Composition and light absorption of nitroaromatic compounds in organic aerosols from laboratory biomass burning

Abstract

<p><strong>Abstract.</strong> This study seeks to understand the compositional details of nitroaromatic compounds (NACs) emitted during biomass burning (BB) and their contribution to light-absorbing organic carbon (OC), also termed brown carbon (BrC). Three laboratory BB experiments were conducted with two U.S. pine forest understory fuels typical of those consumed during prescribed fires. During the experiments, submicron aerosol particles were collected on filter media and subsequently extracted with methanol and examined for their optical and chemical properties. Significant correlations (<i>p</i> < 0.05) were observed between BrC absorption and elemental carbon (EC)/OC ratios for test specific data. However, the pooled experimental data indicated that the BB BrC absorption depends on more than the BB fire conditions as represented by the EC/OC ratio. Fourteen NACs were identified in the BB samples, four of which (C<sub>10</sub>H<sub>11</sub>NO<sub>4</sub>, C<sub>10</sub>H<sub>11</sub>NO<sub>5</sub>, C<sub>11</sub>H<sub>13</sub>NO<sub>5</sub> and C<sub>11</sub>H<sub>13</sub>NO<sub>6</sub>) have not been observed previously in chamber-based secondary organic aerosols, and are expected to have methoxyphenol-type structure specific to the pyrolized biomass lignin based on mass spectral evidence, suggesting these compounds may be unique to BB aerosols. The average total contribution of NACs to organic mass (0.023 ± 0.0089 to 0.18 ± 0.067 %) was 5–10 times lower than the average contribution to the overall BrC absorption at 365 nm (0.12 ± 0.047 to 2.44 ± 0.67 %). The average contributions (%) of total NACs to organic mass and aqueous extracts absorption correlated significantly (<i>p</i> < 0.05) with EC/OC for both test specific and pooled experimental data. These results suggested that the formation of NACs from BB depended more on burn conditions than the bulk absorptive properties of BB BrC.</p>