Aerosol pH estimation at a coastal location in the Eastern Arabian Sea
Garima Shukla*, Ashwini Kumar
CSIR-National Institute of Oceanography
CSIR-NIO Colony, Dona Paula Panaji, Goa, India Pincode: 403004 Mobile: 7237833963 E-mail: firstname.lastname@example.org
Atmospheric aerosols over the marine region play an important role in the Earths atmosphere and subsequently on the climate system. Coastal and remote marine environments are generally dominated by continental and seawater-derived natural aerosols respectively, however, the outflow of polluted atmospheric particles from the continental regions under favourable meteorological conditions can have strong impacts on aerosol composition and characteristics 1, including the aerosol acidity. The effect of the emissions of the acidic species includes the formation of acid rain 2 and a two to threefold increase in the deposition of reactive N to the oceans. The acidity of atmospheric particle can promote specific chemical processes that result in the production of extra condensed phase mass from lesser volatile species (secondary aerosol), change the optical and water absorption characteristics of particles, and solubilize metals that can function as essential nutrients in nutrient-limited environments. The acidity and liquid water content of atmospheric aerosol are key state parameters that profoundly influence the patterns, fluxes, and impacts of atmospheric nutrient deposition to the oceans. Many key multiphase chemical reactions involving aerosols are strongly pH dependent, thus, the measurement and/or estimation of the acidity as well as water content of aerosols are essential. In this study, aerosol pH is estimated indirectly through thermodynamic model (ISORROPIA-II) wherein, we used chemical composition of fine mode (PM2.5) aerosols collected on Tissuquartz filters at a coastal site in the Eastern Arabian Sea (Goa; 15.45°N, 73.20°E, 56m above the sea level) during 2017-19. Our chemical data shows high water-soluble ionic concentration (WSIC) during winter (November-February; 19.6 ± 5.8 μg m-3) and was lowest during the post-monsoon months (September-October; 14.4 ± 8.1 μg m-3). SO42- ion was predominant among anions while NH4+ was major contributor among cations throughout the season. These results suggest contribution from anthropogenic emissions and formation of secondary inorganic species at the study site.
The modelled results showed highest acidity (average pH = 0.80) in the summer season and is well complemented by the lowest aerosol water content (AWC) during the season (AWC=15.58 ± 3.24 μg m-3). The AWC for post monsoon and winter was reported 29.76 ± 5.97 μg m-3 and 42.41 ± 6.05 μg m-3 respectively and corresponding average pH of 1.15 and 1.03. Variation of diurnal relative humidity over the region being at the coast was the primary driver of the AWC variability. From sensitivity analysis it was found that the variation in aerosol pH was impacted significantly with even a small variation in relative humidity and thus AWC. Dominating concentration of SO42- throughout the study period can be attributed as the reason behind high acidity over the region. Source identification using air mass back-trajectory (AMBT) analysis revealed air parcels originated primarily from the maritime region during summer directing that dimethyl sulphide from the phytoplankton as a major contributor of SO42- and thus higher pH. Winds were driven from continental sites of the Indian subcontinent in winter, whereas from both continental and marine sites in the post-monsoon season. High acidity in winter and post-monsoon was possibly due to the higher contributions of SO42- from biomass burning, industrial activities and fossil fuel combustion.