Composition and light absorption of N-containing aromatic compounds in organic aerosols from laboratory biomass burning

• Main Authors: M. Xie, X. Chen, M. D. Hays, A. L. Holder
• Format: Article
• Language: English
• Published: Copernicus Publications 2019-03-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://www.atmos-chem-phys.net/19/2899/2019/acp-19-2899-2019.pdf

<p>This study seeks to understand the compositional details of N-containing aromatic 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 United States 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 (<span class="inline-formula"><i>p</i>&lt;0.05</span>) were observed between BrC absorption and elemental carbon <span class="inline-formula">(EC)∕OC</span> ratios for individual burns data. However, the pooled experimental data indicated that <span class="inline-formula">EC∕OC</span> alone cannot explain the BB BrC absorption. Fourteen NAC formulas were identified in the BB samples, most of which were also observed in simulated secondary organic aerosol (SOA) from photooxidation of aromatic volatile organic compounds (VOCs) with <span class="inline-formula">NO_<i>x</i></span>. However, the molecular structures associated with the identical NAC formula from BB and SOA are different. In this work, the identified NACs from BB are featured by methoxy and cyanate groups and are predominately generated during the flaming phase. The mass concentrations of identified NACs were quantified using authentic and surrogate standards, and their contributions to bulk light absorption of solvent-extractable OC were also calculated. The contributions of identified NACs to organic matter (OM) and BrC absorption were significantly higher in flaming-phase samples than those in smoldering-phase samples, and they correlated with the <span class="inline-formula">EC∕OC</span> ratio (<span class="inline-formula"><i>p</i>&lt;0.05</span>) for both individual burns and pooled experimental data, indicating that the formation of NACs from BB largely depends on burn conditions. The average contributions of identified NACs to overall BrC absorption at 365&thinsp;<span class="inline-formula">nm</span> ranged from <span class="inline-formula">0.087±0.024</span>&thinsp;% to <span class="inline-formula">1.22±0.54</span>&thinsp;%, which is 3–10 times higher than their mass contributions to OM (<span class="inline-formula">0.023±0.0089</span>&thinsp;% to <span class="inline-formula">0.18±0.067</span>&thinsp;%), so the NACs with light absorption identified in this work from BB are likely strong BrC chromophores. Further studies are warranted to identify more light-absorbing compounds to explain the unknown fraction (<span class="inline-formula">&gt;98</span>&thinsp;%) of BB BrC absorption.</p>