Articles written in Journal of Earth System Science

    • Preface

      M Dileep Kumar

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    • Relations between marine biogeochemical indices and cloud properties over the Arabian Sea


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      Knowledge of relations among ocean biogeochemical and cloud properties will help to plan experiments necessary to understand the mechanisms and processes underlying the links between ocean and atmosphere interactions. Here, we explored the associations between ocean biogeochemical and cloud properties in a region that seasonally experiences polluted and pristine atmospheric conditions in winter and summer, respectively. The implications of ocean surface chlorophyll-a and biogeochemical fluxes (sea salt, dimethyl sulphide and organic fraction in sea spray) to cloud properties (cloud effective radii (R$_{e}$), cloud optical thickness, and cloud droplet number concentration(CDNC)) were studied using MODIS (Terra, Collection 6, L3) monthly data from 2001 to 2015 along with reanalysis information. We have adopted a climatological averaging approach in time (monthly, seasonal and annual) and space (coastal, open and total (basin) Arabian Sea). This approach was used to minimize incompatibility, if any, between ocean and cloud properties arising from spatio-temporal lags due to different dynamics in the respective boundary layers. The trends in monthly means suggest decreases in chlorophyll-a and CDNC, while R$_{e}$ increased over the Arabian Sea basin during 2001–2015. Variability at the basin scale (expressed as standard deviation in each month, SD) exceeded mean values of respective months for chlorophyll-a, whereas it was nearly half of the mean values for CDNC. An increase in R$_{e}$ seems facilitated more during warmer 2011–2015 than in the 2001–2010 period, which coincided with the decrease in CDNC. Fifteen-year monthly mean climatologies suggest considerable associations among ocean biogeochemical indices and cloud properties, which is more conspicuous during summer monsoon. Increase in sea salt flux appears to account for the higher values of R$_{e}$ in June–July over the basin due to strong monsoon wind. Inverse relations between chlorophyll-a and R$_{e}$ are indicative of smaller droplets that resulted from new particles formed from and or facilitated by marine biogeochemical emissions. Decline in new particle production due to decrease in surface chlorophyll-a and the growth of particles facilitated by increase in warming, seem responsible for increase in R$_{e}$ and decrease in CDNC from 2001 to 2015. Using chlorophyll-a as the main proxy for ocean biogeochemical indices, we demonstrated that connections between ocean biogeochemistry and clouds are sustained in both small and large scales in space and time over the Arabian Sea.

    • AcidiBcation of the coastal Bay of Bengal by aerosols deposition


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      Polluted aerosol transport from South Asia containing oxides of nitrogen and sulphur and their deposition on surface may acidify coastal waters. To test this hypothesis, we have conducted experiments involving (a) variability in aerosol composition at a coastal station (Visakhapatnam, central east coast of India)during 2013–2014 (monthly) and 2015–2016 (weekly observations), and (b) simultaneous observations of aerosols over land and adjacent coastal water in winter of 2013 and 2016. The annual average composition of aerosols during this study was dominated by SO$_4$$^{-2}$(48%) followed by NO$-3$$^-$ (15%). Sulphate (${\sim}$12 ${\mu}$g m$^{-3}$) exhibited high concentrations in Fall Intermonsoon (FIM) and winter monsoon (WM), whereas higher nitrate (3–4.5 ${\mu}$g m$^{-3}$) concentrations were observed during summer monsoon (SM) and FIM. The mean [NO$_3$$^-$/SO$_4$$^{2-}$] ratio of 0.32 suggests that atmospheric aerosol over the study region is contributed by transportation of fossil fuel burning emissions from the subcontinent by high-altitude large-scale wind circulation. The concentrations of NO$_2$ and SO$_2$ varied from 17.2 to 34.3 and 11.5 to 16.4 ${\mu}$g m$^{-3}$, respectively with mean [SO$_2$/NO$_2$] ratio of 0.57 and [SO$_4$$^{2-}$/SO$_4$$^{2-}$+SO$_2$] ratio of 0.33 indicates coalburning (power plants/industries) and fossil fuel burning may be the major source of atmospheric dust in the study region. Comparison of total cations and anion concentrations indicate aerosols are acidic in FIM and SM and mixed nature (acidic/basic) in WM but near neutral in spring Intermonsoon (SIM).Simultaneous experiments revealed that about 5–45% of the atmospheric aerosols were deposited within 10 km from the coast. The in vitro experiments indicated that the deposition of atmospheric aerosols resulted in a measurable decrease in pH of surface seawater and displayed significant relationship betweendecrease in pH and concentration of NO$_3$$^-$/SO$_4$$^{2-}$, but it was weaker with NO$_2$/SO$_2$ suggesting former ions contribute significantly in lowering pH of coastal waters than latter. The impact of decrease in pH on acid–base equilibrium, carbonate chemistry and gas exchanges need to be assessed.

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