Acidity and oxidative potential of atmospheric aerosols over a remote mangrove ecosystem during the advection of anthropogenic plumes.

To investigate the acidity and the water-soluble oxidative potential of PM10, during the continental biomass-burning plume transport, a three-year (2018-2020) winter-time campaign was conducted over a pristine island (21.35°N, 88.32°E) of Sundarban mangrove ecosystem situated at the shore of Bay of Bengal. The average PM10 concentration over Sundarban was found to be 98.3 ± 22.2 µg m-3 for the entire study period with a high fraction of non-sea-salt- SO42- and water-soluble organic carbons (WSOC) that originated from the regional solid fuel burning. The thermodynamic E-AIM(IV) model had estimated that the winter-time aerosols over Sundarban were acidic (pH:2.4 ± 0.6) and mainly governed by non-sea-salt-SO42-. The volume and mass normalized oxidative potential of PM10 was found to be 1.81 ± 0.40 nmol DTT min-1 m-3 and 18.4 ± 6.1 pmol DTT min-1 µg-1 respectively which are surprisingly higher than several urban atmospheres across the world including IGP. The acid-digested water-soluble transition metals (Cu, Mn) show higher influences in the oxidative potential (under high aerosol acidity) compared to the WSOC. The study revealed that the advection of regional solid fuel burning plume and associated non-sea-salt-SO42- is enhancing aerosol acidity and oxidative stress that in turn alters the intrinsic properties of aerosols over such marine ecosystems rich in ecology and bio-geochemistry.

Identification of most preferable reaction pathways for chloride depletion from size segregated sea-salt aerosols: A study over high altitude Himalaya, tropical urban metropolis and tropical coastal mangrove forest in eastern India.

Depletion of chloride from sea-salt aerosols affects their hygroscopicity, cloud condensation nuclei activity as well as microphysical and chemical properties of aerosols and clouds modifying earth-atmosphere radiative balance. Here, we proposed five possible reaction pathways through which the inorganic acids (H2SO4 and HNO3) could deplete chloride from sea-salt aerosols. We determined "maximum potential contribution" (MPC) of each acid and compared the MPC with actual chloride depletion. This step-by-step approach enables us to identify the most preferable reaction pathway(s) for coarse, superfine, accumulation and ultrafine aerosols over a Himalayan station (Darjeeling), a tropical urban station (Kolkata) and a tropical mangrove forest at the north-east coast of Bay of Bengal (Sundarban) in India. Over Kolkata and Darjeeling, locally generated acids reacted with transported sea-salts. Over Sundarban, the locally generated sea-salts from the Bay of Bengal reacted with the acids of biomass burning plume transported from Eastern Ghat and continental haze transported from upper Indo-Gangetic Plain. The average chloride depletion in PM10 ranged between 70 and 74% over Sundarban and 31-34% over Kolkata and Darjeeling. We observed that HNO3(g) depleted the larger (>1 µm) chlorides whereas H2SO4(g) depleted the smaller (<1 µm) chlorides over Kolkata and Darjeeling. However, in addition to H2SO4(g) and HNO3(g), some other species could be involved in chloride depletion over Sundarban mainly during winter. The study reveals that Sundarban acts as the major sink of the inorganic acids transported from Eastern Ghat biomass burning plume inhibiting their further advection towards inland regions.

Below-cloud scavenging of size-segregated aerosols and its effect on rainwater acidity and nutrient deposition: A long-term (2009-2018) and real-time observation over eastern Himalaya.

The major removal pathway of atmospheric aerosols is the below cloud scavenging. The present study is the first-ever in the world, where long-term (2009-2018) as well as real-time observations on the below-cloud scavenging of ultrafine (<0.4 µm), superfine (0.4-1.0 µm) and coarse mode (>1 µm) aerosols have been made. The study was conducted with 919 rain events over a high altitude Himalayan station (27.01 °N, 88.15 °E, 2200 m amsl) in India. The other factors were normalized in order to investigate the "rain only" effect and therefore 919 rain events were screened and finally 165 events were studied. We determined threshold values of the rain rate (and duration) above which aerosols are scavenged in very high proportion (>75%) irrespective of the duration (and rain rate). These threshold values decrease as the aerosol size increases. For example, threshold rain rate decreases from ~17 mm h-1 to ~8 mm h-1 as the aerosol size increases from ultrafine to coarse mode. We also showed that how the rainwater acidity and the deposition flux of major inorganic nutrients (NH4+ + NO3- + SO42-) vary with the rain rate and duration. We observed that the rains either >12 mm h-1 or >80 min are all acidic. Maximum nutrients were accumulated in the ultrafine aerosols and hence the spectrum of the deposition flux of the nutrients (with rain rate and duration) was similar to the scavenging spectrum of ultrafine aerosol. Such long-term database enables us to quantitatively predict the aerosol scavenging, acid rains and nutrient deposition which showed excellent agreement with the observed results. Such quantitative prediction would in turn help the researchers to predict the rain-induced changes in air quality as well as any bio-geo chemical parameter. The present study bears paramount importance in Himalayan context as well as any ecologically-rich regions.