Mid carbon (C6+-C29+) in refractory black carbon aerosols is a potential tracer of open burning of rice straw: Insights from atmospheric observation and emission source studies

Abstract

Abstract Biomass burning (BB) is one of the major sources of atmospheric aerosols. These aerosols cause global and regional issues such as adverse effects on human health, as well as contributing to climate change. Refractory black carbon (rBC) is a major chemical component emitted from BB. Recent studies indicate that the mass spectra distribution of carbon cluster ions (Cn+) in rBC reflects burning materials and combustion temperatures. Therefore, emissions apportionment is in principle possible with respect to rBC. However, emissions data for Cn+ are still not widely available. In this study, a combustion experiment using rice straw was conducted to identify the mass spectra of Cn+ measured using a soot particle aerosol mass spectrometer. Further, to confirm the results, atmospheric observations were conducted during the harvest season in a suburban area surrounded by rice fields because intentional open burning of rice straw (agricultural residues) is often conducted. Source apportionment of airborne ambient rBC generated from open burning of rice straw is carried out by a positive matrix factorization (PMF) method applied to hourly mean data for rBC and Cn+, as well as organic aerosols. During the atmospheric observation period, there was an intensive day of open burning of rice straw, confirmed by visual inspections around the observation site. Concentrations of PM2.5 reached up to 100 μg/m3 and the contribution of BB to rBC increased by up to 50% on that day. Tracers of BB (C2H4O2+ which is a fragment ion of levoglucosan and refractory K+ ions) were also enriched at that time. This study also found that the signal fraction of mid carbon (C6+-C29+) to total carbon (C1+-C35+) in rBC was 0.21 for dry rice straw burning. Correspondingly, the BB-related PMF factor in atmospheric observations (0.18) was close to the emission source data. Rice straw burning tends to occur at a low combustion temperature and generates amorphous (immature) soot, which was further confirmed by a larger value (0.41) for wet rice straw burning with low modification combustion efficiency. On the other hand, the mid carbon fraction of rBC from automobile exhausts (diesel and gasoline) was low (below 0.02) in laboratory studies and the automobile exhaust-related PMF factor was also relatively low (0.08) in atmospheric observations. These emissions are likely generated as graphite (mature) soot under a high combustion temperature. This study also quantifies and explores the atmospheric aging of BB aerosols. Decay rates of mid carbon were about 2 times slower than that of levoglucosan estimated from atmospheric observations. In sum, the mid carbon fraction can be used to differentiate sources of rBC emissions between open burning of rice straw at low combustion temperatures and other rBC sources at high combustion temperatures (such as internal combustion engines) on the one hand and estimate the decay rates of aerosols emitted from BB on the other hand.