO2) simulations during daytime and the
partition of NO/NO2in the morning. The HONO sources
also accelerate the accumulation of O3concentrations in the
morning by about 2h and subsequently result in a noticeable
enhancement of O3concentrations over the course of the day
with a midday average of about 6ppb. Furthermore, these
HONO sources play a very important role in the formation
of secondary aerosols in the morning. They substantially en-
hance the secondary organic aerosol concentrations by a fac-
tor of 2 on average in the morning, although they contribute
less during the rest of the day. The simulated particle-phase
nitrate and ammonium are also substantially enhanced in the
morning when the four HONO sources are included, in good
agreement with the measurements. The impact of the HONO
sources on the sulfate aerosols is negligible because of the in-
efficient conversion of H2SO4from SO2reacting with OH.
Correspondence to: G. Li
(lgh@mce2.org)
1 Introduction
Nitrous acid (HONO) plays an important role in the photo-
chemistry of the troposphere, especially in the polluted ur-
ban atmosphere, because HONO is a significant photochem-
ical precursor of the hydroxyl radical (OH), the key oxidant
in the degradation of most air pollutants (e.g., Jenkin et al.,
1988; Calvert et al., 1994; Aumont et al., 2003; Stemmler et
al., 2006; Volkamer et al., 2007). Photolysis of HONO by
sunlight in the wavelength range from 300 to 405 nm leads
to the production of NO and OH (e.g., Stutz et al., 2000):
HONO+hv →NO+OH (300nm< λ < 405nm)(R1)
Therefore, the formation and photolysis of HONO could en-
hance the production of ozone (O3) and other secondary pol-
lutants, particularly in the early morning when other OH
sources are less important in the polluted atmosphere (Lam-
mel and Perner, 1988; Harrison et al., 1996; Kurtenbach et
al., 2001).
A wide range of HONO concentrations in the atmosphere
have been reported, with maximum at night and minimum
during daytime (Kleffmann et al., 2006). For example, a
maximum of 15ppb was observed in Los Angeles (Winer
and Biermann, 1994) and 10 ppb was measured in Milan,
Italy (Febo et al., 1996). High values of HONO (100–
400ppt) are also detected during the day in urban areas
(e.g., Febo et al., 1996) and rural regions (53–110ppt) (e.g.,
Staffelbach et al., 1997; Zhou et al., 2002; Acker et al.,
2005).
The main gas phase source of HONO is the reaction of NO
with OH, which frequently fails to explain the observed high
Published by Copernicus Publications on behalf of the European Geosciences Union.
6552 G. Li et al.: Impacts of HONO sources on the photochemistry in Mexico City
daytime and nighttime HONO concentrations (Aumont et al.,
1999; Stafelbach et al., 1997). Recently, gas-phase produc-
tion of HONO has been observed during the photolysis of
different ortho-nitrophenols (Bejan et al., 2006; Kleffmann,
2007), with a noontime HONO production of 100 pptv h−1
in the urban regions. In addition, the gas-phase photolysis
of methyl-substituted nitroaromatics is also proposed to pro-
duce HONO (Kleffmann, 2007). Although HONO formation
mechanism in the atmosphere is still poorly understood at
present, it is well established that heterogeneous conversion
processes of nitrogen dioxide (NO2) on different surfaces are
involved in the troposphere (Febo et al., 1987; Svensson et
al., 1987; Saliba et al., 2000; Kleffmann et al., 1998; Lammel
and Cape, 1996). In the stratosphere, when the temperature
is below 230 K, heterogeneous reactions of HONO on sulfate
aerosols have been observed with an uptake coefficient rang-
ing from 0.03 to 0.1, dependent on the sulfuric acid content
(Zhang et al., 1996). However, considering the high temper-
ature in the troposphere, the heterogeneous loss of HONO
on sulfate aerosols cannot constitute a potentially important
HONO sink. Direct emissions from vehicles may also play a
role in atmospheric HONO levels, depending on vehicle en-
gines and traffic distributions, with a molar emission ratio of
HONO/NOxranging from 10−4to 10−2(Kurtenbach et al.,
2001).
Based on the surface measurements of various radical pre-
cursors, Winer and Biermann (1994) investigated the relative
contribution of HONO to OH sources, showing that HONO
was a major source of OH during the morning in the polluted
Los Angeles atmosphere. Using observations with Differ-
ential Optical Absorption Spectroscopy (DOAS), Alicke et
al. (2002, 2003) found that photolysis of HONO produced
up to 34% of the total OH formed over 24-h period during
BERLIOZ in Germany. Volkamer et al. (2007) also showed
that photo-induced HONO sources made an important con-
tribution to the OH level in the early morning during the
MCMA-2003 field campaign in Mexico City.
Modeling results reported in the literature indicate that the
contribution of HONO to the atmospheric photochemistry
depends on multiple conditions, such as NOxconcentration,
season, etc. In the pioneering modeling study by Harris et
al. (1982) using a trajectory model, various levels of HONO
were included in the early morning at the beginning of the
simulations. The initial HONO input was found to remark-
ably enhance the production rate of O3but not significantly
influence the maximum O3yield, except under high NOx
conditions. Using a box model, Jenkin et al. (1988) found
that the photolysis of HONO in the early morning led to a
50% increase of OH at 0600 GMT, a 14% increase in the
daily maximum OH and a 16% increase in net photochemi-
cal O3production when a heterogeneous HONO source was
added. Similar results were also obtained by Kotamarthi et
al. (2001) using a box model, showing that HONO formation
on aerosols substantially influenced O3, OH, and HO2con-
centrations in the polluted planetary boundary layer (PBL).
Furthermore, employing a photochemical trajectory model,
Clapp and Jenkin (2001) suggested that direct emission of
HONO may have an important impact on the level of oxi-
dant (O3+NO2), especially under more polluted conditions.
Based on the simulations with a chemical transport model,
Lei et al. (2004) concluded that heterogeneous conversion
of NO2to HONO on the surfaces of soot aerosol acceler-
ated the O3production by about 1h in the morning and led
to a noticeable increase of 7ppb on average in the daytime
O3level. However, in the simulations performed by Au-
mont et al. (1999) using a two-layer box model, the photoly-
sis of HONO did not contribute significantly to summertime
O3concentrations of polluted air masses. Furthermore, Au-
mont et al. (2003) conducted simulations with a two-layer
box model to verify the impact of HONO photolysis on
the NOx/HOx/O3chemistry in the polluted boundary layer.
Three HONO sources were added in their simulations, in-
cluding direct emissions of HONO, NO2heterogeneous pro-
duction of HONO on the ground surface and the aerosol sur-
face. Their simulation results varied with the season: HONO
sources were found to slightly influence the NOx/HOx/O3
concentration profiles during summer, but the photolysis of
HONO appeared to be a significant source of OH during win-
ter.
As an important contributor of OH in the polluted atmo-
sphere, HONO sources potentially affect the formation of
secondary aerosols by enhancing the VOC oxidation due to
the increased OH level. However, few studies have been con-
ducted to investigate the impacts of HONO sources on the
formation of secondary aerosols in the polluted atmosphere.
