Smog chamber experiments were conducted to investigate the chemical and physical transformations of organic aerosol (OA) during photo-oxidation of open biomass burning emissions. The experiments were carried out at the US Forest Service Fire Science Laboratory as part of the third Fire Lab at Missoula Experiment (FLAME III). We investigated emissions from 12 different fuels commonly burned in North American wildfires. The experiments feature atmospheric and plume aerosol and oxidant concentrations; aging times ranged from 3 to 4.5 h. OA production, expressed as a mass enhancement ratio (ratio of OA to primary OA (POA) mass), was highly variable. OA mass enhancement ratios ranged from 2.9 in experiments where secondary OA (SOA) production nearly tripled the POA concentration to 0.7 in experiments where photo-oxidation resulted in a 30 % loss of the OA mass. The campaign-average OA mass enhancement ratio was 1.7 ± 0.7 (mean ± 1σ); therefore, on average, there was substantial SOA production. In every experiment, the OA was chemically transformed. Even in experiments with net loss of OA mass, the OA became increasingly oxygenated and less volatile with aging, indicating that photo-oxidation transformed the POA emissions. Levoglucosan concentrations were also substantially reduced with photo-oxidation. The transformations of POA were extensive; using levoglucosan as a tracer for POA, unreacted POA only contributed 17 % of the campaign-average OA mass after 3.5 h of exposure to typical atmospheric hydroxyl radical (OH) levels. Heterogeneous reactions with OH could account for less than half of this transformation, implying that the coupled gas-particle partitioning and reaction of semi-volatile vapors is an important and potentially dominant mechanism for POA processing. Overall, the results illustrate that biomass burning emissions are subject to extensive chemical processing in the atmosphere, and the timescale for these transformations is rapid

Smog chamber experiments were conducted to investigate secondary organic aerosol (SOA) formation from photo-oxidation of low-volatility precursors; n-alkanes were chosen as a model system. The experiments feature atmospherically relevant organic aerosol concentrations (C(OA)). Under high-NO(x) conditions SOA yields increased with increasing carbon number (lower volatility) for n-decane, n-dodecane, n-pentadecane, and n-heptadecane, reaching a yield of 0.51 for heptadecane at a C(OA) of 15.4 microg m(-3). As with other photo-oxidation systems, aerosol yield increased with UV intensity. Due to the log-linear relationship between n-alkane carbon number and vapor pressure as well as a relatively consistent product distribution it was possible to develop an empirical parametrization for SOA yields for n-alkanes between C(12) and C(17). This parametrization was implemented using the volatility basis set framework and is designed for use in chemical transport models. For C(OA) < 2 microg m(-3), the SOA mass spectrum, as measured with an aerosol mass spectrometer, had a large contribution from m/z 44, indicative of highly oxygenated products. At higher C(OA), the mass spectrum was dominated by m/z 30, indicative of organic nitrates. The data support the conclusion that lower volatility organic vapors are important SOA precursors.

The gas-particle partitioning of primary organic aerosol (POA) emissions from a diesel engine and the combustion of hard- and soft-woods in a stove was investigated by isothermally diluting them in a smog chamber or by passing them through a thermodenuder and measuring the extent of evaporation. The experiments were conducted at atmospherically relevant conditions: low concentrations and small temperature perturbations. The partitioning of the POA emissions from both sources varied continuously with changing concentration and temperature. Although the POA emissions are semivolatile, they do not completely evaporate at typical atmospheric conditions. The overall partitioning characteristics of diesel and wood smoke POA are similar, with wood smoke being somewhat less volatile than the diesel exhaust. The gas-particle partitioning of aerosols formed from flash-vaporized engine lubricating oil was also studied; diesel POA is somewhat more volatile than the oil aerosol. The experimental data from the dilution- and thermodenuder-based techniques were fit using absorptive partitioning theory to derive a volatility distribution of the POA emissions from each source. These distributions are suitable for use in chemical transport models that simulate POA concentrations.

Abstract. We quantify the hygroscopic properties of particles freshly emitted from biomass burning and after several hours of photochemical aging in a smog chamber. Values of the hygroscopicity parameter, κ, were calculated from cloud condensation nuclei (CCN) measurements of emissions from combustion of 12 biomass fuels commonly burned in North American wildfires. Prior to photochemical aging, the κ of the fresh primary aerosol varied widely, between 0.06 (weakly hygroscopic) and 0.6 (highly hygroscopic). The hygroscopicity of the primary aerosol was positively correlated with the inorganic mass fraction of the particles. Photochemical processing reduced the range of κ values to between 0.08 and 0.3. The changes in κ were driven by the photochemical production of secondary organic aerosol (SOA). SOA also contributed to growth of particles formed during nucleation events. Analysis of the nucleation mode particles enabled the first direct quantification of the hygroscopicity parameter κ for biomass burning SOA, which was on average 0.11, similar to values observed for biogenic SOA. Although initial CCN activity of biomass burning aerosol emissions are highly variable, after a few hours of photochemical processing κ converges to a value of 0.2 ± 0.1. Therefore, photochemical aging reduces the variability of biomass burning CCN κ, which should simplify analysis of the potential effects of biomass burning aerosol on climate.

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