![Scatter plot of mass fraction of NR-PM1 versus absolute mass of NR-PM1 (Org, NO3−,NH4+,SO4−,\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{NO}}_3^ - ,\,{\mathrm{NH}}_4^ + ,\,{\mathrm{SO}}_4^ - ,$$\end{document} and Cl−\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{Cl}}^ -$$\end{document}) for Delhi
(a) Total NR-PM1 mass versus chloride mass fraction (b) Total NR-PM1 mass versus sulphate mass fraction (c) Total NR-PM1 mass versus nitrate mass fraction, and (d) Total NR-PM1 mass versus organic mass fraction. The square points in each panel represent the mean values for different concentration bins and the error bars extend from lower to upper standard deviation.](https://www.researchgate.net/publication/348753803/figure/fig2/AS:984098248069121@1611638714235/Scatter-plot-of-mass-fraction-of-NR-PM1-versus-absolute-mass-of-NR-PM1-Org_Q320.jpg)
![Averaged diel variations of non-refractory chemical components and respective fraction in submicrometer particulate matter (NR-PM1; Org, NO3−,NH4+,SO4−,\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{NO}}_3^ - ,\,{\mathrm{NH}}_4^ + ,\,{\mathrm{SO}}_4^ - ,$$\end{document} and Cl−\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{Cl}}^ -$$\end{document}) measured in Delhi and Chennai
(a) Stacked diel variations in absolute mass concentrations of chemical components in NR-PM1 measured by ACSM during the field campaign in Delhi. (b) Stacked diel variations in the mass fraction of NR-PM1 for Delhi as derived from absolute mass concentrations. (c) Same as (a) for Chennai. (d) Same as (b) for Chennai.](https://www.researchgate.net/publication/348753803/figure/fig3/AS:984098248077313@1611638714440/Averaged-diel-variations-of-non-refractory-chemical-components-and-respective-fraction-in_Q320.jpg)
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Articles
https://doi.org/10.1038/s41561-020-00677-x
1EWRE Division, Department of Civil Engineering, Indian Institute of Technology Madras, Chennai, India. 2School of Earth and Atmospheric Sciences,
Georgia Institute of Technology, Atlanta, GA, USA. 3John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA,
USA. 4Academy of Scientific and Innovative Research (AcSIR), Department of Environment and Sustainability, CSIR – Institute of Minerals and Materials
Technology, Bhubaneswar, India. 5Department of Earth and Environmental Sciences, School of Natural Sciences, University of Manchester, Manchester,
UK. 6National Centre for Atmospheric Science, University of Manchester, Manchester, UK. 7Lancaster Environment Centre, Lancaster University,
Lancaster, UK. 8Data Science Institute, Lancaster University, Lancaster, UK. 9School of Energy and Environment, City University of Hong Kong, Hong Kong
SAR, China. 10Multiphase Chemistry and Biogeochemistry Department, Max Planck Institute for Chemistry, Mainz, Germany. 11Gangarosa Department
of Environmental Health, Rollins School of Public Health, Emory University, Atlanta, GA, USA. 12Department of Earth and Planetary Sciences, Harvard
University, Cambridge, MA, USA. 13Department of Chemical Engineering, Indian Institute of Technology Madras, Chennai, India. 14Scripps Institution of
Oceanography, University of California San Diego, La Jolla, CA, USA. 15Department of Geology and Geophysics, King Saud University, Riyadh, Saudi Arabia.
