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A parameterization for cloud processing is presented that calculates activation of aerosol particles to cloud drops, cloud drop size, and pH-dependent aqueous phase sulfur chemistry. The parameterization is implemented in the global aerosol-climate model ECHAM5-HAM. The cloud processing parameterization uses updraft speed, temperature, and aerosol...
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Citations
... In addition to the SO 4 2− burden, changes in sulfate formation pathways have implication for aerosol climatic effects [i.e., size distributions, cloud condensation nuclei (CCN) concentrations and aerosol radiative effect]. In-cloud sulfate production is a potentially important source of accumulation mode-sized CCN due to chemical growth of activated Aitken particles and the enhanced coalescence of processed particles (51). Thus, all-sky aerosol radiative forcing (difference between the 1970-1974 and 2005-2009 periods) over the North Atlantic is modeled to be +1.2 ...
After the 1980s, atmospheric sulfate reduction is slower than the dramatic reductions in sulfur dioxide (SO 2) emissions. However, a lack of observational evidence has hindered the identification of causal feedback mechanisms. Here, we report an increase in the oxygen isotopic composition of sulfate ( 17 O SO 4 2−) in a Greenland ice core, implying an enhanced role of acidity-dependent in-cloud oxidation by ozone (up to 17 to 27%) in sulfate production since the 1960s. A global chemical transport model reproduces the magnitude of the increase in observed 17 O SO 4 2− with a 10 to 15% enhancement in the conversion efficiency from SO 2 to sulfate in Eastern North America and Western Europe. With an expected continued decrease in atmospheric acidity, this feedback will continue in the future and partially hinder air quality improvements.
... Assuming an error of 10 % in the effective radius, the cloud droplet numbers are about 27 ± 11 and 47 ± 21 cm −3 . These values are consistent to the annual average close to Barbados as shown in Figure 1 by Roelofs et al. (2006) or to previous studies such as Brenguier et al. (2000). They are also in the range of similar cloud scenes retrieved by Wolf et al. (2019) using non-imaging solar reflectivity observations. ...
Abstract. This paper presents a method to retrieve three-dimensional cumulus cloud macro- and microphysics measured by remote sensing instruments on the German research aircraft HALO. This is achieved by combining our hyper-spectral pushbroom spectrometer specMACS with active and passive remote sensing instruments, such as a lidar, a microwave radiometer, a radar and dropsondes. Two-dimensional cloud information such as cloud size, optical thickness, effective radius and thermodynamic phase are retrieved by specMACS with established remote sensing methods. Information of the other active and passive remote sensing instruments with a smaller field-of-view are mapped to the wider specMACS swath following Barker et al. (2011). The combination of specMACS with passive and active remote sensing quantities, for example, the Cloud Top Height from lidar measurements, allows new possibilities: three-dimensional cloud macrophysics can be reconstructed. Applying a sub-adiabatic microphysical model constrained with measurements allows to extend the measured quantities to a three-dimensional representation of microphysics. A consistency check by means of a three-dimensional radiative transfer simulation of the specMACS observations of these derived three-dimensional cloud fields shows good agreement.
... Various parameterizations of cloud droplet nucleation are applied in general circulation models (GCMs; e.g., Ghan et al., 2011;Jiang et al., 2010;Jiang et al., 2012;Roelofs et al., 2006;Zhang et al., 2016). Some earlier cloud microphysical schemes empirically diagnose cloud droplet number concentration from aerosol mass (e.g., Boucher & Lohmann, 1995;Lohmann & Feichter, 1997;Menon et al., 2002) or aerosol number (e.g., Gultepe & Isaac, 1996) to account for aerosol-cloud interaction. ...
Hygroscopicity of aerosol (κchem) is a key factor affecting its direct and indirect climate effects, however, long‐term observation in Delhi is absent. Here we demonstrate an approach to derive κchem from publicly available data sets and validate it (bias of 5%–30%) with long‐term observations in Beijing. Using this approach, we report the first estimation of κchem in Delhi and discuss its climate implications. The bulk‐averaged κchem of aerosols in Delhi is estimated to be 0.42 ± 0.07 during 2016–2018, implying a higher activation ability as cloud condensation nuclei in Delhi compared with Beijing and continental averages worldwide. To activate a 0.1‐μm particle, it averagely requires just a supersaturation of ~0.18% ± 0.015% in Delhi but ~0.3% (Beijing), 0.28%–0.31% (Asia, Africa, and South America) and ~0.22% (Europe and North America). Our results imply that representing κchem of Delhi using Asian/Beijing average may result in a significant underestimation of aerosol climate effects.
