Characteristics of fresh and aged volatile organic compounds from open burning of crop residues

Abstract

Open burning of crop residues is a major source of volatile organic compounds (VOCs), which contribute substantially to the formation of secondary organic aerosols (SOAs) in the atmosphere. An integrated system of combustion chamber coupled with potential aerosol mass (PAM) reactor was used to demonstrate the emission characteristics of fresh and aged VOCs (corresponding to 2- and 7-day atmospheric aging) from the burning of rice, maize, and wheat straws. The average emission factor (EF) of quantified non-methane VOCs (NMVOCs) emitted from the straw (fresh) was 1.82 ± 0.41 g/kg and wheat straw had the highest EFs. The EF residues of quantified NMVOCs decreased considerably after photo-oxidation in PAM. Stronger oxidation condition (7-day aging) produced a 57.2% decline in NMVOC EFs, compared with 42.3% decline during 2-day atmospheric aging. The largest declines were observed in the alkenes group: 82.6% and 66.2% after 7- and 2-day aging, respectively, which is consistent with their high reactivity toward oxidation with ozone and hydroxyl radical (OH). Aromatic compounds mainly reacted with OH, and their EFs decreased 59.1% on average. Alkanes were much less reactive, and their EFs only decreased an average of 29.8% after the oxidation processes. Considerable SOAs formation was observed in the fine particulate matter (PM2.5) filter samples collected after the oxidation of isoprene, benzene and toluene. The moderate to strong correlations between isoprene and isoprene-derived SOAs, between benzene and toluene with nitrophenols, and between toluene and aromatic acids demonstrate that the VOCs were degraded in the reactions with oxidative radicals, producing active contributors to SOAs formations.