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Mercury (Hg) is a contaminant of global concern, and an accurate understanding of its atmospheric fate is needed to assess its risks to humans and ecosystem health. Atmospheric oxidation of Hg is key to the deposition of this toxic metal to the Earth's surface. Short-lived halogens (SLHs) can provide halogen radicals to directly oxidize Hg and perturb the budget of other Hg oxidants (e.g., OH and O3). In addition to known ocean emissions of halogens, recent observational evidence has revealed abundant anthropogenic emissions of SLHs over continental areas. However, the impacts of anthropogenic SLHs emissions on the atmospheric fate of Hg and human exposure to Hg contamination remain unknown. Here, we show that the inclusion of anthropogenic SLHs substantially increased local Hg oxidation and, consequently, deposition in/near Hg continental source regions by up to 20%, thereby decreasing Hg export from source regions to clean environments. Our modeling results indicated that the inclusion of anthropogenic SLHs can lead to higher Hg exposure in/near Hg source regions than estimated in previous assessments, e.g., with increases of 8.7% and 7.5% in China and India, respectively, consequently leading to higher Hg-related human health risks. These results highlight the urgent need for policymakers to reduce local Hg and SLHs emissions. We conclude that the substantial impacts of anthropogenic SLHs emissions should be included in model assessments of the Hg budget and associated health risks at local and global scales. [ABSTRACT FROM AUTHOR]

Seasonal variability of volatile organic compounds (VOCs) was studied using year-long observations (June 2019–May 2020) over a high-altitude (1380m AMSL) forested site in the Western Ghats of India. Isoprene, a well-known biogenic VOC, peaked during pre-monsoon. Temperature stress and higher photo synthetically active radiation (PAR) were the drivers behind isoprene emissions from the surrounding forested regions. The daytime average isoprene mixing ratio was 1.92 ± 1.51 ppb during summer and the night time average was 0.02 ± 0.01 ppb. A strong relationship between isoprene mixing ratio and ambient temperature was derived. Most of the anthropogenic VOCs like acetonitrile, benzene, toluene and xylene were found to be higher during winter. These anthropogenic VOCs mostly exhibited afternoon to evening peaks, which suggests the contribution by traffic emissions along with other combustion sources (biofuels) and vertical updraft of pollutants from the surrounding valley region. The toluene to benzene ratio (T/B) was >1 during the afternoon and evening hours in monsoon, whereas, aged air masses with lower T/B ratios (<0.7) prevailed during other seasons. Polar bi-variate plots reveal that toluene, xylene and tri-methyl benzene (TMB) were mostly emitted at the south-west of the measurement site, irrespective of the season, indicating a dominant local traffic related source throughout the year. The contribution of local emissions and regional background on two selected aromatic VOCs, toluene and xylene was estimated, revealing the dominance of local anthropogenic sources during monsoon and post monsoon. Positive matrix factorization revealed three source factors in all the seasons with one additional factor during winter and pre-monsoon. The first factor was constituted of toluene, benzene, xylene and TMB, and the T/B ratio was ∼1.2, indicating traffic/fossil fuel contribution. A second 'biomass burning' factor was dominated by acetonitrile, ethanal, acetone, and benzene in which the T/B ratio was 0.45. Factors one & two were further validated by comparison with chamber experiments conducted using different combustion sources and solvents. The third factor was dominated by isoprene and monoterpenes which were attributed to biogenic emissions. The fourth factor comprised mainly of acetonitrile, ethanal, and acetone indicating aged air masses due to long-range transport with contributions from photochemical production and biomass burning. [Display omitted] • VOCs observed during 1 year over a high-altitude site in the Western Ghats, India were reported for the first time. • The site receives predominately continental aged air masses except during monsoon. • Biogenic emissions were dominant during pre-monsoon season. • Isoprene was the largest emitted biogenic VOC in pre-monsoon (1.01 ppb). • Observed monoterpene concentration had mixed biogenic and anthropogenic origin. • Contribution of local traffic sources was evidenced at the SW of observation site. • PMF analysis showed main contributions from biomass burning and aged air masses. [ABSTRACT FROM AUTHOR]

Atmospheric CH 4 observations from two ground-based stations within Indian subcontinent, namely, Sinhagad (SNG) and Cape Rama station (CRI) showed a strong seasonality with a minima (∼1800 ± 20 ppb) during southwest monsoon (SWM; i.e. June–September, JJAS) and a maxima (2000 ± 30 ppb) during northeast monsoon (NEM i.e. December–February, DJF) with a peak-to-peak seasonality close to 200 ppb. The Indian summer (winter) monsoon is characterized with strong southwesterly (northeasterly) winds of oceanic (land) origin at the surface level and strong easterly (westerly) jet streams aloft. The monsoon dynamics has pronounced impact on CH 4 variability over India and is analyzed with winds, Lagrangian trajectories, and 3-dimentional distributions of CH 4 simulated by a general circulation model. The model simulations suggest a consistent annual vertical structure (mean and sub-seasonal uncertainty) of CH 4 over India with a stark contrast in concentration from summer to winter at surface levels (below 750 mb) in confirmation with what is identified by the ground-based observations. During SWM (NEM) the air with comparatively lower (higher) CH 4 concentrations from southern (northern) hemisphere reduces the CH 4 over India by 1814 ± 26 ppb (enhances by 1950 ± 51 ppb). The contribution of local fluxes to this seasonality appears to be albeit weak as the synthesized CH4 fluxes (from EDGAR dataset) of the Indian peninsula itself show a peak in summer and a dip in winter. Similar property of CH 4 is also common to nearby oceanic region (i.e. over Arabian Sea, 1765 ± 10 ppb during summer) suggesting the role of monsoon dynamics as the controlling factor. Further the mixing and convection carries the CH 4 to the upper atmosphere and advect inward or outward aloft according the seasonal monsoon dynamics. [ABSTRACT FROM AUTHOR]

