Unit Affiliation: Ocean and Climate Physics, Lamont-Doherty Earth Observatory (LDEO)
Ambient fine particulate matter (PM2.5) concentrations in India regularly exceed the annual mean national air quality standard of 40 μg/m3 and commonly exceed 100 μg/m3 during peak seasonal pollution episodes. The individual components of PM2.5 differ in their optical properties and thus their radiative effects at the surface. During the fall biomass burning season, total PM2.5 mass is enhanced, but little is known about changes to particle composition and radiative effects during such events. 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 resulting interactions with radiation and clouds during peak pollution episodes compared to annual and seasonal average condition. We simulate annual 2015 and October-December 2016 and 2017 using a 2° x 2.5° global domain, providing boundary conditions to a high resolution (0.25° x 0.3125° ) nested India domain. We focus the nested simulations with TOMAS15 on short-duration extreme haze episodes in fall 2015, 2016, and 2017. We evaluate model output with surface observations from the Central Pollution Control Board and from the SPARTAN (Surface Particulate Matter Network) site in Kanpur, India. Comparing the coarse and fine resolution simulations, average concentrations of total PM2.5 remain approximately the same, but maximum concentrations increase by more than 30%. Higher spatial resolution geographically resolves population dense urban from rural regions, increasing standard deviation of annual average concentrations slightly from 10 μg/m3 to 12 μg/m3. From coarse to high resolution, annual average organic aerosol contributions to total PM2.5 decrease from 40% to 33%, while secondary inorganics increase slightly from 24% to 27% and dust aerosol increases from 33% to 38%. During the fall biomass burning season, organic aerosol is enhanced, and co-located with an increase in cloud condensation nuclei (supersaturation of 0.2%), nearly doubling the total aerosol number concentration and total aerosol mass in northwestern India during this season relative to the annual mean. Finally, we assess climate implications by quantifying differences in the direct radiative effect resulting from changes to aerosol optical properties during select episodes.
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