Emission ratio of carbonaceous aerosols observed near crop residual burning sources in a rural area of the Yangtze River Delta Region, China

Intensive open crop residue burning (OCRB) has a great impact on regional air quality and climate. A field observation campaign in a rural area of the Yangtze River Delta Region (YRDR) was performed during the harvest season, and Elemental carbon (ECa), organic carbon (OC), black carbon (BCe), carbon monoxide (CO), carbon dioxide (CO2) and PM2.5 mass were concurrently measured. During the observation period, urban pollution and OCRB-impact episodes were classified. The emission ratio of ECa mass (defined as the Delta ECa/Delta CO ratio) from OCRB was estimated to be 18.2 +/- 4.6 ng/m(3)/ppbv, much higher than that (3.0 +/- 0.3 ng/m(3)/ppbv) of urban pollution from the YRDR. A significant amount of OC was emitted from OCRB with Delta OC/Delta CO ratio of 101.3 +/- 41.6 ng/m(3)/ppbv. The value found in the present study was near the upper limit of OC emission ratios in the literature, implying great impacts from combustion conditions, types of biomass burned and subsequent evolution. Regarding urban pollution episodes, the Delta OC/Delta CO ratio was found to be 23.7 +/- 2.4 ng/m(3)/ppbv, and secondary organics accounted for the major fraction of OC mass. Combustions phases of OCRB were classified according to a modified combustion efficiency (MCE, defined as Delta CO2/(Delta CO + Delta CO2)). Our results support the view that ECa tend to be produced in flaming combustions (MCE > 0.95) than in smoldering combustions (MCE < 0.95), whereas OC is emitted preferentially from smoldering combustions. Based on our observed carbonaceous aerosol correlations, we estimate that the ECa and OC emissions from OCRB in East Asia might be underestimated by at least 50%.