... Fine particulate matter (PM 2.5 , which has an aerodynamic diameter equal to or smaller than 2.5 μm) has an important influence on air quality (Chow et al., 2009;Moosmüller et al., 2010;Reisen et al., 2013), atmospheric visibility (Bäumer et al., 2008;Wang et al., 2015;Wu et al., 2005), and human health (Tie et al., 2009;Wenzhen et al., 2011;Zheng et al., 2014). It can even affect the global climate change by modulating the radiation balance of the earth-atmosphere system (Roelofs et al., 2006;Solomon et al., 2011;Wang et al., 2011). Knowledge of the composition of PM 2.5 is an important prerequisite for calculating its radiation effects accurately. ...
As the significant components of PM2.5, almost all of previous studies on water-soluble inorganic ions (WSIIs) have been limited by the use of single sampling station, short sampling times or low temporal resolution. This paper focuses on analysing one-year (2012) observations of WSIIs at a regional central (RCEN) site, a coastal urban (CURB) site and a coastal rural (CRUR) site in the Pearl River Delta region. On average, secondary inorganic aerosols (SIA) were the most abundant component and accounted for over 80% of the total WSIIs. The ratio among sulfate, nitrate and ammonium mass concentrations was close to 2:1:1 (5:2:1) at the RCEN and CURB sites (CRUR site). Most components (except Na⁺) showed higher concentrations in the dry season. The diurnal variations of different ions showed obvious differences, which were partially controlled by photochemical reactions and diffusion conditions in the boundary layer. Ionic formation patterns were different among the three sites. Secondary inorganic pollution was much more serious in the northwestern PRD, and it had a significant effect on pollution in the coastal areas. High SO42 − concentrations at the CRUR site may be associated with local emissions, such as dimethysulfide (DMS). Long-range transport along the southeastern coastline also played an important role in SO42 − pollution over the PRD region. Sea salt aerosols were an important source in coastal regions; they contributed large amounts of Cl⁻, Na⁺, Mg2 + at the CRUR site and large amounts of Na⁺, Mg2 + at the CURB site. The case studies found that sea salt aerosols concentrations increased obviously during the heavy precipitation period of typhoon. The presence of warm-wet air masses before continuous moist weather (CMW) was favourable for the formation of SIA. On the other hand, during CMW periods, SIA concentrations decreased rapidly.
... The model and its substance specific parametrizations had been applied and validated for studies on the global cycling of organochlorine pesticides, 40 semivolatile and nonvolatile PAHs, 26,43 and aerosol components in general. 41,44 Atmospheric chemistry of PAHs and PCDDs is considered by reaction with the hydroxyl radical and ozone in the gasphase (kinetic data in SI, Table S1), while the reactivity of the particulate phase mass fraction of the substances is accounted for in the context of a sensitivity study (SI, S1.5.1). For PAHs, such a simplification appears justified as reactivity in the particulate phase must be significantly less than in the gaseous phase in order to explain levels at remote sites. ...
Forest, savannah and agricultural fires in the tropics and subtropics are sources for wide spread pollution and release many organic substances into air and soil, including persistent organic pollutants, i.e. polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) and polycyclic aromatic hydrocarbons (PAHs). The significance of this source for the exposure of humans and the environment in Africa towards phenanthrene, fluoranthene, pyrene, benzo(a)pyrene, 2,3,7,8-tetrachlorodibenzo-p-dioxin, 1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin, and octachlorodibenzo-p-dioxin is studied using daily global emissions from vegetation fires observed by satellite and a global multicompartment chemistry-transport model. Near-ground atmospheric concentrations of model-predicted vegetation fire related concentrations of PAHs and PCDDs were in the 10-1000 and 10-5-10-3 pg m-3 ranges, respectively. Vegetation fires in Africa are found to emit 180±25 kg yr-1 of PCDD/Fs. By comparison with observations it is found that fires explain 1-10% of the PCDD (5% of 2,3,7,8- tetrachlorodibenzo-p-dioxin) concentrations in the rural and background atmosphere of sub-Saharan Africa. The contribution of vegetation fires to exposure to PAH is probably > 10%, but cannot be quantified due to lack of knowledge with regard to both emission factors and photochemistry. A sensitivity analysis suggests that the heterogeneous reaction of PAHs with ozone is effectively limiting atmospheric lifetime and long-range transport.
... Due to the limited oxidant concentration for winter, the oxidation of SO 2 generally yields the largest amount of sulphate for summer (cf. Roelofs et al., 2006). Moreover, during summer, the SO 2 is distributed over a larger volume of air that also has higher oxidant availability making its transformation more rapid ( Rasch et al., 2000). ...