A network of air quality and weather monitoring stations was established under the System of Air Quality Forecasting and Research (SAFAR) project in Delhi. We report observations of ozone (O 3 ), nitrogen oxides (NO x ), carbon monoxide (CO) and particulate matter (PM 2.5 and PM 10 ) before, during and after the Diwali in two consecutive years, i . e ., November 2010 and October 2011. The Diwali days are characterised by large firework displays throughout India. The observations show that the background concentrations of particulate matter are between 5 and 10 times the permissible limits in Europe and the United States. During the Diwali-2010, the highest observed PM 10 and PM 2.5 mass concentration is as high as 2070 µg/m3 and 1620 μg/m 3 , respectively (24 hr mean), which was about 20 and 27 times to National Ambient Air Quality Standards (NAAQS). For Diwali-2011, the increase in PM 10 and PM 2.5 mass concentrations was much less with their peaks of 600 and of 390 μg/m 3 respectively, as compared to the background concentrations. Contrary to previous reports, firework display was not found to strongly influence the NO x , and O 3 mixing ratios, with the increase within the observed variability in the background. CO mixing ratios showed an increase. We show that the large difference in 2010 and 2011 pollutant concentrations is controlled by weather parameters. [ABSTRACT FROM AUTHOR]

Atmospheric methane (CH 4) is considered to be one of the most important greenhouse gases due to its increasing atmospheric concentrations and the fact that it has a warming potential 28 times that of atmospheric carbon dioxide (CO 2). Over the Indian sub-continent, fluxes and transport both contribute towards CH 4 seasonal variability. Its intra-seasonal variability however is more complex as it is additionally influenced by monsoonal activity during the Asian Summer Monsoon (ASM) period. In this study, the intra-seasonal variability of atmospheric CH 4 is examined using ground-based observations at two sites located in the Southern Indian Peninsula, Sinhagad (SNG) and Cape Rama (CRI); and outputs from three different model simulations. Both, the ground based observations and multi-model simulations show that the dominant spectral variability of CH 4 is coherent with 20–90 day oscillations in the dynamics of the monsoon (termed hereafter as Intra-Seasonal Oscillations, ISOs). The multi-model analysis revealed that CH 4 is heavily influenced by advection due to this intra-seasonal variability. The simulations also display a clear northward propagation of CH 4 anomalies over India. The co-evolution of CH 4 , outgoing long wave radiation (to represent convection) and OH radicals (proxy to CH 4 sinks) is presented. The study quantifies CH 4 variability at intra-seasonal timescales and also its spatial extent. The results suggest that the effect of ISOs on CH 4 needs to be considered along with the corresponding observations for future inverse modeling. Image 1 Composite analysis shows a clear northward propagation of atmospheric CH 4 concentration anomalies with amplitude of ±10 ppb and at a speed of approximately 1.5° latitude per day. On the day when the active spell commences (i.e. at zero on the time axis), the CH 4 concentrations switches from positive to negative at surface, middle and upper troposphere. This is visible in three different model outputs analyzed here although with some differences from model to model.Figure: Active-break composite evolution of atmospheric CH4 anomalies (ppb) over India, as simulated by models LMDz (upper left panel), ACTM (upper right panel), and CAM-Chem (lower panel). The data was averaged from 55 °E to 110 °E and is shown from the equator to 30 °N. • Surface CH 4 observations in India show coherence of 30–90 day oscillations. • Advected CH 4 signal shows clear northward propagation of anomalies during JJAS. • The co-evolutions of CH 4 , OLR, and OH radicals are presented for the JJAS period. [ABSTRACT FROM AUTHOR]

Mercury (Hg) is a contaminant of global concern, and an accurate understanding of its atmospheric fate is needed to assess its risks to humans and ecosystem health. Atmospheric oxidation of Hg is key to the deposition of this toxic metal to the Earth's surface. Short-lived halogens (SLHs) can provide halogen radicals to directly oxidize Hg and perturb the budget of other Hg oxidants (e.g., OH and O 3 ). In addition to known ocean emissions of halogens, recent observational evidence has revealed abundant anthropogenic emissions of SLHs over continental areas. However, the impacts of anthropogenic SLHs emissions on the atmospheric fate of Hg and human exposure to Hg contamination remain unknown. Here, we show that the inclusion of anthropogenic SLHs substantially increased local Hg oxidation and, consequently, deposition in/near Hg continental source regions by up to 20%, thereby decreasing Hg export from source regions to clean environments. Our modeling results indicated that the inclusion of anthropogenic SLHs can lead to higher Hg exposure in/near Hg source regions than estimated in previous assessments, e.g., with increases of 8.7% and 7.5% in China and India, respectively, consequently leading to higher Hg-related human health risks. These results highlight the urgent need for policymakers to reduce local Hg and SLHs emissions. We conclude that the substantial impacts of anthropogenic SLHs emissions should be included in model assessments of the Hg budget and associated health risks at local and global scales., Competing Interests: Competing interests statement:The authors declare no competing interest.