Carbonaceous aerosols cause strong atmospheric heating and large surface cooling that is as important to South Asian climate forcing as greenhouse gases, yet the aerosol sources are poorly understood. Emission inventory models suggest that biofuel burning accounts for 50 to 90% of emissions, whereas the elemental composition of ambient aerosols points to fossil fuel combustion. We used radiocarbon measurements of winter monsoon aerosols from western India and the Indian Ocean to determine that biomass combustion produced two-thirds of the bulk carbonaceous aerosols, as well as one-half and two-thirds of two black carbon subfractions, respectively. These constraints show that both biomass combustion (such as residential cooking and agricultural burning) and fossil fuel combustion should be targeted to mitigate climate effects and improve air quality.

Organic carbon (OC) comprises a large fraction of fine particulate matter (PM_2.5) in Mexico City. Daily and select 12-h PM_2.5 samples were collected in urban and peripheral sites in Mexico City from 17–30 March 2006. Samples were analyzed for OC and elemental carbon (EC) using thermal-optical filter-based methods. Real-time water-soluble organic carbon (WSOC) was collected at the peripheral site. Organic compounds, particularly molecular markers, were quantified by soxhlet extraction with methanol and dichloromethane, derivitization, and gas chromatography with mass spectrometric detection (GCMS). A chemical mass balance model (CMB) based on molecular marker species was used to determine the relative contribution of major sources to ambient OC. Motor vehicles, including diesel and gasoline, consistently accounted for 49% of OC in the urban area and 32% on the periphery. The daily contribution of biomass burning to OC was highly variable, and ranged from 5–26% at the urban site and 7–39% at the peripheral site. The remaining OC unapportioned to primary sources showed a strong correlation with WSOC and was considered to be secondary in nature. Comparison of temporally resolved OC showed that contributions from primary aerosol sources during daylight hours were not significantly different from nighttime. This study provides quantitative understanding of the important sources of OC during the MILAGRO 2006 field campaign.

Water‐soluble organic carbon (WSOC) is typically a large component of carbonaceous aerosols with a high propensity for inducing cloud formation. The sources of WSOC, which may be both of primary and secondary origins, are in general poorly constrained. This study assesses the concentrations and dual‐carbon isotope (¹⁴C and ¹³C) signatures of South Asian WSOC during a 15‐month continuous campaign in 2008–2009. Total suspended particulate matter samples were collected at Sinhagad (SINH) India and at the Maldives Climate Observatory at Hanimaadhoo (MCOH). Monsoon‐driven meteorology yields significant WSOC concentration differences between the dry winter season (0.94 ± 0.43 µg m⁻³ MCOH and 3.6 ± 2.3 µg m⁻³ SINH) and the summer monsoon season (0.10 ± 0.04 µg m⁻³ MCOH and 0.35 ± 0.21 µg m⁻³ SINH). Radiocarbon‐based source apportionment of WSOC not only shows the dominance of biogenic/biomass combustion sources but also a substantial anthropogenic fossil‐fuel contribution (17 ± 4% MCOH and 23 ± 4% SINH). Aerosols reaching MCOH after long‐range over‐ocean transport were enriched by 3–4‰ in δ¹³C‐WSOC relative to SINH. This is consistent with particle‐phase aging processes influencing the δ¹³C‐WSOC signal in the South Asian regional receptor atmosphere.