Ambient fine particulate matter (PM2.5) concentrations in India frequently exceed 100 mu g/m(3) 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 degrees x 0.3125 degrees) nested-domain simulations over India for short-duration, high-PM2.5 episodes in the fall of 2015 and 2017. The simulations capture the magnitude and spatial patterns of pollution episodes measured by surface monitors (r(PM2.5)(2) = 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 mu g/m(3), respectively. Episodic aerosol increases enhance cooling by lowering the top-of-atmosphere clear-sky direct radiative effect (DRETOA) during the 2015 episode (-6 W/m(2)), with a smaller impact during the 2017 episode (-1 W/m(2)). Differences in DRETOA reflect larger increases in scattering aerosols in the column during the 2015 episode (+17 mg/m(2)) than in 2017 (+13 mg/m(2)), while absorbing aerosol column enhancements are smaller (+3 mg/m(2)) in both years. Changes in shortwave radiation at the surface (SWsfc) are spatially similar to DRETOA and mostly negative during both episodes. CCN enhancements (0.2% supersaturation) 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.

Detailed examination of the impact of modern space launches on the Earth's atmosphere is crucial, given booming investment in the space industry and an anticipated space tourism era. We develop air pollutant emissions inventories for rocket launches and re-entry of reusable components and debris in 2019 and for a speculative space tourism scenario based on the recent billionaire space race. This we include in the global GEOS-Chem model coupled to a radiative transfer model to determine the influence on stratospheric ozone (O-3) and climate. Due to recent surge in re-entering debris and reusable components, nitrogen oxides from re-entry heating and chlorine from solid fuels contribute equally to all stratospheric O-3 depletion by contemporary rockets. Decline in global stratospheric O-3 is small (0.01%), but reaches 0.15% in the upper stratosphere (similar to 5 hPa, 40 km) in spring at 60-90 degrees N after a decade of sustained 5.6% a(-1) growth in 2019 launches and re-entries. This increases to 0.24% with a decade of emissions from space tourism rockets, undermining O-3 recovery achieved with the Montreal Protocol. Rocket emissions of black carbon (BC) produce substantial global mean radiative forcing of 8 mW m(-2) after just 3 years of routine space tourism launches. This is a much greater contribution to global radiative forcing (6%) than emissions (0.02%) of all other BC sources, as radiative forcing per unit mass emitted is similar to 500 times more than surface and aviation sources. The O-3 damage and climate effect we estimate should motivate regulation of an industry poised for rapid growth.

The absorption characteristics and source processes of aerosols are investigated at two nearby distinct altitude sites: Nainital, located over the central Himalayas (similar to 1958m amsl) and Pantnagar, in the adjacent foothill region (similar to 231m amsl) in the Indo-Gangetic Plain region (IGP); based on in-situ measurements and model (GEOS-Chem) simulations. The study reveals the significant influence of biomass burning sources over both the locations during spring, indicating the efficiency of the vertical transport of biomass burning aerosols during the peak of the fire activity period over the northern Indian region. On the other hand, the dominance of fossil fuel emission sources is seen during most part of the year at the mountain site, while biomass/biofuel sources are prevalent at the foothill site. Simulations of different aerosol components in the GEOS-Chem model have revealed that dust aerosols, in addition to carbonaceous aerosols from fossil fuel and biomass burning sources, significantly influence aerosol burden over this broad region covering both high-altitude site Nainital and adjacent foothill site Pantnagar in IGP. Examination of dominant aerosol types and their contribution to the columnar abundance of aerosols is performed. During spring, the contribution of dust aerosols is as high as 22%, even though inorganic aerosols (42%), organic carbon (29%) play a dominant role in modulating aerosol absorption characteristics in the column over this region. This study highlights the importance of absorbing aerosol, their types and quantification for better estimates of radiative forcing of aerosols over this region. This might also provide valuable information for the regional impact assessment of aerosols over the Himalayan region.