Measurements of reactive trace gases and variable O3 formation rates in some South Carolina biomass burning plumes
In October-November 2011 we measured trace gas emission factors from\nseven prescribed fires in South Carolina (SC), US, using two Fourier\ntransform infrared spectrometer (FTIR) systems and whole air sampling\n(WAS) into canisters followed by gas-chromatographic analysis. A total\nof 97 trace gas species were quantified from both airborne and\nground-based sampling platforms, making this one of the most detailed\nfield studies of fire emissions to date. The measurements include the\nfirst emission factors for a suite of monoterpenes produced by heating\nvegetative fuels during field fires. The first quantitative FTIR\nobservations of limonene in smoke are reported along with an expanded\nsuite of monoterpenes measured by WAS including alpha-pinene,\nbeta-pinene, limonene, camphene, 4-carene, and myrcene. The known\nchemistry of the monoterpenes and their measured abundance of 0.4-27.9%\nof non-methane organic compounds (NMOCs) and similar to 21% of organic\naerosol (mass basis) suggests that they impacted secondary formation of\nozone (O-3), aerosols, and small organic trace gases such as methanol\nand formaldehyde in the sampled plumes in the first few hours after\nemission. The variability in the initial terpene emissions in the SC\nfire plumes was high and, in general, the speciation of the initially\nemitted gas-phase NMOCs was 13-195% different from that observed in a\nsimilar study in nominally similar pine forests in North Carolina\nsimilar to 20 months earlier. It is likely that differences in stand\nstructure and environmental conditions contributed to the high\nvariability observed within and between these studies. Similar factors\nmay explain much of the variability in initial emissions in the\nliterature. The Delta HCN/Delta CO emission ratio, however, was found to\nbe fairly consistent with previous airborne fire measurements in other\nconiferous-dominated ecosystems, with the mean for these studies being\n0.90+/-0.06 %, further confirming the value of HCN as a biomass burning\ntracer. The SC results also support an earlier finding that C3C4 alkynes\nmay be of use as biomass burning indicators on the time-scale of hours\nto a day. It was possible to measure the downwind chemical evolution of\nthe plume on four of the fires and significant O-3 formation (Delta\nO-3/Delta CO from 1090 %) occurred in all of these plumes within two\nhours. The slowest O-3 production was observed on a cloudy day with low\nco-emission of NOx. The fastest O-3 production was observed on a sunny\nday when the downwind plume almost certainly incorporated significant\nadditional NOx by passing over the Columbia, SC metropolitan area. Due\nto rapid plume dilution, it was only possible to acquire high-quality\ndownwind data for two other trace gas species (formaldehyde and\nmethanol) during two of the fires. In all four of these cases,\nsignificant increases in formaldehyde and methanol were observed in < 2\nh. This is likely the first direct observation of post-emission methanol\nproduction in biomass burning plumes. Post-emission production of\nmethanol does not always happen in young biomass burning plumes, and its\noccurrence in this study could have involved terpene precursors to a\nsignificant extent.