Atmospheric deposition of mineral dust and associated nutrients over the Equatorial Indian Ocean

Aerosols are potential supplier of nutrients to the surface water of oceans and can impact biogeochemical processes particularly in the remote locations. The nutrient data from atmospheric supply is poorly reported from the Indian Ocean region. In this study, we present atmospheric nutrients such as reactive nitrogen species (Nitrate, Ammonium, Organic nitrogen), micro-nutrients (e.g. Fe, Mn and Cu) concentration along with mineral dust in the aerosol samples collected over meridional transect during summer (April-May 2018) and monsoon (June-July 2019) months. A significant spatial variation of dust was observed during summer (0.6-22.8 μg m-3) and monsoon (2.8-25.1 μg m-3) months with a decreasing trend from north to south. Dust as well as other nutrient species shows a general north to south decreasing trend, however, no such trend was seen in the soluble trace elements (TEs) concentration. Anthropogenic species like NH4+ and nss-K+ were found below detection limit during monsoon campaign. The fractional solubility (in percentage) of Fe, Mn and Cu were estimated by measuring their concentration in ultrapure water leach which averaged around 0.99 ± 1.12, 31.0 ± 14.9 and 31.1 ± 25.4, respectively during summer and 0.09 ± 0.08, 6.0 ± 8.9, 16.7 ± 9.6, respectively, during monsoon period. Correlation of soluble Fe with total Fe and total acidic species suggest varying dust sources is an important controlling factor for the fraction solubility of Fe with negligible contribution from the chemical processing. However, a significant correlation was observed between total acid and fractional solubility of Mn and Cu suggest role of chemical processing in enhancement of their solubility. Dry deposition flux of aeolian dust was estimated for both campaign using Al concentration and relatively higher fluxes were observed for summer (12.6 ± 8.4 mg·m-2·d-1) and monsoon (8.7 ± 8.4 mg·m-2·d-1) months as compared to model based estimates reported in the literature. Contrastingly, estimated deposition flux of soluble Fe from both campaign displays relatively lower values as compared to model based results which underscores a need for re-evaluation of biogeochemical models with real-time data.

Continental outflow of anthropogenic aerosols over Arabian Sea and Indian Ocean during wintertime: ICARB-2018 campaign

Chemical characterisation of atmospheric aerosols over Arabian Sea (AS) and Indian Ocean (IO) have been carried out during the winter period (January to February 2018) as part of the Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB-2018). Mass concentrations of organic carbon (OC), elemental carbon (EC), water soluble and insoluble OC (WSOC, WIOC), primary and secondary OC (POC, SOC), water-soluble inorganic ions and trace metals have been estimated with a view to identify and quantify the major anthropogenic pollutants affecting the oceanic environments. Aerosol mass loading was found to exhibit strong spatial heterogeneity (varying from 13 to 84 μg m^-3), significantly modulated by the origin of air-mass trajectories. Chemical analysis of aerosols revealed the presence of an intense pollution plume over south-eastern coastal Arabian Sea, near to south-west Indian peninsula (extending from ~ 12°N to 0° at 75°E) with a strong latitudinal gradient (~3 μg m^-3/deg. from north to south) dominated by anthropogenic species contributing as high as 73% (38% nss-SO₄^2-, 24.2% carbonaceous aerosols (21% Organic Matter, 3.2% EC) and 10% NH₄⁺). Anthropogenic signature over oceanic environment was also evident from the dominance and high enrichment of elements like Zn, Cu, Mn and Pb in trace metals. Long-range transport of air-masses originating from Indo Gangetic Plains and its outflow regions in Bay of Bengal, has been seen over Arabian Sea during winter, that imparted such strong anthropogenic signatures over this oceanic environment. Comparison with previous cruise studies conducted nearly two decades ago shows a more than two-fold increase in the concentration of nss-SO₄^2-, over the continental outflow region in Arabian Sea.

Characteristics of indoor and outdoor fine particles in heating period at urban, suburban, and rural sites in Harbin, China

Concurrent indoor-outdoor fine particulate matter (PM_2.5) measurements were conducted at urban, suburban, and rural sites in Harbin, a megacity in the northeast of China. Chemical constituents of indoor-outdoor PM_2.5 were determined. Infiltration factors (F_INF) of all sites were calculated according to the indoor to outdoor (I/O) ratios of PM_2.5 based on the regression analysis. Linear discriminant analysis (LDA) is applied to determine the indoor-outdoor relationship. Secondary organic carbon (SOC) was calculated on the basis of organic carbon to elemental carbon (OC/EC) ratios. The mean concentrations of indoor and outdoor PM_2.5 were 166.4 ± 32.5 μg/m³ and 228.4 ± 83.7 μg/m³, respectively, during the heating period. OC/EC and potassium ion to elemental carbon (K⁺/EC) ratios verified that biomass was an important source in Harbin especially for rural sites. The nitrate to sulfate (NO₃^-/SO₄^2-) ratio indicates the higher contribution of traffic emissions in urban sites. Cr was the only species that exceeded the guidelines of WHO 2002, which was mainly emitted from coal and oil combustion. SOC/OC and NO₃^-/SO₄^2- ratios, and ion-balanced acidity (the ratio of cation to anion, R_+/-) showed a large urban-rural and indoor-outdoor difference. The highest SOC/OC ratio was found at urban sites, up to 38.3% for indoors. SOC/OC ratios and R_+/- values of indoor environments were higher, which is attributed to the conducive condition of forming the secondary pollutants during the heating period. The results of LDA indicated that the distributions of the chemical components of PM_2.5 at three sites were statistically dissimilar. Graphical abstract.

