Assessment of submicron aerosol liquid water content and mass-based growth factors in South Asian outflow over the Indian Ocean

Aerosol-bound water, a ubiquitous and abundant component of atmospheric aerosols, has an impact on regional climate, visibility, human health, the hydrological cycle, and atmospheric chemistry. Yet, the intricate relationship between aerosol liquid water (ALWC) and chemical composition and relative humidity (RH) was not well understood. The present study explores ALWC derived from the ISORROPIA II model using real-time, high-resolution data of non-refractory submicron chemical species and meteorological parameters (temperature and RH) collected over the Indian Ocean as part of the ICARB (Integrated Campaign for Aerosols, Gases, and Radiation Budget)-2018 experiment. Results show that ALWC values over the South Eastern Arabian Sea (SEAS) were found to be higher by 4-6 times than those observed over the Equatorial Indian Ocean (EIO) due to a large decrease in aerosol loading from SEAS to EIO. ALWC peaked in the early morning hours (4:00-7:00), with greater values during the nighttime and lower values during the daytime across SEAS, which is comparable with RH variation. While the ratio of organics-to-SO42- mass fraction linearly decreased with increasing mass-based growth factors (MGFs) over EIO, such a scenario was not observed over SEAS. The latitudinal gradient of mass fraction of ALWC had shown a decrease towards EIO, consistent with organic fraction. The extinction coefficient of the dry mass of submicron particles is noticeably increased by 40 % by ALWC over SEAS and EIO. Moreover, ALWC could enhance the aerosol negative forcing by an average of 66 % (64 %) over SEAS (EIO) at the top of the atmosphere during the cruise period. These inferences imply that ALWC is the key factor in assessing the role of aerosols on atmospheric radiative forcing. Overall, the present study highlights the serious need to consider the ALWC in climate forcing simulations, particularly in moist tropical environments where their effect can be significant.

Decadal variations in natural and anthropogenic aerosol optical depth over the Bay of Bengal: the influence of pollutants from Indo-GangeticPlain

Perennial increase in atmospheric pollution over the Bay of Bengal (BoB) and South China Sea is reported due to increase in human population and industrial activity in South and Southeast Asia. Based on total aerosol optical depth (AOD) derived from MODIS (moderate resolution imaging resolution imaging spectroradiometer), natural and anthropogenic fractions were derived. The seasonality and spatial variability in rate of increase in total, natural, and anthropogenic AOD fractions were examined over the BoB using data collected between 2001 and 2019. Both total and anthropogenic AOD displayed statistically significant rate of increase in the northwest BoB (NWB) and western coastal BoB (WCB) regions during 2001 to 2019 whereas the long-term changes are insignificant in the other regions of BoB. Significant increase in AOD in the NWB and WCB regions is mainly contributed by dominant outflow of anthropogenic emissions from Indo-Gangetic Plain (IGP) area of Indian subcontinent. The magnitude of AOD decreased by half from northern BoB to equatorial region due to increase in distance from the source region. The contribution of anthropogenic AOD was >70% to total AOD with higher contribution during winter and lower during summer. The rate of increase in both total and anthropogenic AOD was close to 0.104 and 0.099 per decade in the NWB and 0.069 and 0.059 per decade in the WCB region between 2001 and 2019. The rate of increase in total and anthropogenic AOD decreased from 2001-2009 (0.164 and 0.115 per decade respectively) to 2010-2019 (0.068 and 0.076 per decade respectively) in the NWB region. Significant increase in anthropogenic AOD by 50 and 30% was observed during El Niño and La Niña periods respectively than normal year in both northwest BoB (NWB) and western coastal (WCB) regions due to change in strength and direction of winds. Although some fraction of anthropogenic AOD is found over the entire BoB, significant rate of increase in anthropogenic AOD is found only about 23% of the area of BoB than hitherto reported as entire BoB. The impact of atmospheric deposition of anthropogenic aerosols on biogeochemical processes, such as primary production and ocean acidification, needs further evaluation.

Characterization of particulate matter and black carbon over Bay of Bengal during summer monsoon: results from the OMM cruise experiment

Total and size-segregated particulate matter (PM) and black carbon (BC) concentrations over the Bay of Bengal (BoB) have been measured in the summer monsoon (August-September 2014) onboard a scientific cruise conducted as a part of the Ocean Monsoon and Mixing (OMM) experiment. Role of long-range transport and prevailing meteorology in producing the observed spatio-temporal features is inferred by synthesizing the results of in situ observations in conjunction with the wind components from Modern Era Retrospective Analysis for Research and Applications (MERRA), rainfall data from Tropical Rainfall Measuring Mission (TRMM), surface BC concentration and BC Aerosol Optical Thickness (AOT_BC) from MERRA2 and HYSPLIT back trajectory and dispersion model analysis. Mean values of total PM and BC mass concentrations are observed to be ~ 21.4 μgm^-3 and ~ 393ngm^-3 respectively. The study has revealed significant influence of monsoon rainfall (over the measurement locations and regions through which transport occurred) on the concentration of both PM and BC over northern BoB. Results also indicate transport of aerosols with significant anthropogenic fraction, from the land regions at west to the BoB. A comprehensive analysis showed that while an eastward wind (westerly) from the Indian mainland produced an increase in PM_2.5 over northern BoB, a southerly wind, mostly from the Indian Ocean, caused a decrease in concentration of PM_2.5. Spectral variation of absorption coefficients of aerosols reveals that most of the BC over BoB is associated with fossil fuel combustion. Prevailing strong surface-level convergence (associated with a low-level anticyclone) resulted in accumulation and consequent enhancement of aerosol concentration over central and northern BoB during the study period. In addition, horizontal flow rates estimated across western boundary of BoB using AOT_BC from MERRA2 for 10 years revealed an increasing trend in BC transport from the mainland leading to a gradual buildup in BC concentration over the regions of BoB.

The South Asian monsoon-pollution pump and purifier

Air pollution is growing fastest in monsoon-affected South Asia. During the dry winter monsoon, the fumes disperse toward the Indian Ocean, creating a vast pollution haze, but their fate during the wet summer monsoon has been unclear. We performed atmospheric chemistry measurements by aircraft in the Oxidation Mechanism Observations campaign, sampling the summer monsoon outflow in the upper troposphere between the Mediterranean and the Indian Ocean. The measurements, supported by model calculations, show that the monsoon sustains a remarkably efficient cleansing mechanism by which contaminants are rapidly oxidized and deposited to Earth's surface. However, some pollutants are lofted above the monsoon clouds and chemically processed in a reactive reservoir before being redistributed globally, including to the stratosphere.