Dual carbon isotope characterization of total organic carbon in wintertime carbonaceous aerosols from northern India

Abstract

Large‐scale emissions of carbonaceous aerosols (CA) from South Asia impact both regional climate and air quality, yet their sources are not well constrained. Here we use source‐diagnostic stable and radiocarbon isotopes (δ13C and Δ14C) to characterize CA sources at a semiurban site (Hisar: 29.2°N, 75.2°E) in the NW Indo‐Gangetic Plain (IGP) and a remote high‐altitude location in the Himalayan foothills (Manora Peak: 29.4°N, 79.5°E, 1950 m above sea level) in northern India during winter. The Δ14C of total aerosol organic carbon (TOC) varied from −178‰ to −63‰ at Hisar and from −198‰ to −1‰ at Manora Peak. The absence of significant differences in the 14C‐based fraction biomass of TOC between Hisar (0.81 ± 0.03) and Manora Peak (0.82 ± 0.07) reveals that biomass burning/biogenic emissions (BBEs) are the dominant sources of CA at both sites. Combining this information with δ13C, other chemical tracers (K+/OC and SO42−/EC) and air mass back trajectory analyses indicate similar source regions in the IGP (e.g., Punjab and Haryana). These results highlight that CA from BBEs in the IGP are not only confined to the atmospheric boundary layer but also extend to higher elevations of the troposphere, where the synoptic‐scale circulations could substantially influence their abundances both to the Himalayas and over the downwind oceanic regions such as the Indian Ocean. Given the vast emissions of CA from postharvest crop residue combustion practices in the IGP during early Northeast Monsoon, this information is important for both improved process and model understanding of climate and health effects, as well as in guiding policy decision aiming at reducing emissions.

Carbon isotope‐constrained seasonality of carbonaceous aerosol sources from an urban location (Kanpur) in the Indo‐Gangetic Plain

“…A comparison of f bio‐TC from this study with other sampling sites in South Asia (Figure ) reveals that biomass combustion is a major source of carbonaceous aerosols. The mean percentage contribution of 14 C‐based f bio‐TC is highest over the IGP source sites (e.g., Hisar, ~81% and Manora peak, ~82%, [ Bikkina et al ., ]); and Kanpur (~77%; this study) except for Delhi (~53% [ Kirillova et al ., ]), owing to their proximity to biomass combustion source regions. Since Delhi is a megacity, traffic emissions contribute more significantly at that location, which explains the low f bio‐TC despite having influence from BBEs in Punjab and Haryana.…”

“…Geographical variability of 14 C‐based fraction biomass ( f bio‐TC ) of total carbon (TC) from South Asia (Kanpur, this study; Hisar and Manora Peak, Bikkina et al . []; Delhi, Kirillova et al . []; Sinhagad and Maldives Climate Observatory at Hanimaadhoo (MCOH), Sheesley et al .…”

“…The loading of TC on blank filter was ~3.4 μg cm −2 , which accounts for no more than ~21% of sample concentration. The Δ 14 C blank of TC at Kanpur was consistent with that reported from another urban site in the IGP (Hisar, −667 ± 6‰ [ Bikkina et al ., ]) and remote Arctic aerosols (−776 ± 0.2‰ and −617 ± 5‰ [ Barrett et al ., ]). However, the Δ 14 C blank of TC at Kanpur is somewhat more fossil than those observed at other South Asian receptor sites (Sinhagad, −433 ± 12‰ [ Sheesley et al ., ]; MCOH, −309 ± 9‰ [ Sheesley et al ., ]; and South Asian aerosols, 0.57 [ Gustafsson et al ., ]).…”

“…However, we focused on the cluster output files to those PM 10 analyzed for radiocarbon-stable carbon isotopes of TC to derive the CWT analysis-based f bio-TC . The trajectory-weighted concentration/isotopic signatures were estimated by the following mathematical expression [Bikkina et al, 2016a;Kirillova et al, 2013;Sheesley et al, 2012]:…”

“…The major sources of carbonaceous aerosols over the IGP include wood/crop residue burning emissions and vehicular/coal‐fired power plant emissions [ Bikkina et al ., ; Ram and Sarin , ; Srinivas and Sarin , ]. The prevailing meteorology over the IGP during NE monsoon together with shallow atmospheric boundary layer facilitates the advection of carbonaceous aerosols and other pollutants to the Indian Ocean [ Bikkina et al ., ; Lawrence and Lelieveld , ]. In fact, the vast emissions of carbonaceous aerosols from the postharvest crop residue burning period (October–November) in the IGP set up a gradient of atmospheric outflow from NW to NE India along the Himalayan range and enters into the Bay of Bengal (referred to as IGP outflow) [ Kaskaoutis et al ., ; Srinivas and Sarin , ].…”