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Air pollution caused by various anthropogenic activities and biomass burning continues to be a major problem in India. To assess the effectiveness of current air pollution mitigation measures, we used a 3D global chemical transport model to analyze the projected optical depth of carbonaceous aerosol (AOD) in India under representative concentration pathways (RCP) 4.5 and 8.5 over the period 2000–2100. Our results show a decrease in future emissions, leading to a decrease in modeled AOD under both RCPs after 2030. The RCP4.5 scenario shows a 48%–65% decrease in AOD by the end of the century, with the Indo‐Gangetic Plain (IGP) experiencing a maximum change of ∼ ${\sim} $25% by 2030 compared to 2010. Conversely, RCP8.5 showed an increase in AOD of ∼ ${\sim} $29% by 2050 and did not indicate a significant decrease by the end of the century. Our study also highlights that it is likely to take three decades for current policies to be effective for regions heavily polluted by exposure to carbonaceous aerosols, such as the IGP and eastern India. We emphasize the importance of assessing the effectiveness of current policies and highlight the need for continued efforts to address the problem of air pollution from carbonaceous aerosols, both from anthropogenic sources and biomass burning, in India. Plain Language Summary: Air pollution from human activities and biomass burning is a significant issue in India. To understand the efficacy of current efforts, a computer model is used to study the projected levels of carbonaceous aerosols (measured as optical depth) from 2000 to 2100. The results suggest that emissions are expected to decrease after 2030, leading to a drop in modeled aerosol levels. In an optimistic scenario of RCP4.5, aerosol levels could decrease by 48%–65% by the end of the century, with the Indo‐Gangetic Plain (IGP) showing the most improvement by 2030. However, in a scenario without any significant measures, aerosol levels may increase by 29% by 2050 and not improve significantly by the end of the century. The study indicates that it might take around 30 years for current pollution control measures to make a noticeable difference in heavily polluted regions like the IGP and eastern India. These findings underscore the importance of evaluating the effectiveness of current policies and the need to address air pollution in India caused by carbonaceous aerosols emitted from both, human activities and biomass burning. Results presented here are crucial for policymakers and the public to understand the progress made and the challenges that persist in combating air pollution over Indian region. Key Points: The future changes in carbonaceous aerosols carry significant consequences for air quality together with climate change in IndiaMitigation of carbonaceous aerosols is essential to maximize the co‐benefits for future air qualityFuture shifts in emissions will affect the degree of mitigation needed for anthropogenic sources over India after the year 2030 [ABSTRACT FROM AUTHOR]

Trichodesmium plays a key role in the biogeochemical cycling of carbon, nitrogen and phosphorus in the Tropical Atlantic Ocean. A complex interplay of physicochemical factors control the growth of Trichodesmium. However, owing to the large spatial and temporal variability, the relative influence of these factors in controlling Trichodesmium distribution and abundance remains unclear. In this study, we examined the basin‐scale distribution pattern of Trichodesmium in the upper 200 m water column of the Atlantic Ocean (25°N–30°S and 70°W–20°E) using a large data set (n = 33,235) and tried to constrain the distribution based on various physicochemical parameters. We suggest that the combined effect of warm temperatures and phosphate (PO43−) availability determines the zonal spatial extent and the abundance of Trichodesmium in the Tropical North Atlantic Ocean. However, the availability of dissolved iron, along with high sea surface temperatures and meteorological parameters such as the wind direction and precipitation, likely govern the meridional distribution of Trichodesmium across the Atlantic Ocean. Excess PO43− at the surface rules out the possibility of PO43− limitation in regulating the meridional distribution of the Trichodesmium. Depth‐integrated nitrogen fixation rates, based on a multiple linear regression, vary from 0.07 to 306 μmol N m−2 d−1. The presence of Trichodesmium colonies down to a depth of 200 m and the depth‐integrated nitrogen fixation rates reflect the pivotal role of Trichodesmium in the nitrogen budget of this region. Plain Language Summary: Microbial nitrogen fixation is the key to carbon sequestration in the ocean. Trichodesmium, being the ubiquitous nitrogen fixing microbe in the Tropical Atlantic Ocean, contributes significantly to nitrogen inputs. Limited availability of data, however, restricts our understanding of environmental parameters in controlling the distribution and abundance of Trichodesmium. To address this, we conducted a comprehensive analysis using large‐scale field‐based data of Trichodesmium abundance to investigate the role of various physical, chemical, and meteorological parameters on the distribution and abundance of Trichodesmium along the zonal and meridional transects of the Tropical Atlantic Ocean. We conclude that Trichodesmium distribution is governed by a complex interplay of environmental factors. Along the zonal transect, Trichodesmium abundance is primarily governed by the availability of PO43− and high sea surface temperatures. Conversely, the inter‐hemispheric variability seems to be influenced by dust deposition (a proxy for iron inputs) and high sea surface temperatures. Furthermore, our estimation of high modeled depth‐integrated nitrogen fixation rates based on Trichodesmium underscores its crucial role in the nitrogen budget. These findings provide valuable insight into the role of environmental factors driving Trichodesmium abundance and its significance toward the global nitrogen budget. Key Points: Trichodesmium distribution across the east‐west continuum in the Tropical North Atlantic Ocean is governed by the availability of phosphate and elevated sea surface temperaturesThe interhemispheric variability in Trichodesmium distribution in the Atlantic Ocean is regulated by the cumulative effect of high temperature and dust depositionTrichodesmium niche may have been overlooked in the past, potentially leading to underestimation of associated N2 fixation rates [ABSTRACT FROM AUTHOR]