Chemical Constituents of Carbonaceous and Nitrogen Aerosols over Thumba Region, Trivandrum, India

Aerosol filter samples collected at a tropical coastal site Thumba over Indian region were analysed for water-soluble ions, total carbon and nitrogen, organic carbon (OC), elemental carbon (EC), and water-soluble organic carbon/nitrogen and their sources for different seasons of the year. For the entire study period, the order of abundance of ions showed the dominance of secondary ions, such as SO₄^2-, NO₃^-, and NH₄⁺. On average, Mg²⁺ (56%), K⁺ (11%), SO₄^2- (8.8%), and Ca²⁺ (8.1%) contributions were from maritime influence. There was significant chloride depletion due to enhanced levels of inorganic acids, such as SO₄^2- and NO₃^-. Total carbon contributed 21% of the aerosol total suspended particulate matter in which 85% is organic carbon. Primary combustion-generated carbonaceous aerosols contributed 41% of aerosol mass for the entire study period. High average ratios of OC/EC (5.5 ± 1.8) and WSOC/OC (0.38 ± 0.11) suggest that organic aerosols are predominantly comprised of secondary species. In our samples, major fraction (89 ± 9%) was found to be inorganic nitrate in total nitrogen (TN). Good correlations (R ² ≥ 0.82) were observed between TN with NO₃^- plus NH₄⁺, indicating that nitrate and ammonium ions account for a significant portion of TN. The temporal variations in the specific carbonaceous aerosols and air mass trajectories demonstrated that several pollutants and/or their precursor compounds are likely transported from north western India and the oceanic regions.

Photochemistry of iron in simulated crustal aerosols with dimethyl sulfide oxidation products

Iron contained in dust-derived aerosol particles deposited into remote oceans is essential for phytoplankton productivity, which controls photosynthesis rate and the uptake and release of climate forcing gases. Understanding chemical mechanisms that control iron bioavailability, that is, its speciation, is therefore crucial for global climate predictions. In the present study, the photoredox chemistry of iron in marine atmospheric aerosol particles was investigated by using ferrihydrite as a surrogate iron phase in the presence of dimethyl sulfide (DMS) derived oxidation products: dimethyl sulfoxide (DMSO), dimethyl sulfone (DMS02), methane sulfinic acid (MSIA), and methane sulfonic acid (MSA). Reactants and products were analyzed with UV-vis absorption spectroscopy, ion chromatography, and a hydrogen peroxide sensitive electrode. Results show that MSIA enhances the photoreductive dissolution of iron in a ligand-to-metal charge transfer reaction producing Fe(II), MSA, and H2O2. The rate law for Fe(II) is close to first order (0.79) with regard to adsorbed MSIA and has an empirical rate constant of 1.4 x 10(-4) s(-1). This mechanism may represent a significant pathway through which iron becomes more bioavailable, and it contributes to models of iron and sulfur chemistries in the marine atmosphere.

Major ionic composition of aerosol, rainwater and its impact on surface and sub-surface waters, in and around Mangalore, west coast of India

Physicochemical characteristics of dissolved components from environmental samples have been studied in order to understand the impact of atmospheric pollution from aerosols and rain on surface and sub-surface waters, at industrialised urban areas in and around Mangalore, southwest coast of India. Comparatively, the H(+) ion deposition from the atmosphere was moderately higher than that for other places of the west coast of India, excepting some highly industrialised locations. The major ions in aerosols and rainwater were found to be derived predominantly from the sea-salt owing to strong winds prevailing over the sea over this region. From rain to river substantial enrichment of cations was observed due to leaching processes. Further leaching of ions was well noticed from river water to the sub-surface water, as major cations are higher by factors of 2.6, 2.8, 4.2 and 5.0, respectively. Sulphate and NO(-)(3) concentrations were higher in the sub-surface waters, particularly in the densely populated areas around Mangalore probably due to sewage/effluent contamination. The pH of sub-surface waters (5.98 +/- 1.03) were significantly lower than that in rain and river waters (6.16 +/- 0.75 and 7.12 +/- 0.79), implying fallout of acidic species particularly during the non-monsoon seasons.