Rohit Srivastava

Atmospheric dust influences global climate and ocean biogeochemistry. Here we present a high‐resolution ice core dust record (1905–2005 CE) from coastal Dronning Maud Land (71°20′S, 11°35′E), East Antarctica, to understand dust flux variability, its causes, and potential contribution to radiative forcing during the 20th century in the South Atlantic sector of East Antarctica (SASA). The dust flux profile (sum of 1–25 μm size fractions) reveals three stepwise increase during 1905–1929, 1930–1979, and 1980–2005 CE time with an average of 0.83, 4.7, and 12.88 mg·m⁻²·year⁻¹, respectively. Our investigation for such dramatic dust flux increase reveals that the increased aridity and favorable wind conditions over Southern South America (SSA), a potential dust source to Antarctica, caused increase in dust production and transport during the late 20th century. The dust flux variability in Southern Hemisphere is concomitant with in‐phase relationship between El‐Niño Southern Oscillation and Pacific Decadal Oscillations, which influenced the spatial distribution of global wet‐dry phase (precipitation pattern). Further, shifting of Southern Annular Mode to positive phase made wind conditions more conducive for long‐range dust transport from SSA to coastal Dronning Maud Land region. To the best of our knowledge, this is the first report of past radiative forcing estimates for Antarctica using ice core dust record into a radiative transfer model. Our estimates of radiative forcing corresponding to the enhanced dust flux yield nearly 30% increase in aerosol forcing during the late 20th century. This has resulted 40% decrease in atmospheric cooling rate, which contributed to net atmospheric warming over SASA.

Aerosol concentrations and their properties strongly depend on dynamics of atmosphere. Effects of physical and dynamical parameterizations on meteorology and black carbon (BC) mass in Weather Research and Forecasting model coupled with Chemistry (WRF-CHEM) are investigated over India. Simulations are performed in ten experiments considering two boundary layer, three cumulus parameterization, and five microphysics schemes during winter and monsoon of 2008. Morrison double-moment physical parameterization, Yonsei University boundary layer parameterization with Kain-Fritsch and Grell-Freitas cumulus parameterization schemes are found suitable to simulate meteorology and BC mass over India. BC mass is found to be underestimated in almost all experiments during winter; while, BC mass is overestimated in monsoon over Ahmedabad, Delhi, and Kanpur, which suggests inefficient wet scavenging of BC in monsoon, while lower emission rate may cause differences in winter. The results will be useful in understanding parameterizations and their impact on aerosols.

The spatio‐temporal variation in black carbon (BC) mass concentration and total aerosol optical properties simulated from regional climate model RegCM Versions 4.3, 4.4 and 4.5 are investigated and compared with in situ and remote sensing measurements. Higher BC mass concentrations (>3 µg m⁻³) are found over the Indo‐Gangetic Basin (IGB) in all versions. BC mass in RegCM 4.4 and 4.5 are closer to observations. BC mass estimated from RegCM 4.4 and 4.5 is found to agree within 20% with respect to observation during pre‐monsoon and monsoon seasons. The differences in observed and simulated mass are >50% during winter over all locations and during winter and post‐monsoon over western India, which suggests the need of improvement in emission inventory over the region, as emissions from the biofuel and crop residue burning in agriculture are very low in the emission inventories. RegCM 4.3, 4.4 and 4.5‐simulated aerosol optical depths (AODs) are found to have more than 40% underestimation during winter and the post‐monsoon over Kanpur, while 4.3 overestimates AOD during the pre‐monsoon and monsoon seasons over Kanpur, and during all seasons except winter over Gandhi College. Single scattering albedo (SSA) values range from 0.75 to 0.95 in 4.4 and 4.5. Lower SSA values are found over the IGB where anthropogenic emissions dominate. SSA simulated by RegCM 4.4 and 4.5 are lowered by 2–10% with respect to aerosol robotic network (AERONET)‐derived SSA. Moreover, during winter such differences are higher (>10%). The RegCM 4.4 and 4.5‐simulated BC mass concentrations are nearer to observations, while for columnar optical properties, RegCM 4.3 is more close to measurement. BC mass is 10–30% closer to observations when model for ozone and related chemical tracers‐chemical transport model chemical boundary condition are utilized in addition to CAM + EC‐EARTH.

Mixing can influence the optical, physical, and chemical characteristics of aerosols, which in turn can modify their life cycle and radiative effects. Assumptions on the mixing state can lead to uncertain estimates of aerosol radiative effects. To examine the effect of mixing on the aerosol characteristics, and their influence on radiative effects, aerosol mixing states are determined over four environmentally distinct locations (Karachi, Gwangju, Osaka, and Singapore) in Asia, an aerosol hot spot region, using measured spectral aerosol optical properties and optical properties model. Aerosol optical depth (AOD), single scattering albedo (SSA), and asymmetry parameter (g) exhibit spectral, spatial, and temporal variations. Aerosol mixing states exhibit large spatial and temporal variations consistent with aerosol characteristics and aerosol type over each location. External mixing of aerosol species is unable to reproduce measured SSA over Asia, thus providing a strong evidence that aerosols exist in mixed state. Mineral dust (MD) (core)-Black carbon (BC) (shell) is one of the most preferred aerosol mixing states. Over locations influenced by biomass burning aerosols, BC (core)-water soluble (WS, shell) is a preferred mixing state, while dust gets coated by anthropogenic aerosols (BC, WS) over urban regions influenced by dust. MD (core)-sea salt (shell) mixing is found over Gwangju corroborating the observations. Aerosol radiative forcing exhibits large seasonal and spatial variations consistent with features seen in aerosol optical properties and mixing states. TOA forcing is less negative/positive for external mixing scenario because of lower SSA. Aerosol radiative forcing in Karachi is a factor of 2 higher when compared to Gwangju, Osaka, and Singapore. The influence of g on aerosol radiative forcing is insignificant. Results emphasize that rather than prescribing one single aerosol mixing state in global climate models regionally and temporally varying aerosol mixing states should be included for more accurate assessment of aerosol radiative effects