Emissions of nonmethane volatile organic compounds from open crop residue burning in the Yangtze River Delta region, China

Open crop residue burning is one of the major sources of air pollutants including the precursors of photooxidants like ozone and secondary organic aerosol. We made measurements of trace gases including nonmethane volatile organic compounds (NMVOCs) in a rural area in central East China in June 2010. During the campaign, we identified six biomass burning events in total through the simultaneous enhancement of carbon monoxide and acetonitrile. Four cases represented fresh plumes (<2 h after emission), and two cases represented aged plumes (>3 h after emission), as determined by photochemical age. While we were not able to quantify formic acid, we identified an enhancement of major oxygenated volatile organic compounds (OVOCs) as well as low molecular alkanes and alkenes, and aromatic hydrocarbons in these plumes. The observed normalized excess mixing ratios (NEMRs) of OVOCs and alkenes showed dependence on air mass age, even in fresh smoke plumes, supporting the view that these species are rapidly produced and destructed, respectively, during plume evolution. Based on the NEMR data in the fresh plumes, we calculated the emission factors (EFs) of individual NMVOC. The comparison to previous reports suggests that the EFs of formaldehyde and acetic acid have been overestimated, while those of alkenes have been underestimated. Finally, we suggest that open burning of wheat residue in China releases about 0.34 Tg NMVOCs annually. If we applied the same EFs to all crops, the annual NMVOC emissions would be 2.33 Tg. The EFs of speciated NMVOCs can be used to improve the existing inventories.