Emission of organic carbon, elemental carbon and water-soluble ions from crop straw burning under flaming and smoldering conditions

Abstract

Emissions from major agricultural residues were measured using a self-designed combustion system. Emission factors (EFs) of organic carbon (OC), elemental carbon (EC), and water-soluble ions (WSIs) (K+, NH4+, Na+, Mg2+, Ca2+, Cl−, NO3−, SO42–) in smoke from wheat and rice straw were measured under flaming and smoldering conditions. The OC1/TC (total carbon) was highest (45.8% flaming, 57.7% smoldering) among carbon fractions. The mean EFs for OC (EFOC) and EC (EFEC) were 9.2±3.9 and 2.2±0.7g/kg for wheat straw and 6.4±1.9 and 1.1±0.3g/kg for rice straw under flaming conditions, while they were 40.8±5.6 and 5.8±1.0g/kg and 37.6±6.3 and 5.0±1.4g/kg under smoldering conditions, respectively. Higher EC ratios were observed in particulate matter (PM) mass under flaming conditions. The OC and EC for the two combustion patterns were significantly correlated (p<0.01, R=0.95 for wheat straw; p<0.01, R=0.97 for rice straw), and a higher positive correlation between OC3 and EC was observed under both combustion conditions. WSIs emitted from flaming smoke were dominated by Cl− and K+, which contributed 3.4% and 2.4% of the PM mass for rice straw and 2.2% and 1.0% for wheat straw, respectively. The EFs of Cl− and K+ were 0.73±0.16 and 0.51±0.14g/kg for wheat straw and 0.25±0.15 and 0.12±0.05g/kg for rice straw under flaming conditions, while they were 0.42±0.28 and 0.12±0.06g/kg and 0.30±0.27 and 0.05±0.03g/kg under smoldering conditions, respectively. Na+, Mg2+, and NH4+ were vital components in PM, comprising from 0.8% (smoldering) to 3.1% (flaming) of the mass. Strong correlations of Cl− with K+, NH4+, and Na+ ions were observed in rice straw and the calculated diagnostic ratios of OC/EC, K+/Na+ and Cl−/Na+ could be useful to distinguishing crop straw burning from other sources of atmospheric pollution.