Characterization of emissions from South Asian biofuels and application to source apportionment of carbonaceous aerosol in the Himalayas

Abstract

This study focuses on improving source apportionment of carbonaceous aerosol in South Asia and consists of three parts: (1) Development of novel molecular marker–based profiles for real‐world biofuel combustion, (2) Application of these profiles to a year‐long data set and (3) Evaluation of profiles by an in‐depth sensitivity analysis. Emissions profiles for biomass fuels were developed through source testing of a residential stove commonly used in South Asia. Wood fuels were combusted at high and low rates, which corresponded to source profiles high in Organic Carbon (OC) or high in Elemental Carbon (EC), respectively. Crop wastes common to the region, including rice straw, mustard stalk, jute stalk, soybean stalk and animal residue burnings, were also characterized. Biofuel profiles were used in a source apportionment study of OC and EC in Godavari, Nepal. This site is located in the foothills of the Himalayas and was selected for its well‐mixed and regionally impacted air masses. At Godavari, daily samples of fine particulate matter (PM_2.5) were collected throughout the year of 2006, and the annual trends in particulate mass, OC and EC followed the occurrence of a regional haze in South Asia. Maximum concentrations occurred during the dry winter season and minimum concentrations occurred during the summer monsoon season. Specific organic compounds unique to aerosol sources, molecular markers, were measured in monthly composite samples. These markers implicated motor vehicles, coal combustion, biomass burning, cow dung burning, vegetative detritus and secondary organic aerosol as sources of carbonaceous aerosol. A molecular marker–based Chemical Mass Balance (CMB) model provided a quantitative assessment of primary source contributions to carbonaceous aerosol. The new profiles were compared to widely used biomass burning profiles from the literature in a sensitivity analysis. This analysis indicated a high degree of stability in estimates of source contributions to OC when different biomass profiles were used. The majority of OC was unapportioned to primary sources and was estimated to be of secondary origin, while biomass combustion was the next‐largest source of OC. The CMB apportionment of EC to primary sources was unstable due to the diversity of biomass burning conditions in the region. The model results suggested that biomass burning and fossil fuel were important contributors to EC, but could not reconcile their relative contributions.