A direct method to quantify methanol-soluble organic carbon for brown carbon absorption studies.

The current research provides a newly developed method to quantify methanol-soluble organic carbon (MeS_OC) in aerosol samples. This analytical procedure allows an accurate separation of MeS-OC component, which is critical for the calculation of mass absorption efficiency (MAE) of ambient Brown Carbon (BrC) and consequently its climate relevant potential. The method includes extraction, filtering and condensation stages, leading to the preparation of a highly concentrated product in which MeS-OC can be precisely quantified by a Sunset Carbon Analyzer in a single analysis step. This method can be applied on aerosol collected using either high or low volume samplers, since a relatively small filter area is required for the determination. Furthermore, it eliminates any misestimation of the MeS-OC mass that may appear in other reported techniques that don't seem to include the precise separation of methanol-soluble fraction in their quantification process.•The mass quantification of methanol-soluble organic carbon is essential, contributing up to 50% to the absorptivity of organic aerosol (BrC) at shorter wavelengths.•The method provides a direct measurement of methanol-soluble aerosol components, resolving any potential uncertainties of previously applied methods.•The adoption of this direct quantification approach leads to a rationalization of past MAE estimates for BrC with implications for radiative transfer models.

Brown carbon absorption and radiative effects under intense residential wood burning conditions in Southeastern Europe: New insights into the abundance and absorptivity of methanol-soluble organic aerosols.

Biomass burning is a major source of Brown Carbon (BrC), strongly contributing to radiative forcing. In urban areas of the climate-sensitive Southeastern European region, where strong emissions from residential wood burning (RWB) are reported, radiative impacts of carbonaceous aerosols remain largely unknown. This study examines the absorption properties of water- and methanol-soluble organic carbon (WSOC, MeS_OC) in a city (Ioannina, Greece) heavily impacted by RWB. Measurements were performed during winter (December 2019 - February 2020) and summer (July - August 2019) periods, characterized by RWB and photochemical processing of organic aerosol (OA), respectively. PM2.5 filter extracts were analyzed spectrophotometrically for water- and methanol-soluble BrC (WS_BrC, MeS_BrC) absorption. WSOC concentrations were quantified using TOC analysis, while those of MeS_OC were determined using a newly developed direct quantification protocol, applied for the first time to an extended series of ambient samples. The direct method led to a mean MeS_OC/OC of 0.68 and a more accurate subsequent estimation of absorption efficiencies. The mean winter WS_BrC and MeS_BrC absorptions at 365 nm were 13.9 Mm-1 and 21.9 Mm-1, respectively, suggesting an important fraction of water-insoluble OA. Mean winter WS_BrC and MeS_BrC absorptions were over 10 times those observed in summer. MeS_OC was more absorptive than WSOC in winter (mean mass absorption efficiencies - MAE365: 1.81 vs 1.15 m2 gC-1) and especially in summer (MAE: 1.12 vs 0.27 m2 gC-1) due to photo-dissociation and volatilization of BrC chromophores. The winter radiative forcing (RF) of WS_BrC and MeS_BrC relative to elemental carbon (EC) was estimated at 8.7 % and 16.7 %, respectively, in the 300-2500 nm band. However, those values increased to 48.5 % and 60.2 % at 300-400 nm, indicating that, under intense RWB, BrC forcing becomes comparable to that of soot. The results highlight the consideration of urban BrC emissions in radiative transfer models, as a considerable climate forcing factor.

Impact of atmospheric anthropogenic nitrogen on new production in the northern Indian Ocean: constrained based on satellite aerosol optical depth and particulate nitrogen levels.

Aerosols are one of the significant external sources of soluble reactive nitrogen to the surface ocean and their deposition affects the primary productivity. Owing to rapid industrialization over South and Southeast Asia, an increasing trend in atmospheric pollutants was observed over the northern Indian Ocean (NIO). To assess the contribution of the aeolian supply of inorganic nitrogen to the NIO, the available compositional data of marine aerosols collected over this basin between 2001 and 2020 were compiled. Based on the observed relationship of mass load, and particulate nitrate and ammonium concentrations with the corresponding satellite-derived anthropogenic aerosol optical depth (AAOD), the temporal, spatial, and long-term variabilities were derived for the past two decades. In particular, high aerosol mass load, nitrate and ammonium levels were observed in the coastal aerosols of peninsular India during fall and winter and they were low in summer. The atmospheric input of inorganic nitrogen to the Arabian Sea is higher (AS; 1.7 TgN per year) compared to that of the Bay of Bengal (BoB; 0.9 TgN per year) and accounts for ∼30% of the total external sources of nitrogen to the NIO. The new production, supported by external sources of nitrogen, contributes to ∼23 and 53% of export production to the oxygen minimum zone (OMZ) in the AS and BoB respectively. A significant rate of increase in the aerosol mass load (0.05-1.67 µg per m3 per year), and nitrate (0.003-0.04 µg per m3 per year) and ammonium (0.006-0.11 µg per m3 per year) concentrations was observed between 2001 and 2020, likely because of the increased emission of anthropogenic pollutants over South and Southeast Asia and their subsequent long-range atmospheric transport to the NIO. Overall, these results suggest that an enhanced contribution of atmospheric nitrogen may potentially increase (1) the N/P ratio of the surface ocean that impacts phytoplankton composition, (2) export production to the OMZ leads to intensification, and (3) sequestration of atmospheric CO2. A decrease in primary production due to global warming is reported due to a decrease in vertical nutrient supply; however, the increase in atmospheric deposition of nutrients may compensate for this. Therefore, ocean models must be coupled with atmospheric models to better constrain the oceanic response to climate change in the NIO.