Atmospheric radical chemistry determines the atmospheric composition, fate of trace species, secondary production including formation of organic aerosols and harmful tropospheric ozone (O3). Chlorine radicals (Cl•) have a pivotal role in air quality under contrasting urban atmospheric chemistry. Even in trace concentration Cl• is a critical oxidant in urban atmosphere.  Cl• has analogous reaction mechanism to OH• and having noted that Cl• have much faster reaction rates than initiated by OH•. The nocturnal reactions of N2O5 on Cl-rich aerosols affect NOx recycling, oxidation of VOCs and increases levels in particulate matter in winter mornings, thus posing serious threat to human population and reduces visibility. Abundance of Cl• highly depends on regional emissions and Indian region is prone to high chlorine rich PM and notably, northern India falls under most polluted regions globally.  Despite its importance, chlorine chemistry is often overlooked in atmospheric models, underestimating pollution levels.The work utilizes 3D GEOS-Chem model, integrated with anthropogenic HCl emissions coupled with heterogeneous N2O5 + Cl chemistry to evaluate the impact of chlorine chemistry on air quality over Indian region. It includes model's capability in reproducing observations and their distributions, quantifying the changes in total PM2.5 and surface O3. The model successfully reproduces observations, quantifying the effects of chlorine emissions on regional air chemistry. This study provides insights into the distribution of reactive chlorine species, limiting processes, impact on atmospheric oxidative capacity, chlorine-initiated oxidation of VOCs and changes in the levels of atmospheric pollutants. It underscores the necessity of incorporating chlorine emissions and mechanisms into models to accurately predict and understand air quality in India. Further results will be shared at the later stage providing a detailed and its potential effects regional air quality.

Air pollution caused by various anthropogenic activities and biomass burning continues to be a major problem in India. To assess the effectiveness of current air pollution mitigation measures, we used a 3D global chemical transport model to analyze the projected optical depth of carbonaceous aerosol (AOD) in India under representative concentration pathways (RCP) 4.5 and 8.5 over the period 2000-2100. Our results show a decrease in future emissions, leading to a decrease in modeled AOD under both RCPs after 2030. The RCP4.5 scenario shows a 48-65% decrease in AOD by the end of the century, with the Indo-Gangetic Plain (IGP) experiencing a maximum change of ~25% by 2030 compared to 2010. Conversely, RCP8.5 showed an increase in AOD of ~29% by 2050 and did not indicate a significant decrease by the end of the century. Our study also highlights that it is likely to take three decades for current policies to be effective for regions heavily polluted by exposure to carbonaceous aerosols, such as the IGP and eastern India. We emphasize the importance of assessing the effectiveness of current policies and highlight the need for continued efforts to address the problem of air pollution from carbonaceous aerosols, both from anthropogenic sources and biomass burning, in India.

Covid lockdown presented an important opportunity to study relatively cleaner conditions in India. The complex factors of power production, industry, and transportation could be more carefully dissected because of the extreme reduction in the influence of the latter two emission sources. Measurements of cloud condensation nuclei (CCN) activity and other chemical properties of atmospheric aerosols showed that newly formed aerosol particles were produced in the SO2 plume from a large coal-fired power plant, contrary to normal conditions of heavy pollution. The sulfate-rich particles had high CCN activity and number concentration, indicating high cloud-forming potential. Examining the sensitivity of CCN properties under relatively clean conditions over India provides important new constraints on the perturbations of past and future climate forcing by anthropogenic emissions. Because most sensitive regime of aerosol climate forcing on cloud development is the midpoint of relatively clean conditions afforded by the Covid lockdown between background and polluted conditions.

Aerosol Liquid Water Content (ALWC), a ubiquitous component of atmospheric aerosols, contributes to total aerosol mass burden, modulating atmospheric chemistry through aerosol surface reactions and reducing atmospheric visibility. However, the complex dependency of ALWC on aerosol chemistry and relative humidity (RH) in the Indian region remains poorly characterized. Here, we combine available measurements of aerosol chemical composition with thermodynamic model ISORROPIA2.1 to reveal a comprehensive picture of ALWC in fine mode aerosols during the winter season in the Indian region. The fac-tors modulating ALWC are primarily dependent on the RH, such that the effect of aerosol dry mass and hygroscopicity are significant at high RH while the effect of hygroscopicity loses its significance as RH is lowered. ALWC, depending upon the particle hygroscopicity, displays a sharp non-linear rise beyond a critical value of ambient RH. Further analysis coupling WRF-Chem simulation with ISORROPIA2.1 revealed significant spatial heterogeneity in ALWC over India, strongly associating with regions of high aerosol loading and RH. The Indo-Gangetic Plain is consequently observed to be a hotspot of higher ALWC, which explains the prevalent conditions of haze and smog during winter in the region. Our findings re-emphasize that high aerosol mass resulting from intense pollution is vital in modulating aerosol–climate interaction under favorable meteorological conditions. They suggest the need for pollution control strategies to be directed at the reduction in emissions of specific species like NH3 and NOx, which were observed to contribute to the enhancement of PM and ALWC during wintertime in the region.