During the MCMA-2006 field campaign as part of the MI-
LAGRO (Megacity Initiative: Local and Global Research
Observations) project in March 2006, an extensive data set
was obtained, including highly time-resolved ambient gas
phase species and aerosols (Molina et al.,2008; 2010), which
provides a unique opportunity to investigate the impacts of
HONO sources on the photochemistry in a polluted urban
area. The objective of the present study is to examine the
relative importance of different HONO sources and evaluate
their contributions to the HOx/NOx/O3chemistry and sec-
ondary aerosols using the WRF-CHEM model based on the
measurements taken during MCMA-2006 field campaign.
The WRF-CHEM model and the model configuration are de-
scribed in Sect. 2. Results of the modeling experiments and
comparisons are presented in Sect. 3. The discussion and
summary are given in Sect. 4.
2 Model description
2.1 WRF-CHEM Model
The WRF Model (Skamarock et al., 2005) is a next-
generation mesoscale numerical weather prediction (NWP)
system designed to serve both operational forecasting and
Atmos. Chem. Phys., 10, 6551–6567, 2010 www.atmos-chem-phys.net/10/6551/2010/
G. Li et al.: Impacts of HONO sources on the photochemistry in Mexico City 6553
atmospheric research needs. A detailed description of the
WRF model can be found in the WRF web-site http://www.
wrf-model.org/index.php.
Additionally, a chemical model is also fully implemented
into the WRF model (WRF-CHEM) (Grell et al., 2005;
Tie et al., 2007). Based on the framework of the cur-
rent WRF-CHEM model and the available emissions in-
ventory in Mexico City (Song et al., 2010), a new flexi-
ble gas phase chemical module has been developed and im-
plemented into the WRF-CHEM model, which can be uti-
lized with different chemical mechanisms, including CBIV,
RADM2, and SAPRC. The chemistry is solved by an Eule-
rian backward Gauss-Seidel iterative technique with a num-
ber of iterations, inherited from NCAR-HANK (Hess et al.,
1999). The short-lived species, such as OH and O (1D),
are assumed to be in steady state. The solution is iterated
until all species are within 0.1% of their previous iterative
values. For the aerosol simulations, the CMAQ/models3
(version 4.6) aerosol module developed by US EPA (Envi-
ronmental Protection Agency), which is designed to be an
efficient and economical depiction of aerosol dynamics in
the atmosphere, has also been incorporated into the WRF-
CHEM model (Binkowski and Roselle, 2003). In this aerosol
component, the particle size distribution is represented as
the superposition of three lognormal sub-distributions called
modes. The processes of coagulation, particle growth by the
addition of mass, and new particle formation are included. In
addition, the wet deposition also follows the method used in
the CMAQ/Models3. Surface deposition of chemical species
is parameterized following Wesely (1989). The photolysis
rates are calculated using the FTUV (Tie et al., 2003; Li et al.,
2005). Anthropogenic emissions used in the WRF-CHEM
model are constructed from the official emissions inventory
for the MCMA-2006 (Song et al., 2010). Biogenic emissions
are estimated using the MEGAN v2.04 model (Model of
Emissions of Gases and Aerosols from Nature) developed by
Guenther et al. (2006, 2007); the on-line biogenic emissions
calculation is turned off. In the present study, the SAPRC 99
gas phase chemical mechanism is employed according to the
available emission inventory in Mexico City.
2.2 Parameterization of HONO Sources
In addition to the homogeneous reaction of NO with OH,
four sources of HONO are included: secondary HONO for-
mation from NO2heterogeneous reaction with semivolatile
organics, NO2reaction with freshly emitted soot, NO2het-
erogeneous reaction on aerosols and ground surface.
With regard to the secondary HONO formation from NO2
heterogeneous reaction with semivolatile organics, we fol-
low the results of a recent experimental study performed by
Gutzwiller et al. (2002) and assume that a fraction of 0.023
of the NOxemitted is heterogeneously converted to HONO.
It is worthy to note that the fraction suggested by Gutzwiller
et al. (2002) was obtained from diesel exhaust. It might be
more appropriate to use the value of 244mg of secondary
HONO formation per kg of diesel fuel burnt as suggested
by Gutzwiller et al. (2002). However, in their experiment,
the semivolatile organics in diesel exhaust react with NO2on
time scales of hours, much longer than the time scale of the
NO2to HONO conversion on freshly soot particles, which is
in the order of one minute (Kalberer et al., 1999). Therefore,
using the fraction of NOxor diesel fuel burnt to calculate the
HONO formation is a highly parameterized method based on
the experiment of Gutzwiller et al. (2002). In Mexico City,
diesel vehicles contribute almost all the organic aerosol emis-
sions (Zavala et al., 2009) and very high levels of organic
aerosols have been observed (Aiken et al., 2009), indicating a
large amount of semivolatile organics are emitted from diesel
vehicles, which could participate in the conversion of NO2
from other sources, such as gasoline engines and biomass
burning. Considering the emission contributions from diesel
engines and the variation in the fraction of NO2in NOxemis-
sions in Mexico City, the fraction of 0.023 used in the study
is likely an upper limit for the HONO formation from NO2
heterogeneous reaction with semivolatile organics. Further-
more, it is necessary to emphasize that further studies need
to be conducted to more appropriately parameterize the NO2
reaction with semivolatile organics.
Ammann et al. (1998) found an efficient conversion of
NO2to HONO on suspended soot particles with a reac-
tion probability ranging from 1.1×10−2to 3.3×10−4, but
they did not conclude whether the conversion process is
catalytic or not. Subsequently, Kalberer et al. (1999) con-
cluded that the NO2to HONO conversion on soot parti-
cles rapidly saturates because of consumption of the reac-
tive sites. They suggested an upper limit for HONO for-
mation of 1018 HONO molecules mg−1freshly emitted soot.
However, Arens et al. (2001) estimated that the total HONO
formation potential of soot particles was about 1.3×1017
HONOmoleculesmg−1of diesel soot, and suggested that
the reaction between NO2and diesel soot particles does not
provide a significant secondary HONO source in the atmo-
sphere. We employ the value of 1.3×1017 HONOmg−1of
freshly emitted soot to calculate the conversion of NO2to
HONO based on the black carbon emissions in the WRF-
CHEM model, considering that the reaction rate of NO2with
freshly emitted soot is dramatically reduced after the first
seconds (Kalberer et al., 1999), which is less than the chem-
ical integration time step in simulations.
Several heterogeneous reactions on different surfaces in-
volving NO2as a reactant have been proposed as potential
contributors to HONO formation in the troposphere (Febo et
al., 1987; Svensson et al., 1987; Saliba et al., 2000; Kleff-
mann et al., 1998; Lammel and Cape, 1996):
2NO2+H2O→HONO+HNO3(R2)
NO+NO2+H2O→2HONO (R3)
NO+HNO3→NO2+HONO (R4)
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6554 G. Li et al.: Impacts of HONO sources on the photochemistry in Mexico City
However, most laboratory studies suggest that the reaction
of NO2to HONO is first order in NO2concentration, thus
the HONO formation can be parameterized by the following
reactions (Aumont et al., 2003):
NO2→HONO, ka(R5)
NO2→HONO, kg(R6)
Where, kaand kgare the first order rate constants for aerosol
and ground surface reactions, respectively.
