Investigating drivers of particulate matter pollution over India and the
implications for radiative forcing with GEOS-Chem-TOMAS15
Abstract
Ambient fine particulate matter (PM2.5) concentrations in India
frequently exceed 100 μg/m3 during fall and winter pollution episodes.
We use the GEOS-Chem chemical transport model with the TwO-Moment
Aerosol Sectional microphysics scheme with 15 size bins (TOMAS15) to
assess PM2.5 composition and impacts on radiation and cloud condensation
nuclei (CCN) during pollution episodes as compared to the seasonal
(October-December) average. We conduct high resolution (0.25 degree
x0.3125 degree) nested-domain simulations over India for short-duration,
high-PM2.5 episodes in fall 2015 and 2017. The simulations capture the
magnitude and spatial patterns of pollution episodes measured by surface
monitors (r2PM2.5=0.69) although aerosol optical depth is
underestimated. During the episodes, near-surface organic matter (OM),
black carbon (BC), and secondary inorganic aerosol concentrations
increase from seasonal averages by up to 36, 7, and 7 µg/m3,
respectively. Episodic aerosol increases enhance cooling by lowering the
top-of-atmosphere clear-sky direct radiative effect (DRETOA) during the
2015 episode (-6 W/m2), with a smaller impact during the 2017 episode
(-1 W/m2). Differences in DRETOA reflect larger increases in scattering
aerosols in the column during the 2015 episode (+17 mg/m2) than in 2017
(+13 mg/m2), while absorbing aerosol column enhancements are smaller (+3
mg/m2) in both years. Changes in shortwave radiation at the surface
(SWsfc) are spatially similar to DRETOA and mostly negative during both
episodes. CCN enhancements during these episodes occur across the
western Indo-Gangetic Plain, coincident with higher PM2.5
concentrations. Changes in DRETOA, SWsfc, and CCN during high-PM2.5
episodes may have implications for crops, the hydrologic cycle, and
surface temperature.