Studies of atmospheric trace gases in remote, pristine environments are critical for assessing the accuracy of climate models and advancing our understanding of natural processes and global changes. We investigated the surface ozone (O 3) variability over East Antarctica during the austral summer of 2015–2017 by combining surface and balloon-borne measurements at the Indian station Bharati (69.4 ∘ S, 76.2 ∘ E, ∼ 35 m above mean sea level) with EMAC (ECHAM5/MESSy Atmospheric Chemistry) atmospheric chemistry–climate model simulations. The model reproduced the observed surface O 3 level (18.8 ± 2.3 nmol mol -1) with negligible bias and captured much of the variability (R = 0.5). Model-simulated tropospheric O 3 profiles were in reasonable agreement with balloon-borne measurements (mean bias: 2–12 nmol mol -1). Our analysis of a stratospheric tracer in the model showed that about 41 %–51 % of surface O 3 over the entire Antarctic region was of stratospheric origin. Events of enhanced O 3 (∼ 4–10 nmol mol -1) were investigated by combining O 3 vertical profiles and air mass back trajectories, which revealed the rapid descent of O 3 -rich air towards the surface. The photochemical loss of O 3 through its photolysis (followed by H 2 O + O(1 D)) and reaction with hydroperoxyl radicals (O 3 + HO 2) dominated over production from precursor gases (NO + HO 2 and NO + CH 3 O 2) resulting in overall net O 3 loss during the austral summer. Interestingly, the east coastal region, including the Bharati station, tends to act as a stronger chemical sink of O 3 (∼ 190 pmol mol -1 d -1) than adjacent land and ocean regions (by ∼ 100 pmol mol -1 d -1). This is attributed to reverse latitudinal gradients between H 2 O and O(1 D), whereby O 3 loss through photolysis (H 2 O + O(1 D)) reaches a maximum over the east coast. Further, the net photochemical loss at the surface is counterbalanced by downward O 3 fluxes, maintaining the observed O 3 levels. The O 3 diurnal variability of ∼ 1.5 nmol mol -1 was a manifestation of combined effects of mesoscale wind changes and up- and downdrafts, in addition to the net photochemical loss. The study provides valuable insights into the intertwined dynamical and chemical processes governing the O 3 levels and variability over East Antarctica. [ABSTRACT FROM AUTHOR]

Studies of atmospheric trace gases in remote, pristine environments are critical for assessing the accuracy of climate models and advancing our understanding of natural processes and global changes. We investigated the surface ozone (O3) variability over East Antarctica during the austral summer of 2015-2017 by combining surface and balloon-borne measurements at the Indian station Bharati (69.4° S, 76.2° E, ~35 m above mean sea level) with EMAC atmospheric chemistry-climate model simulations. The model reproduced the observed surface O3 level (18.8 ± 2.3 nmol mol-1) with negligible bias and captured much of the variability (R=0.5). Model simulated tropospheric O3 profiles were in reasonable agreement with balloon-borne measurements (mean bias: 3-11 nmol mol-1). Our analysis of a stratospheric tracer in the model showed that about 40-50% of surface O3 over the entire Antarctic region was of stratospheric origin. Events of enhanced O3 (~4-10 nmol mol-1) were investigated by combining O3 vertical profiles and air mass back trajectories, which revealed the rapid descent of O3-rich air towards the surface. The photochemical loss of O3 through its photolysis (followed by H2O+O(1D)) and reaction with hydroperoxyl radicals (O3+HO2) dominated over production from precursor gases (NO+HO2 and NO+CH3O2) resulting in overall net O3 loss during the austral summer. Interestingly, the east coastal region, including the Bharati station, tends to act as a stronger chemical sink of O3 (~190 pmol mol-1 d-1) than adjacent land and ocean regions (by ~100 pmol mol-1 d-1). This is attributed to reverse latitudinal gradients between H2O and O(1D), whereby O3 loss through photolysis (H2O+O(1D)) reaches a maximum over the east coast. Further, the net photochemical loss at the surface is counterbalanced by downward O3 fluxes, maintaining the observed O3 levels. The O3 diurnal variability of ~1.5 nmol mol-1 was a manifestation of combined effects of mesoscale wind changes and up- and downdrafts, in addition to the net photochemical loss. The study provides valuable insights into the intertwined dynamical and chemical processes governing the O3 levels and variability over East Antarctica. [ABSTRACT FROM AUTHOR]

Tropospheric chlorine chemistry can strongly impact the atmospheric oxidation capacity and composition, especially in urban environments. To account for these reactions, the gas- and aqueous-phase Cl chemistry of the community atmospheric chemistry box model CAABA/MECCA has been extended. In particular, an explicit mechanism for ClNO2 formation following N2O5 uptake to aerosols has been developed. The updated model has been applied to two urban environments with different concentrations of NOx (NO and NO2): New Delhi (India) and Leicester (United Kingdom). The model shows sharp build-up of Cl at sunrise through Cl2 photolysis in both environments. Besides Cl2 photolysis, ClO+NO reaction, and photolysis of ClNO2 and ClONO are prominent sources of Cl in Leicester. High-NOx conditions in Delhi tend to suppress the night-time build-up of N2O5 due to titration of O3 and thus lead to lower ClNO2, in contrast to Leicester. Major loss ClNO2 is through its uptake on chloride, producing Cl2, which consequently leads to the formation of Cl through photolysis. The reactivities of Cl and OH are much higher in Delhi, however, the Cl/OH ratio is up to -7 times greater in Leicester. The contribution of Cl to the atmospheric oxidation capacity is significant and even exceeds (by -2.9 times) that of OH during the morning hours in Leicester. Sensitivity simulations suggest that the additional consumption of VOCs due to active gasand aqueous-phase chlorine chemistry enhances OH, HO2, RO2 near the sunrise. The simulation results of the updated model have important implications for future studies on atmospheric chemistry and urban air quality. [ABSTRACT FROM AUTHOR]