This paper describes the potential effects of present-day aerosol pollution from North America (USA, Canada) on the climate of the North Atlantic region. The study has been performed by applying the comprehensive atmospheric general circulation model ECHAM5-HAM, which is coupled to a mixed-layer ocean with an embedded thermodynamic sea ice module. The model includes a microphysical aerosol model (HAM), which allows for the assessment of aerosol impacts on climate. Sulphate, black and organic carbon, sea salt and mineral dust are considered as aerosol species. Two equilibrium simulations with two different aerosol pollutant scenarios are compared for each season. We investigate the effect on radiation, temperature, hydrological quantities and dynamics, when human-induced aerosol emissions from North America were omitted. The decrease of both direct and indirect aerosol effects induces a positive change in top of the atmosphere (TOA) radiative fluxes resulting in an overall warming in the whole region. Our results demonstrate the vulnerability especially of the Arctic to the reduction in aerosol load. For fall we find an increase in precipitation over the North Atlantic, associated with a tendency to a larger number of cyclones with high-pressure gradients and a higher frequency in storm days.
... Aqueous chemistry calculations are done for the small and large drop bin regions. An analogous separation into small and large droplet bin regions for aqueous chemistry calculations has been used with 1-D size resolved aerosol and cloud microphysics [Feingold and Kreidenweis, 2002] and with a two-moment modal aerosol and bulk cloud microphysics scheme [Roelofs et al., 2006]. ...
... The majority of regional and global models uses bulk aqueous chemistry and thus ignores these chemical dependencies. An exception is the study of Roelofs et al. [2006], which does separate aqueous chemistry calculations for the smaller (more dilute) and larger (more concentrated) portions of several aerosol modes (which have different compositions), while using the p = 0 cloud-water partitioning assumption. Creating a parameterization with improved and consistent treatments of both the chemical and microphysical aspects of size-dependent aerosol processing seems desirable. ...
The modification of sulfate aerosol by the activation, aqueous chemistry, resuspension cycle in a shallow marine stratiform cloud is modeled using a large-eddy simulation (LES) model and a trajectory ensemble model (TEM). Both dynamical frameworks are coupled with a new microphysics module based on a 2-D joint size distribution function representing both interstitial and cloud particles. Particle mixing, which is represented in LES but not TEM, leads to slightly broader droplet spectra and 6% lower droplet number on average. TEM simulations with a 1-D moving-bin microphysics module, also included in the study, predict far narrower droplet spectra and 15% higher droplet number that are closer to observations except near the cloud boundaries where mixing appears to be important. Despite these differences in simulated droplet spectra, predicted changes in aerosol spectra due to aqueous chemistry are consistent among all three model configurations. The TEM model is used to evaluate assumptions about liquid water partitioning among activated cloud condensation nuclei (CCN). These assumptions are used in large-scale models to map the bulk aqueous chemistry sulfate production to the changes in the aerosol size distribution. Previously used assumptions, such as droplet mass being independent of CCN size or droplet mass being proportional to CCN mass, do not perform well in the considered case. The aerosol spectra changes from aqueous chemistry using these two assumptions differ markedly from the spectra changes with the explicitly predicted water partitioning and have root-mean-squared deviations normalized by the mean (RMSDnorm) of 0.51 and 1.17. These deviations greatly exceed the uncertainties because of the treatment of mixing in the TEM and numerical diffusion in the fixed-bin representation for which the RMSDnorm is about 0.15. Instead, the explicitly predicted water partitioning suggests that in the considered case the mean droplet mass is proportional to CCN dry size.
... Most global aerosol models do not treat the in-cloud aerosol explicitly, but account for the effects of clouds on the aerosol size distribution in simplified ways. For example, sulfate that is formed in the aqueous phase, can for example be added to the accumulation and coarse mode particles (Stier et al., 2005;Roelofs et al., 2006). The pH-dependent aqueous phase chemistry in droplets of different sizes is approximated by Roelofs et al. (2006) by the use of two bins for concentrated and diluted droplets. ...
... For example, sulfate that is formed in the aqueous phase, can for example be added to the accumulation and coarse mode particles (Stier et al., 2005;Roelofs et al., 2006). The pH-dependent aqueous phase chemistry in droplets of different sizes is approximated by Roelofs et al. (2006) by the use of two bins for concentrated and diluted droplets. Tost et al. (2007) explicitly compute the pH-values of clouds and precipitation with the help of a prescribed precipitation size distribution and size-dependent transfer coefficients for atmospheric gases. ...
An explicit and detailed representation of in-droplet and in-crystal aerosol particles in stratiform clouds has been introduced in the global aerosol-climate model ECHAM5-HAM. The new scheme allows an evaluation of the cloud cycling of aerosols and an estimation of the relative contributions of nucleation and collision scavenging, as opposed to evaporation of hydrometeors in the global aerosol processing by clouds. On average an aerosol particle is cycled through stratiform clouds 0.5 times. The new scheme leads to important changes in the simulated fraction of aerosol scavenged in clouds, and consequently in the aerosol wet deposition. In general, less aerosol is scavenged into clouds with the new prognostic treatment than what is prescribed in standard ECHAM5-HAM. Aerosol concentrations, size distributions, scavenged fractions and cloud droplet concentrations are evaluated and compared to different observations. While the scavenged fraction and the aerosol number concentrations in the marine boundary layer are well represented in the new model, aerosol optical thickness, cloud droplet number concentrations in the marine boundary layer and the aerosol volume in the accumulation and coarse modes over the oceans are overestimated. Sensitivity studies suggest that a better representation of below-cloud scavenging, higher in-cloud collision coefficients, or a reduced water uptake by seasalt aerosols could reduce these biases. A possible future extension of the new scheme to convective clouds will be discussed.