For the reaction on aerosols, the first order rate constant ka
is estimated by
ka=1
4vNO2(S
V)ra(R7)
where vNO2is the mean molecular speed of NO2,S/ V is
the surface to volume ratio, and rais the reactive uptake co-
efficient. We use a low value of ra=1×10−6in the night-
time (Aumont et al., 2003) and increase it to 5×10−6in
the daytime with the assumption that aerosols are mixed or
covered with organic compounds, humid acids, soot or dust,
which dramatically enhance the HONO formation when the
sunlight is available (George, 2005; Stemmler et al., 2007;
Monge et al., 2010; Ndour et al., 2008). With regard to the
NO2reaction on ground surface, the rate constant is calcu-
lated by
kg=1
8vNO2(S
V)rg(R8)
where rgis the reactive uptake coefficient of ground sur-
face. According to tunnel measurements and experiments
by Kurtenbach et al. (2001), we use a value of rg=1×10−6
at the nighttime. Stemmler et al. (2006) showed that NO2
is effectively reduced to HONO on light activated surfaces
containing humic acids, soils or selected synthetic aromatic
compounds. The dust and soot particles deposited on the
ground surface also enhance the HONO production when the
sunlight is available (Ndour et al., 2008; Monge et al., 2010).
A relatively large uptake coefficient of 2×10−5is suggested,
with dependence on the light intensity. When the sunlight
is available and the light density is less than 400Wm−2, the
uptake coefficient is set to 2×10−5. When the light density
is more than 400Wm−2, the uptake coefficient of 2×10−5
is scaled by a factor defined as (light density)/400. We use
an effective surface of 1.7m2per geometric surface in the
model surface layer to calculate the surface to volume ratio
(Vogel et al., 2003).
2.3 Model configuration
Two three-day episodes from 24 to 26 and from 27 to 29
March 2006 are selected in the present study, representing
typical “O3-Convection South” and “O3-Convection North”
meteorological conditions in Mexico City, respectively (de
Fig. 1. WRF-CHEM simulation domain. Black squares represent
the RAMA (Mexico City Ambient Air Monitoring Network) sites
and red filled circle is the T0 super site at the Instituto del Petroleo
(IMP) laboratories (Mexico, DF).
Foy et al., 2008). The WRF-CHEM model is configured with
one grid with spacing of 3 km (99×99 grid points) centered at
19.538◦N and 99◦E (Fig. 1). Thirty-five vertical levels are
used in a stretched vertical grid with spacing ranging from
50m near the surface, to 500 m at 2.5 km Above Ground
Level (AGL) and 1 km above 14km. The modeling system
employs the microphysics scheme of Lin et al. (1983), the
Yonsei University (YSU) PBL scheme (Noh et al., 2001),
the Noah land-surface model (Chen and Dudhia, 2000), the
RRTM longwave radiation parameterization (Mlawer et al.,
1997), and a shortwave radiation parameterization (Dudhia,
1989). Meteorological initial and boundary conditions are
from NCEP 1◦×1◦reanalysis data. Chemical initial and
boundary conditions are interpolated from MOZART 3-h
output (Horowitz et al., 2003).
Atmos. Chem. Phys., 10, 6551–6567, 2010 www.atmos-chem-phys.net/10/6551/2010/
G. Li et al.: Impacts of HONO sources on the photochemistry in Mexico City 6555
(LT)
Fig. 2. Comparison of HONO measurements from three LP-
DOAS instruments during two field campaigns in the MCMA.
(open circles) HONO mean concentration during MCMA-2003
from DOAS#2 at 70m above ground; (solid circles) MCMA-2003
mean from DOAS#1 at 18m above ground; (solid squares with
dashed line) reference case: MCMA-2006 mean, excluding data
from the model period (24–29 Apri), data measured 16 m above
ground; (open squares) mean during the model period (24–29
April); (crossed squares) data from 29 April 2006. Also shown
on the right scale are ratios of HONO concentrations for data sub-
sets: (green line) MCMA-2003 average of two DOAS instruments,
divided by reference case; (red line) model period divided by refer-
ence case; (black line) 29 April 2006 divided by the reference case.
Two case simulations are performed in this study. In the
homogeneous case (hereafter referred to as H-case), only
the gas phase reaction of NO with OH is considered as the
HONO source. For the enhanced case (hereafter referred to
as E-case), the four HONO sources described in Section 2.2
are included together with the gas phase reaction. We com-
pare the model results from the H-case and E-case with mea-
surements obtained at the T0 urban supersite (Table 1). In
addition, based on the E-case, several experimental studies
have been performed to verify the HONO contribution from
the photolysis of different ortho-nitrophenols and evaluate
the sensitivity of HONO sources to the input parameters.
3 Results
3.1 HONO measurements
Figure 2 compares diurnal profiles of HONO concentra-
tions measured by LP-DOAS during the MCMA-2003 and
MCMA-2006 field campaigns. Two LP-DOAS instruments
were deployed at a mean height of 16m and 70m above
ground at CENICA during MCMA-2003 (Volkamer et al.,
2007). During MCMA-2006, the LP-DOAS was located at
about 14m above ground at T0. Mean diurnal profiles are
shown for the entire campaign duration for the two instru-
ments from MCMA-2003, and for different data subsets for
MCMA-2006. We define the reference case as the mean di-
urnal profile calculated from measurements for all MCMA-
2006 campaign days except the modeling period. If measure-
ments are compared at a comparable mean height of the LP-
DOAS light path above ground, the mean HONO diurnal pro-
files during MCMA-2003 and the reference case agree within
10% at night. Somewhat larger differences are observed dur-
ing daytime, with up to 50% lower daytime HONO observed
during MCMA-2003 compared to the reference case. Ver-
tical concentration gradients can explain up to 40% lower
HONO at 70 m compared to 16m above ground at night dur-
ing MCMA-2003; this is consistent with HONO formation
on urban surfaces as it has been invoked in other studies
(Olaguer et al., 2009). The onset of convective vertical mix-
ing of air shortly after sunrise quickly reduced any vertical
concentration gradients of HONO in the early morning, and
no evidence for vertical gradients was observable 2–3 h after
sunrise during MCMA-2003. Figure 2 shows that on a mean
basis the combination of horizontal and vertical HONO gra-
dients can explain a variability of up to 50% in HONO be-
tween the reference case and the MCMA-2003 data.