16Present address: Department of Civil and Infrastructure Engineering, Indian Institute of Technology Jodhpur, Karwar, Jodhpur, India. 17Present address:
The Conflict and Environment Observatory, Hebden Bridge, West Yorkshire, UK. ✉e-mail: s.gunthe@iitm.ac.in; pengfei.liu@eas.gatech.edu
The 2018 World Air Quality Report mentioned New Delhi
as the “world air pollution capital”. In 2017–2018, particu-
late matter (PM) concentrations exceeded 200 μg m−3 and
600 μg m−3 for PM with diameter less than 1.0 µm and 2.5 µm, respec-
tively (that is, PM1 and PM2.5)1,2. The severe pollution, also evidenced
by high aerosol optical depth (AOD) over the Indo-Gangetic Plain
(IGP; Fig. 1), is associated with increased respiratory and cardiovas-
cular diseases, poor visibility and economic damages3. For example,
persistent poor visibility over the New Delhi airport due to fog and
haze has incurred significant financial losses to airline industries4
and led to increasing vehicular deaths3. According to one estimate,
in 2017 over Delhi alone, ~12,000 excess deaths can be attributed to
exceedingly high PM2.5 concentrations5.
Numerous observational studies focusing on surface PM concen-
trations have been conducted over India during the past decade2,6–17.
Although much progress has been made in measuring the seasonal
variability of PM and its chemical comp osition, understanding of the
underlying mechanisms responsible for the reduced visibility and
deterioration of air quality over Delhi and the IGP remains limited.
For example, it is unclear why the PM in Delhi has a higher potential
to form haze and fog than in other polluted Asian cities18, although
a large fraction of Delhi PM is primary organic matter2,19, which is
less hygroscopic20.
Here we present new chemical composition measurements of
non-refractory PM smaller than 1 µm (NR-PM1) from Delhi and
a relatively cleaner Chennai (Fig. 1). Measurements are combined
with thermodynamic modelling (Methods) to elucidate sources
of the observed high chloride concentrations and the implica-
tions for PM concentrations and visibility. We report that high
non-refractory particulate chloride in Delhi probably results from
gas–particle partitioning of HCl gas into aerosol water under typi-
cal winter haze conditions of high relative humidity (RH), low
temperatures and excess ammonia. The HCl is apparently emit-
ted from continental anthropogenic sources, including industrial
and combustion processes. In the process of co-condensation, the
particulate chloride can take up more water, enhancing haze and
fog formation. Our field observations and model calculations show
that 50% visibility reduction in Delhi during the winter can be
Enhanced aerosol particle growth sustained by
high continental chlorine emission in India
Sachin S. Gunthe 1 ✉ , Pengfei Liu 2,3 ✉ , Upasana Panda1,4, Subha S. Raj1, Amit Sharma1,16,
Eoghan Darbyshire 5,17, Ernesto Reyes-Villegas 5, James Allan 5,6, Ying Chen 7,8, Xuan Wang 9,
Shaojie Song3, Mira L. Pöhlker10, Liuhua Shi 11, Yu Wang5, Snehitha M. Kommula1, Tianjia Liu 12,
R. Ravikrishna13, Gordon McFiggans 5, Loretta J. Mickley3, Scot T. Martin3,12, Ulrich Pöschl10,
Meinrat O. Andreae 10,14,15 and Hugh Coe 5
Many cities in India experience severe deterioration of air quality in winter. Particulate matter is a key atmospheric pollutant
that impacts millions of people. In particular, the high mass concentration of particulate matter reduces visibility, which has
severely damaged the economy and endangered human lives. But the underlying chemical mechanisms and physical processes
responsible for initiating haze and fog formation remain poorly understood. Here we present the measurement results of chem-
ical composition of particulate matter in Delhi and Chennai. We find persistently high chloride in Delhi and episodically high
chloride in Chennai. These measurements, combined with thermodynamic modelling, suggest that in the presence of excess
ammonia in Delhi, high local emission of hydrochloric acid partitions into aerosol water. The highly water-absorbing and soluble
chloride in the aqueous phase substantially enhances aerosol water uptake through co-condensation, which sustains particle
growth, leading to haze and fog formation. We therefore suggest that the high local concentration of gas-phase hydrochloric
acid, possibly emitted from plastic-contained waste burning and industry, causes some 50% of the reduced visibility. Our work
implies that identifying and regulating gaseous hydrochloric acid emissions could be critical to improve visibility and human
health in India.
NATURE GEOSCIENCE | VOL 14 | FEBRUARY 2021 | 77–84 | www.nature.com/naturegeoscience 77
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