... Modellers now recognize the fact that aerosol activation and cloud processing effects are complex and should be effectively integrated within global aerosolclimate model studies (Roelofs et al. 2006). In a recent study, Fountoukis & Nenes (2005) have developed parametrizations to include sulphate and sea-salt aerosol. ...
Aerosols are known to influence significantly the radiative budget of the Earth. Although the direct effect (whereby aerosols scatter and absorb solar and thermal infrared radiation) has a large perturbing influence on the radiation budget, the indirect effect (whereby aerosols modify the microphysical and hence the radiative properties and amounts of clouds) poses a greater challenge to climate modellers. This is because aerosols undergo chemical and physical changes while in the atmosphere, notably within clouds, and are removed largely by precipitation. The way in which aerosols are processed by clouds depends on the type, abundance and the mixing state of the aerosols concerned. A parametrization with sulphate and sea-salt aerosol has been successfully integrated within the Hadley Centre general circulation model (GCM). The results of this combined parametrization indicate a significantly reduced role, compared with previous estimates, for sulphate aerosol in cloud droplet nucleation and, consequently, in indirect radiative forcing. However, in this bicomponent system, the cloud droplet number concentration, N(d) (a crucial parameter that is used in GCMs for radiative transfer calculations), is a smoothly varying function of the sulphate aerosol loading. Apart from sea-salt and sulphate aerosol particles, biomass aerosol particles are also present widely in the troposphere. We find that biomass smoke can significantly perturb the activation and growth of both sulphate and sea-salt particles. For a fixed salt loading, N(d) increases linearly with modest increases in sulphate and smoke masses, but significant nonlinearities are observed at higher non-sea-salt mass loadings. This non-intuitive N(d) variation poses a fresh challenge to climate modellers.
... Aerosol particles co-determine cloud optical characteristics and cloud lifetime, so anthropogenic activities may have altered the role of clouds in the climate of the Earth compared to the pre-industrial atmosphere (e.g., Penner et al., 2001). To estimate the extent of the anthropogenic modification of cloud characteristics on a global scale, coupled aerosol climate models are applied (e.g., Ghan et al., 2001;Roelofs et al., 2005). However, the coupled cloud dynamical, microphysical and chemical system is characterized by spatial and temporal scales spanning several orders of magnitude.Therefore,cloud microphysics have to be parameterized in large scale atmospheric models.In recent years parameterizations of cloud activation have been developed that calculate the cloud drop number concentration (CDNC) based on detailed aerosol size and chemical characteristics and meteorological parameters (e.g., Fountoukis and Nenes, 2005). ...
... The parameterization is implemented in the global aerosol-climate model ECHAM5-HAM. Roelofs et al.(2005;further referred to as R05) present a study of simulated annual and global distributions of CDNC and in-cloud sulfate formation. The model produces realistic distributions for CDNC although CDNC is underestimated in the marine atmosphere.CDNC is generally dominated by particles from the accumulation mode,with smaller contributions from the Aitken and coarse modes.The simulations indicate that in-cloud sulfate production is an important source of accumulation mode sized cloud condensation nuclei (CCN), due to chemical growth of activated Aitken particles and to enhanced coalescence of processed particles.The magnitude of the source depends on the distribution of produced sulfate over the activated modes and is influenced by many uncertainties in the parameters associated with particle activation, such as the updraft velocity, the aerosol chemical composition and the organic solubility. ...
Summary We present simulations of CDNC and cloud drop effective radii over Europe, carried out with the coupled aerosol-climate model ECHAM5-HAM and a detailed cloud processing parameterization. Simulated effective cloud drop radii agree relatively well with measurements, except in winter and over the Mediterranean Sea. Comparison with a simulation without anthropogenic emissions from Europe indicates that anthropogenic activities con- tribute upto 70% to CDNC over continental Europe and in the Mediterranean region. Pollution transports export the European anthropogenic influ- ence towards Asia and to the west coast of Northern Africa.The simulation results suggest an increase of the average cloud optical depth of ~35% over land and ~55% over the Mediterranean. These values, however, carry a large uncertainty because the simulated CDNC is highly sensitive to inaccura- cies in parameters that play a role in cloud activation and cloud chemistry.