During the period from 24–29 April 2006 HONO was sys-
tematically higher than could be explained based on typi-
cal concentration gradients. While nighttime HONO agreed
reasonably well with that observed in the reference scenario
(with the exception of April 29), significantly higher HONO
was observed primarily during daytime. The mean daytime
HONO during 24–29 April was 2–3 times higher than dur-
ing the rest of the MCMA-2006 field campaign, and 4–5
times higher than during MCMA-2003. Differences were
particularly large on 29 April when nighttime HONO was 2-
3 times higher, and daytime HONO was up to 7 times higher
than observed during the reference scenario. The largest en-
hancement in daytime HONO over the reference case was
observed in the mid-morning, but differences persisted for
most of the day. This modeling paper does not attempt to
explain why periods of enhanced daytime HONO are ob-
served in the MCMA. Rather, the focus is on the study of
the role that such periods can play to accelerate the forma-
tion of secondary pollutants such as ozone and secondary
aerosol formation within the MCMA. However, it should be
emphasized that no events of elevated daytime HONO were
observed during MCMA-2003 (Volkamer et al., 2007), and
for the most part of MCMA-2006. The frequency with which
events of enhanced daytime HONO occur within the MCMA
is presently not well established.
3.2 HONO simulations
Figure 3 shows the diurnal variation of the modeled HONO
concentrations in the H-case and E-case compared with the
measurement at T0 from 24 to 29 March 2006. In the H-
case with only the gas phase reaction as the HONO source,
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6556 G. Li et al.: Impacts of HONO sources on the photochemistry in Mexico City
Table 1. Species used for comparison with simulations at T0.
Species Instrumentation Institution Reference
Radical species
OH LIF-FAGE Indiana University Dusanter et al., 2009a
HO2LIF-FAGE Indiana University Dusanter et al., 2009a
Inorganic species
O3UV absorption Indiana University Dusanter et al., 2009a
LP-DOAS (DOAS2) MIT, U. of Heidelberg
NO Chemiluminescence RAMA Dusanter et al., 2009a
NO2Chemiluminescence RAMA Dusanter et al., 2009a
LP-DOAS (DOAS1) MIT, U. of Heidelberg Merten, 2008
LP-DOAS (DOAS2) MIT, U. of Heidelberg Merten, 2008
HNO2LP-DOAS (DOAS1) MIT, U. of Heidelberg Merten, 2008
LP-DOAS (DOAS2) MIT, U. of Heidelberg Merten, 2008
SO2LP-DOAS (DOAS1) MIT, U. of Heidelberg Merten, 2008
LP-DOAS (DOAS2) MIT, U. of Heidelberg Merten, 2008
HNO3ID-CIMS Texas A&M U. Zheng et al., 2008
Aerosols
Sulfate
HR-ToF-AMS U. of Colorado Aiken et al., 2009
Nitrate
Ammonium
SOA
Fig. 3. Simulated and observed HONO at T0 during 24–29 March
2006. The black dots denote the observation, and the red and blue
lines represent the simulations in the E-case and H-case, respec-
tively.
the HONO concentrations are underestimated by a factor of
10 on average compared with the observations, particularly
during nighttime. The simulation results are fairly consistent
with previous studies, which show that the pure gas phase
reaction fails to explain the observed high HONO concentra-
tions (Aumont et al., 1999; Stafelbach et al., 1997; Vogel et
al., 2003). Additionally, the ROx(OH+HO2+RO2) modeling
study for T0 performed by Dusanter et al. (2009b) using a
box model has also demonstrated a systematic underpredic-
tion of the measured HONO concentrations during daytime
if only the homogeneous gas-phase reaction is considered as
the HONO source. Moreover, the simulated peak HONO
concentrations in the H-case occur at 09:00LT (local time),
one or two hours later than the observed value.
When the four additional sources of HONO are consid-
ered in the E-case, the WRF-CHEM model tracks reasonably
well the HONO diurnal variability compared with the mea-
surement. For example, the occurrence of peak HONO con-
centrations is well replicated, and the accumulation of night-
time HONO concentrations and the rapid falloff of morn-
ing HONO concentrations are also reproduced. However,
the E-case frequently underestimates the HONO levels dur-
ing the afternoon, particularly on 28 and 29 March indicat-
ing either unawareness of unknown sources or uncertainties
from meteorological field simulations. For example, Zhou et
al. (2002) speculated that the photolysis of HNO3might con-
tribute significantly to the missing daytime HONO sources.
In the simulations of Vogel et al. (2003), although the di-
rect emission of HONO and the NO2heterogeneous reaction
on aerosol and ground surfaces were included, the simulated
daytime HONO concentrations were also underestimated by
30–50% compared with the observations. An artificial pho-
tolytic HONO source had to be introduced in order to im-
prove the HONO simulations during daytime.
In the present study, a photochemical source of HONO
on ground surfaces has been incorporated into the WRF-
CHEM model with a relatively large uptake coefficient. Ad-
ditionally, in the HONO parameterization, we do not in-
clude the HONO production from the photolysis of different
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G. Li et al.: Impacts of HONO sources on the photochemistry in Mexico City 6557
Fig. 4. Simulated and observed photolysis rate of HONO at T0
during 24–29 March 2006. The black dots denote the observation
and the red represents the simulations in the E-case.
ortho-nitrophenols (Bejan et al., 2006; Kleffmann, 2007),
because all nitrophenols are represented by one lumped
species (NPHE) in the SAPRC 99 mechanism and the ab-
sorption cross section of ortho-nitrophenols is not available
yet. In order to evaluate possible HONO contribution from
the photolysis of different ortho-nitrophenols, we have con-
ducted a sensitivity study in the E-case, assuming that all
nitrophenols are photolyzed to form HONO and the photol-
ysis frequency is interpolated from J[NO2] as suggested by
Bejan et al. (2006). The study provides an upper limit of
HONO production from the photolysis of different ortho-
nitrophenols. The enhancement of HONO concentrations
from the photolysis of nitrophenols is less than 3 ppt on aver-
age during daytime, which is probably due to the low level of
predicted nitrophenols (less than 0.3ppb) in the simulation.
Therefore, the photolysis of ortho-nitrophenols is less likely
to be a significant HONO source in Mexico City.
Meteorological conditions play also a key role in air pollu-
tion simulations principally through determining the disper-
sion or accumulation of pollutant emissions and also influ-
encing their lifetime via changing chemical processes (Bei et
al., 2008). Convections were active during the period from
24 to 29 March (de Foy et al., 2009), which may signifi-
cantly influence the HONO simulation in the WRF-CHEM
model. The frequent cumulus cloud formation and develop-
ment remarkably affect the photolysis rates of HONO. Fig-
ure 4 presents the comparison of the observed and simulated
photolysis rates of HONO at T0. The photolysis rates for
HONO were directly measured using a spectroradiometer at
T0 (Volkamer et al., 2007). The reduction of HONO pho-
tolysis rates due to clouds is salient in the afternoon during
the two episodes, particularly on 28 and 29 March. However,
unfortunately, the WRF-CHEM model frequently fails to re-
solve the formation and development of cumulus clouds us-
ing 3-km horizontal resolution. Therefore, in the afternoon,
the WRF-CHEM model often overestimates the HONO pho-
tolysis rates, and likely underestimates HONO concentra-
Fig. 5. Comparison between simulated and observed HONO at T0
during (a) daytime and (b) nighttime. The red and blue dots repre-
sent the simulations in the E-case and H-case, respectively.
tions during that time. Moreover, the uncertainties in the
parameterization of HONO sources used in the study also
influence the HONO simulations.
