Carbonaceous Aerosols over Lachung in the Eastern Himalayas: Primary Sources and Secondary Formation of Organic Aerosols in a Remote High-Altitude Environment

Abstract

The Himalayan and Tibetan Plateau, containing the largest ice mass outside the polar region, is very sensitive to the influence of carbonaceous aerosols. In this regard, year-round measurements of carbonaceous aerosols, along with major ionic species, were made over a remote high-altitude (2700 m a.s.l.) site Lachung in the eastern Himalayas to elucidate seasonal source signatures, transport, and secondary organic aerosol (SOA) formation pathways. The observation showed the dominance of organic carbon (OC) in winter (7.6 ± 2.6 μg m–3), having its highest fractional share (32%) to PM10 during both winter and summer. Elemental carbon (EC) concentrations as high as 1 μg m–3 and EC/PM10 > 5% indicated significant anthropogenic influence over this remote site. High OC/EC (5.5 ± 2.5) and the WSOC/OC (0.74 ± 0.15) ratios indicated the dominance of water-soluble secondary organic aerosol (SOA) throughout the year. During spring, the aqueous phase formation (APF) of aerosols was prominent, which is indicated by the strong correlation of aerosol liquid water content with WSOC and SO4 2–. Further, the positive matrix factorization model demonstrated the dominant contribution by biomass burning sources (>25%), followed by primary emission including mineral dust (22%) and vehicular and industrial emissions (20.5%). The role of north-westerly advection (∼88%) was highest in spring, increasing the OC and EC concentrations (∼70%). These observations univocally support the dominant contributions by anthropogenic aerosols to the eastern Himalayas.

Atmospheric Abundance of PM2.5 Carbonaceous Matter and Their Potential Sources at Three High-Altitude Glacier Sites over the Indian Himalayan Range

“…As shown in Tables S1, S3, and S5, the average percentage of SOC in OC was ∼50% (ranging 19–80%) at the WHR, 43% at the CHR (ranging 6–61%), and ∼31% at the EHR (ranging 13–66%). Moreover, the estimated percentage of SOC–OC was 41% in all the three studied sites, which is quite comparable with the earlier reported study of the Himalayan region. , The SOC/OC ratio was the highest at the WHR (0.51) followed by the CHR (0.48) and the EHR (0.32).…”

“…Sandeep et al reported annual averaged PM 2.5 concentrations (73.14 ± 20.76 μg m –3 ) at Srinagar (a town in CHR), which is ∼16% lower than that reported in the present study. Furthermore, the present study reported 2–4 times higher PM 2.5 concentration in the EHR than those reported by Sharma et al and Arun et al …”

“…Sandeep et al 17 reported annual averaged PM 2.5 concentrations (73.14 ± 20.76 μg m −3 ) at Srinagar (a town in CHR), which is ∼16% lower than that reported in the present study. Furthermore, the present study reported 2−4 times higher PM 2.5 concentration in the EHR than those reported by Sharma et al 23 and Arun et al 21 Seasonal mean concentrations during summer and winter periods are 48.51 ± 13.79 and 63.39 ± 16.06 μg m −3 at the WHR and 73.77 ± 39.61 and 102.14 ± 28.99 μg m −3 at the CHR, whereas summer and winter season concentrations were 63.98 ± 26.57 and 82.41 ± 20.66 μg m −3 at the EHR. Highest seasonal average mass concentrations were found in the CHR with higher winter concentrations than nonwinter concentrations (Figure 3).…”

“…Moreover, the estimated percentage of SOC−OC was 41% in all the three studied sites, which is quite comparable with the earlier reported study of the Himalayan region. 21,23 The SOC/OC ratio was the highest at the WHR (0.51) followed by the CHR (0.48) and the EHR (0.32).…”

“…However, the emergence of atmospheric brown clouds (ABCs) over south Asia raised environmental concerns. − Numerous studies have suggested that long-range transport of pollutants from the Indo-Gangetic plain to the Himalayan region during premonsoon is the vital factor. , In addition, local sources from low lands of the Himalayan region also contribute to air pollution . Most of studies were conducted in the foothills − rather than high altitudes of the Himalayan region. − This study measures ambient PM 2.5 and carbonaceous matter (OC and EC) over three subregions of Himalayan glacier locations to evaluate associated spatiotemporal variations and seasonal changes. The study’s findings have important implications for appropriately assessing the effects of carbonaceous matter on glacial melting and water availability in high-altitude Himalayan glacier regions.…”