Open burning of agricultural biomass: Physical and chemical properties of particle-phase emissions

摘要

We present the physical and chemical characterization of particulate matter (PM_(2.5)) emissions from simulated agricultural fires (AFs) of surface residuals of two major grain crops, rice (Oryza saliva) and wheat (Triticum aestivum L.). The O_2 levels and CO/CO_2 ratios of the open burn simulations are typical of the field fires of agricultural residues. In the AF plumes, we observe predominantly accumulation mode (100-1000 nm) aerosols. The mean PM_(2.5) mass emission factors from replicate burns of the wheat and rice residuals are 4.7 ± 0.04 and 13.0±0.3gkg~(-1) of dry biomass, respectively. The combustion-derived PM emissions from wheat are enriched in K (31 % weight/weight, w/w) and Cl (36% w/w), whereas the PM emissions from rice are largely carbonaceous (84% w/w). Molecular level gas chromatography/mass spectrometry analysis of PM_(2.5) solvent extracts identifies organic matter that accounts for as much as 18% of the PM mass emissions. A scarcity of detailed PM-phase chemical emissions data from AFs required that comparisons among other biomass combustion groups (wildfire, woodstove, and fireplace) be made. Statistical tests for equal variance among these groups indicate that the degree to which molecular emissions vary is compound dependent. Analysis of variance testing shows significant differences in the mean values of certain n-alkane, polycyclic aromatic hydrocarbon (PAH), oxy-PAH, and sugar marker compounds common to the biomass combustion types. Individual pairwise comparisons of means at the combustion group level confirm this result but suggest that apportioning airborne PM to these sources may require a more comprehensive use of the chemical emissions fingerprints. Hierarchical clustering of source test observations using molecular markers indicates agricultural fuels as distinct from other types of biomass combustion or biomass species. Rough approximations of the total potential PM2.5 emissions outputs from the combustion of the wheat and rice surface residues are given. This agricultural activity could significantly contribute to emissions inventories at regional, national, and global geographic levels.