Figure 5a and b present the comparisons between simu-
lated and observed HONO concentrations (hourly averaged)
during nighttime and daytime. In order to further evaluate
the performance of the HONO simulation in the H-case and
E-case against the measurements at T0, an “index of agree-
ment” (IOA) to describe the relative difference between the
model and observation is calculated during nighttime and
daytime (Willmott and Wicks, 1980). IOA is defined as:
IOA=1−
N
P
i=1(pi−oi)2
N
P
i=1(|pi−o|+|oi−o|)2(R9)
where piand oiare the simulated and observed variable, re-
spectively. Nis the number of cases and odenotes the av-
erage of the observation. The model index ranges from 0 to
1, with 1 indicating perfect agreement between model and
observation. During daytime, the IOA in the E-case is 0.94,
showing good performance in the HONO simulation in the
E-case. However, the E-case has difficulties in the nighttime
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6558 G. Li et al.: Impacts of HONO sources on the photochemistry in Mexico City
HONO simulations, with IOA of 0.62. In contrast, irregard-
less of daytime or nighttime, the small IOA in the H-case
shows the failure of the homogeneous source to interpret the
observed high levels of HONO.
The most important HONO source in the E-case is
the secondary HONO formation from NO2reaction with
semivolatile organics, contributing about 75% of the HONO
concentrations near the ground level during daytime. When a
large reactive uptake coefficient is used during daytime, the
ground surface also plays an important role in the HONO
formation, providing 18% of the HONO concentrations in
the surface level. The contribution from the reaction of
NO2with the freshly emitted soot is small, less than 2%.
The model simulations conducted by Aumont et al. (1999),
which considered the deactivation of fresh soot surfaces, also
showed that the NO2+soot reaction is not a significant atmo-
spheric source of HONO (Arens et al., 2001). The simu-
lated maximum aerosol surface to volume ratio (S/V ) ex-
ceeds 1.0×10−3m−1, due to the high pollution in Mexico
City. We have used a relatively higher NO2reaction proba-
bility on aerosol surfaces during daytime, but the contribu-
tion of HONO at the aerosol surface is still negligible com-
pared to the ground surface, which is in good agreement with
the results obtained by Vogel et al. (2003).
Near the ground level, the contribution of HONO from the
homogeneous source in the E-case is about 5% during day-
time, which is inconsistent with the results presented by Du-
santer et al. (2009b). Using a box model constrained by mea-
surements at T0, they calculated the loss rate of HONO from
photolysis to be 11.4×106moleculecm−3s−1while the pro-
duction rate from OH+NO is 6.5×106moleculecm−3s−1on
average between 08:40 a.m. and 18:40 LT. They concluded
that the homogeneous source accounts for more than 50%
of the HONO formation rate during this time period. In
the WRF-CHEM model, HONO is transported and dispersed
due to its relatively long lifetime ranging from half an hour in
the early morning to around 10 min at noontime. Particularly,
HONO is efficiently mixed in the PBL by the vertical tur-
bulent exchange. The HONO sources of secondary HONO
formation from NO2reaction with semivolatile organics and
NO2reactions with the freshly emitted soot and ground sur-
face are all included in the surface level in the WRF-CHEM
model, hence the three sources play a dominant role in the
ground level. If the three sources are assumed to be dis-
tributed in the PBL, and considering that the simulated mean
PBL height between 08:40 and 18:40 LT is about 1.5 km and
the height of the model ground level is about 50 m, the ho-
mogeneous source contributes more than 50% to the HONO
production, comparable to the box model results from Du-
santer et al. (2009b).
As the most important HONO source in the surface level,
the NO2reaction with the semivolatile organics is highly pa-
rameterized in the simulations, using a fraction of 0.023 of
the NOxemitted to calculate the HONO formation. Based on
the experiment of Gutzwiller et al. (2002), the semivolatile
organics in diesel exhaust react with NO2within hours un-
der the condition of sufficient water content. Considering the
massive emissions of semivolatile organics from diesel ve-
hicles in Mexico City and the high relative humidity due to
convective activities during the simulation period, the frac-
tion of 0.023 used is reasonable, which is demonstrated by
the comparison of the simulations in the E-case with the ob-
servations at T0. However, the parameterization of the NO2
reaction with the semivolatile organics in the study might not
be fit for moderately polluted areas where the emissions of
semivolatile organics are not high enough. Further studies
are needed to directly parameterize the NO2reaction with
semivolatile organics in models. The uncertainty of the ef-
fective surface used in the WRF-CHEM model may also
considerably influence the simulated HONO levels because
the ground surface plays an important role in the HONO for-
mation in the surface layer. In addition, the HONO forma-
tion from NO2reaction with freshly emitted soot remains
highly uncertain, ranging from 1.3×1017 to 1018 HONO
moleculesmg−1freshly emitted soot and also depending on
the water content (Kalberer et al., 1999; Arens et al., 2001).
We have performed three experimental studies to evaluate
the sensitivities of HONO concentrations to the parameters in
the above three sources based on the E-case. In the first study,
we used a fraction of 0.013 of the emitted NOxto parame-
terize NO2reaction with semivolatile organics in the E-case.
The simulated HONO concentrations are decreased by about
0.67ppb or 34% on six-day average at T0 when the fraction
of the emitted NOxis reduced from 0.023 to 0.013. In the
second study, when only the effective surface is increased
50% in the E-case, the HONO concentrations are increased
by 0.16ppb or 8% on average at T0, and the simulations in
the afternoon are improved but overestimations frequently
occur in the morning. When the value of 1018 HONOmg−1
of freshly emitted soot is used to calculate the conversion of
NO2to HONO in the E-case, the HONO concentrations are
enhanced by about 0.13ppb or 7% on average at T0, and the
NO2-soot reaction become a non-negligible HONO source.
As discussed above, there are large uncertainties in the
HONO simulations due to its poorly understood formation
mechanisms, especially during daytime. However, the addi-
tional sources in the E-case yield fairly good agreement with
the measurement generally. Hence, based on the parameteri-
zation, we will evaluate the impact of HONO sources on the
photochemistry in the following section.
3.3 Impacts on HOx/NOx/O3chemistry
In the polluted urban atmosphere, the major reactions that are
responsible for the O3formation can be described as follows
(Brasseur et al., 1999):
OH+RH →R+H2O (R10)
R+O2+M→RO2+M(R11)
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G. Li et al.: Impacts of HONO sources on the photochemistry in Mexico City 6559
RO2+NO→RO+NO2(R12)
RO+O2→HO2+R0CHO (R13)
HO2+NO→OH+NO2(R14)
2(NO2+hv →NO+O)(R15)
2(O+O2+M→O3+M)(R16)
Overall:
RH+4O2+hv →R0CHO+H2O+2O3(R17)
where RH represents non-methane hydrocarbons, R0CHO
denotes carbonyl products, and R0designates an organic
fragment having one carbon atom fewer than R. The OH
plays a key role in the initialization of the O3formation pro-
cess. The most important primary sources of OH in the pol-
luted atmosphere includes the reaction of H2O with O(1D)
which comes from the photolysis of O3, the reaction of O3
with alkenes and biogenic VOCs-like isoprene and monoter-
penes, and the photolysis of HONO. When the additional
HONO sources are considered in the WRF-CHEM model,
the OH concentration will be increased to further impact the
HO2/RO2concentration, the NO/NO2partition, and the O3
formation.
Due to the variability and uncertainties of observed HOx
the six-day averaged modeled and observed HOx(2-h av-
eraged for OH and 1-h averaged for HO2) at T0 are pre-
sented in Fig. 6. Although the WRF-CHEM model follows
well the variation of OH concentrations, it underestimates
the OH concentrations in the late morning and early after-
noon, regardless of whether the HONO sources are consid-
ered. However, when the HONO sources are accounted for,
the simulation of OH concentrations in the E-case substan-
tially improves compared to the measurement, especially in
the morning. The E-case yields about 86% of the observed
OH concentration averaged during the daytime from 08:00 to
18:00LT; but in the H-case, the simulated OH concentration
is only about 52% of the observation on average. In addition,
the HONO sources play a dominant role in the OH concen-
tration in the morning, consistent with previous model results
(Aumont et al., 2003; Vogel et al., 2003). The inclusion of
the HONO sources in the WRF-CHEM model also increases
the HO2concentrations in the E-case, particularly in the early
morning and late afternoon, but still underestimates the HO2
concentration by a factor of 2 during noontime (Fig. 5b). On
average, the E-case produces 55% of the observed HO2con-
centration during the daytime from 08:00 to 18:00LT, but
the H-case only simulates 34% of the observed HO2concen-
tration. Although the E-case simulates the HOxconcentra-
tions more consistently with the measurements than the H-
case, the modeled HO2concentrations in the E-case remain
about 45% lower than the observations during daytime, indi-
cating either missing radical sources or incomplete chemical
Fig. 6. Influence of the HONO sources on the (a) HO and (b) HO2
diurnal cycle at T0 averaged during the 24–29 March 2006. The
black dots correspond to the observations, and the solid red and blue
lines are the simulations in the E-case and H-case, respectively.
mechanism (Hofzumahaus et al., 2009). The underestima-
tion of VOC concentrations in the model is able to lead to
the underestimation of HO2concentrations. It would cause
an underestimation of the reactivity of OH with VOCs which
in turn would result in an underestimation of peroxy radi-
cals. Dusanter et al. (2009b) have calculated a median noon-
time OH reactivity of 20–22s−1due to VOCs at T0 using a
box model, whereas the mean noontime OH reactivity in the
WRF-CHEM model is around 16s−1. Sheehy et al. (2008)
reported that, during MCMA-2003, the measured OH reac-
tivity at noon was about 20–25s−1, and the MCM box model
explained around 20 s−1. Volkamer et al. (2007) also showed
that a significant portion of the HOxsources is from photol-
ysis of oxygenated VOCs, which may potentially help re-
solve the lack of peroxy radicals. In addition, the uncertain-
ties from measurements also need to be taken into account
when the comparison is performed between simulations and
observations. Dusanter et al. (2009a) have reported that the
accuracy of the HOxmeasurements is about 18% (1σ).
The enhancement of HO2will increase NO2concentra-
tions and decrease NO concentrations by accelerating the
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6560 G. Li et al.: Impacts of HONO sources on the photochemistry in Mexico City
Fig. 7. Simulated and observed (a) NO and (b),(c) NO2at T0 dur-
ing the 24–29 March, 2006. The black dots denote the observation
and the red and blue lines represent the simulations in the E-case
and H-case, respectively. NO2measurement in (b) is from Chemi-
luminescence and (c) is from LP-DOAS.
conversion from NO to NO2in the polluted atmosphere.
When the four HONO sources are considered, in the early
morning, the WRF-CHEM model performs well in the simu-
lation of the NO concentrations compared with the measure-
ment at T0 in the E-case; the NO concentrations are also con-
siderably reduced compared to that in the H-case (Fig. 7a).
However, the WRF-CHEM model overestimates the NO con-
centrations in the late morning and afternoon in both the E-
case and H-case, which may be caused by the underestima-
tion of HO2concentrations or other reasons, such as PBL
evolution and emission temporal variations. Although the
peak NO2concentrations in the morning are comparable to
the measurements in the E-case in Fig. 7b and c, the occur-
rence of the peak NO2concentrations is about one hour later
than the observation. Without the HONO sources, the H-
case substantially underestimates the peak NO2concentra-
tions from 27 to 29 March. It is worth noting that, on 25
and 26 March, the WRF-CHEM model fails to follow the
observed NO2variation during the nighttime and also signif-
icantly overestimates the observation in the early morning,
Fig. 8. Simulated and observed O3(a) averaged over all RAMA
monitoring sites and (b),(c) at T0 site during the 24–29 March
2006. The black dots denote the observation and the red and blue
lines represent the simulations in the E-case and H-case, respec-
tively. O3measurement in (b) is from Chemiluminescence and (c)
is from LP-DOAS.
which is probably caused by the failure of boundary layer
simulations in these two days.
Figure 8a presents the impact of the HONO sources on
the diurnal variation of near-surface O3concentrations av-
eraged over all monitoring sites from 24 to 29 March 2006.
Apparently, the additional HONO sources accelerate the ac-
cumulation of O3concentrations in the morning by about
2h generally in the E-case and subsequently result in around
7ppb enhancement averaged from 08:00 to 14:00LT. How-
ever, during the late afternoon when the HONO sources do
not play a dominant role in the OH production and the plume
formed in the morning move out of Mexico City, the O3in-
crease in the E-case become negligible. The 6-day midday
average of O3enhancement is about 6ppb, which is compa-
rable to the previous CTM study from Lei et al. (2004) in
Houston. They included the HONO source from NO2con-
version on the surfaces of soot aerosol in a CTM and found
that the HONO source speeds up the build-up of morning
O3concentrations by about 1h and leads to an appreciable
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G. Li et al.: Impacts of HONO sources on the photochemistry in Mexico City 6561
Fig. 9. Pattern comparison of simulated vs. observed O3at 14:00LT over Mexico City in the E-case (left) and H-case (right) during the
24–29 March 2006. Colored dots: observations; color contour: simulations.
increase of the O3concentration over the course of the day
with a midday average of 7ppb. In addition, both the E-case
and H-case considerably overestimate the O3concentrations
in the late afternoon, which is attributed to the slow move-
ment of the simulated plume and the overestimation of pho-
tolysis rates in the afternoon (Fig. 4). Figures 8b and c show
the comparisons of modeled and observed diurnal variation
of O3concentrations at T0. The results are similar to that at
ambient monitoring sites, except that inclusion of the HONO
sources in the WRF-CHEM model improves the simulations
of peak O3concentrations. Figure 9 displays the spatial dis-
tributions of calculated (in color contours) and observed (in
colored dots) near surface O3concentrations at 14:00 LT in
the E-case and H-case, respectively. In general, the simulated
and observed O3patterns are well matched, but, clearly, the
predicted plumes moves more slowly than the observation.
For example, at 14:00LT on 27th, the observed plume has
moved to the north of Mexico City, but part of the modeled
plume remains stagnant in the center of the city. Further-
more, the HONO sources do not alter the O3pattern, but con-
siderably enhance the O3level in the center of the plumes.
3.4 Impacts on secondary aerosols
3.4.1 Secondary organic aerosols
In the WRF-CHEM model, a traditional 2-product secondary
organic aerosol (SOA) module is applied to predict the
aerosol distribution and variation. The SOA module accounts
for the SOA formation from the oxidation of six primary or-
ganic groups (POG): alkanes, alkenes, cresol, high-yield aro-
matics, low-yield aromatics, and monoterpenes. Ten semi-
volatile, gaseous SOA precursors (hereafter referred to as
to SVOC) are produced via these reactions – two each for
olefins, monoterpenes, and aromatics, and one each for alka-
nes and cresol.
POG+oxidant →α1SVOC1+α2SVOC2(R18)
The oxidants mainly include the OH radical, the nitrate rad-
ical NO3, and O3. The saturation vapor pressures and mass-
based stoichiometric yield coefficients (αi) of semi-volatile
precursors are obtained from either smog chamber exper-
iments or from published estimates in cases where smog
chamber data are unavailable. The SOA is calculated us-
ing the method developed by Schell et al. (2001), which was
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6562 G. Li et al.: Impacts of HONO sources on the photochemistry in Mexico City
Fig. 10. Simulated submicron SOA and observed submicron OOA
at T0 during the 24–29 March 2006. The black dots denote the
observation and the red and blue lines represent the simulations in
the E-case and H-case, respectively.
based on the absorptive partitioning model of Pankow (1994)
that was extended by Odum et al. (1996). When the or-
ganic gas/aerosol equilibrium has been established, gas and
aerosol-phase concentrations of each SVOC are calculated
iteratively using a globally convergent variation of Newton’s
method. Therefore, the enhancement of the oxidants will
augment the production of SVOCs, especially in the morn-
ing when the VOCs accumulate in the PBL. The increase of
SVOCs is apt to partition more to aerosol phase and further
escalate the SOA concentrations.
Figure 10 shows the comparison of submicron modeled
SOA and observed OOA diurnal variation at T0. In the morn-
ing, the SOA concentrations are substantially enhanced in the
E-case compared to the H-case, with an increase of more than
100% on average. However, during the rest of the day, the en-
hancement of the SOA concentrations is not very significant,
particularly during nighttime. The observed OOA exhibits
clearly two peaks around 10:00 and 14:00LT from 25 to 29.
The E-case and H-case replicates reasonably well these two
peaks, but the simulated second peak in the afternoon is not
obvious in 25 and 28. Although the E-case yields about 40%
more SOA than the H-case averaged in the six days, both the
E-case and H-case significantly underestimate the observed
OOA concentrations. The E-case produces about 13% of the
observed OOA concentrations from the oxidation of anthro-
pogenic precursors, consistent with the previous study per-
formed by Volkamer et al. (2006). They reported much larger
amounts of SOA formed from reactive anthropogenic VOCs
in Mexico City than the estimation from a SOA model based
on empirical parameterization of chamber experiments. In
addition, although the updated aromatic SOA yields from re-
cent chamber experiments (Ng et al., 2007) have been em-
ployed in a photochemical box model study by Dzepina et
al. (2009), the traditional 2-product SOA model still fails
to yield sufficient SOA concentrations to match the obser-
vations by a factor of 7. Hodzic et al. (2009) also reported
that less than 15% of the observed SOA within Mexico City
can be explained by the traditional mechanism based on oxi-
dation of anthropogenic precursors.
The large discrepancies between the modeled SOA and ob-
served OOA concentrations demonstrate that there are other
mechanisms that contribute to the SOA formation, in addi-
tion to the one used in this study. Robinson et al. (2007)
suggested the formation of SOA from primary semivolatile
and intermediate volatility species. Dzepina et al. (2009)
have shown that the mechanism can help to close the gap
in SOA mass between measurements and models in Mexico
City. Additionally, Volkamer et al. (2007) pointed out that
glyoxal is also a source of SOA. In the future study, we will
evaluate the contributions of the above mechanisms to the
SOA concentrations.
3.4.2 Inorganic aerosols
The inorganic aerosols are predicted in the WRF-CHEM
model using ISORROPIA Version 1.7 (http://nenes.eas.
gatech.edu/ISORROPIA/). ISORROPIA calculates the com-
position and phase state of an ammonium-sulfate-nitrate-
chloride-sodium-calcium-potassium-magnesium-water inor-
ganic aerosol in thermodynamic equilibrium with gas phase
precursors. The kind of thermodynamic equilibrium is deli-
cately dependent on the environmental humidity and temper-
ature. In this study, ISORROPIA is mainly utilized to predict
the thermodynamic equilibrium between the ammonium-
sulfate-nitrate-water aerosols and their gas phase precursors
of H2SO4-HNO3-NH3-water vapor. Accordingly, the in-
crease of the OH level due to the HONO sources will impact
the inorganic aerosols in the WRF-CHEM model through
varying H2SO4and HNO3concentrations.
Figure 11a provides the diurnal variation of HNO3con-
centrations in the E-case and H-case compared with the mea-
surements at T0. In the early morning, the E-case and H-case
reproduce well the observed slow accumulation of HNO3
concentrations from about 0.01ppb to 1ppb, but in the af-
ternoon, the WRF-CHEM model overestimates the obser-
vation, which is partially caused by the slow movement of
the simulated plume. The impacts of the HONO sources on
the HNO3concentrations are insignificant in the morning, al-
though the enhancement of the OH concentrations due to the
HONO sources is supposed to increase the HNO3concentra-
tions in the E-case. These results indicate that there are suf-
ficient NH3to balance the enhanced HNO3into the aerosol
phase through the thermodynamic equilibrium. Figure 11b
shows the comparison of NH3concentrations between the E-
case and H-case at T0. Both the E-case and H-case obtain
high levels of NH3concentrations in the morning and the
NH3concentrations in the E-case are also decreased com-
pared to that in the H-case. Therefore, as shown in Fig. 12,
the particle-phase nitrate and ammonium are significantly en-
hanced in the E-case compared to that in the H-case in the
morning. The observed particle-phase nitrate and ammo-
nium show two peaks around 10:00 and 14:00LT from 25
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G. Li et al.: Impacts of HONO sources on the photochemistry in Mexico City 6563
Fig. 11. Same as Fig. 8, except for (a) HNO3and (b) NH3.
to 29 and the concentrations in the first peak are much higher
than those in the second one. When the HONO sources are
considered in the E-case, the WRF-CHEM model reason-
ably well simulates the first peak but remains underestima-
tion from 27 to 29 compared to the observation. In addition,
the HONO sources cause overestimation of the first peak on
25 and 26. The particle-phase nitrate and ammonium are
increased by almost 100% in the E-case compared to the H-
case in the morning, but the enhancement is not significant
in the afternoon; the slow movement of plumes also results
in the overestimation of the particle-phase nitrate and ammo-
nium in the late afternoon.
The formation of H2SO4in the atmosphere through the gas
phase reaction of SO2with OH is not efficient due to the slow
reaction rate. Therefore, the enhancement of the OH concen-
trations due to the HONO sources cannot efficiently convert
SO2into H2SO4to increase the sulfate aerosols. Figure 13a
displays the diurnal variations of SO2concentrations in the
E-case and H-case compared with the measurement at T0.
The impacts of the HONO sources on the SO2concentra-
tions are negligible and the reduction of SO2concentrations
in the E-case is less than 1% during the daytime on aver-
age in comparison with that in the H-case. The WRF-CHEM
model basically tracks the observed variation of SO2concen-
trations during the daytime, but fails to reproduce the large
fluctuations of the observations during the afternoon and at
night. The impacts of the HONO sources on the sulfate at
T0 are also negligible, indicating that the gas phase reaction
of SO2with OH plays a minor role in the sulfate formation
(Fig. 13b). Other sources, such as the oxidation of SO2in
cloud droplets as well as the transport of direct emissions
of sulfate from the Tula industrial complex and the volcano,
may play a key role in the sulfate concentrations in Mexico
City (de Foy et al., 2009).
Fig. 12. Same as Fig. 8, except for submicron (a) nitrate and (b)
ammonium aerosols.
Fig. 13. Same as Fig. 8, except for (a) SO2and (b) submicron
sulfate aerosols.
4 Conclusions
The present study investigates the contribution of HONO
sources to the photochemistry in Mexico City during the
MCMA-2006/MILAGO Campaign using the WRF-CHEM
model. In addition to the gas phase reaction of NO with OH,
four additional HONO sources are considered in the WRF-
CHEM model, including secondary HONO formation from
NO2heterogeneous reaction with semivolatile organics, NO2
www.atmos-chem-phys.net/10/6551/2010/ Atmos. Chem. Phys., 10, 6551–6567, 2010
6564 G. Li et al.: Impacts of HONO sources on the photochemistry in Mexico City
reaction with freshly emitted soot, NO2heterogeneous re-
action on aerosol and ground surfaces. When only the gas
phase reaction is considered, the WRF-CHEM model fails to
predict the high levels of HONO concentrations observed at
T0 by a factor of 10, consistent with previous studies. In con-
trast, the WRF-CHEM model with the four additional HONO
sources performs well in tracking the diurnal variation of the
HONO concentrations, although it frequently underestimates
HONO concentrations in the afternoon. Quantification of
the contribution from individual sources in the ground level
shows that secondary HONO formation from NO2hetero-
geneous reaction with semivolatile organics is the most im-
portant sources in Mexico City, providing about 75% of the
HONO formation. The NO2reaction on ground surfaces also
plays an important role in the HONO formation, with about
18% contribution near the surface. The contributions from
the NO2reaction with newly emitted soot and heterogeneous
reaction on aerosol surfaces is negligible.
Based on the simulations with only the homogeneous reac-
tion and with the additional HONO sources, we evaluate the
impacts of the HONO sources on the HOx/NOx/O3chem-
istry and secondary aerosols. When the additional HONO
sources are considered, the WRF-CHEM model significantly
increases the HOxconcentration, yielding better agreement
with the measurements than if only the gas phase reac-
tion source is considered. Inclusion of additional HONO
sources alters the partition of NO/NO2and converts more
NO into NO2, particularly in the morning. Although the
WRF-CHEM model including the five HONO sources can-
not improve the simulations of the occurrence of peak NO2
concentrations, it performs quite well in modeling the NO
variability compared to the observations at T0. The HONO
sources accelerate the accumulation of O3concentrations av-
eraged over all the ambient monitoring sites in the morning
by about 2h and subsequently lead to a noticeable enhance-
ment of O3concentrations over the course of the day with
a midday average of about 6ppb. The WRF-CHEM model
considerably overestimates O3concentrations in the late af-
ternoon whether the additional HONO sources are consid-
ered or not, which is mainly caused by the slow movement
of the simulated plumes and overestimation of the photolysis
rates.
The HONO sources also play a very important role in the
formation of secondary aerosols in the morning. The HONO
sources substantially enhance the SOA concentrations by a
factor of 2 on average in the morning, but contribute less
during the rest of the day. It is worthy to note that the tradi-
tional 2-product SOA mechanism fails to predict high levels
of SOA mass comparable to the observations at T0, which
agrees well with the other studies in Mexico City (Volka-
mer et al., 2006; Dzepina et al., 2009; Hodzic et al., 2009).
The particle-phase nitrate and ammonium are significantly
enhanced in the morning when all five HONO sources are
included, and agree better with the measurements at T0 com-
pared to the results with only gas phase HONO source. Due
to the inefficient conversion of H2SO4from SO2reaction
with OH, the impact of the HONO sources to the sulfate
aerosols is negligible.
Generally speaking, the HONO sources play an important
role in the photochemistry in the morning by enhancing the
OH concentration in Mexico City. If the air quality in the
morning is of much concern in urban regions, it may be nec-
essary to include these HONO sources in air quality models.
However, except the gas-phase reaction of NO with OH, the
other four HONO sources are all associated with NO2, so
the importance of these four HONO sources is dependent on
the NO2level, i.e., there is a NOxthreshold to activate the
importance of these HONO sources.
Furthermore, it is important to emphasize that the model
period is not representative of all the MCMA-2006 cam-
paign duration, but rather exemplifies the periods with ele-
vated daytime HONO observed in the MCMA, and that dur-
ing these periods the formation of secondary pollutants has
a particular sensitivity to HONO concentrations. Addition-
ally, meteorological conditions also play a key role in de-
termining the accumulation or dispersion of HONO (as well
as other species), also influencing its lifetime via changing
chemical processes. Therefore, meteorological conditions
significantly influence the evaluation of the impact of HONO
sources on the photochemistry in the polluted urban atmo-
sphere.
Acknowledgements. We are indebted to the large number of people
involved in the MILAGRO field campaign as well as those involved
in long-term air quality monitoring and the emissions inventory in
the Mexico City metropolitan area, which made this study possible.
This work was supported by the US National Science Foundation’s
Atmospheric Chemistry Program (ATM-0528227) and the Molina
Center for Energy and the Environment. Acknowledgment is also
made to the National Center for Atmos. Res., which is sponsored
by the National Science Foundation, for the computing time used
in this research.
Edited by: S. Madronich
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