Chromophores and chemical composition of brown carbon characterized at an urban kerbside by excitation–emission spectroscopy and mass spectrometry

<p>The optical properties, chemical composition, and potential chromophores of brown carbon (BrC) aerosol particles were studied during typical summertime and wintertime at a kerbside in downtown Karlsruhe, a city in central Europe. The average absorption coefficient and mass absorption efficiency at 365 nm (Abs<span class="inline-formula">₃₆₅</span> and MAE<span class="inline-formula">₃₆₅</span>) of methanol-soluble BrC (MS-BrC) were lower in the summer period (1.6 <span class="inline-formula">±</span> 0.5 Mm<span class="inline-formula">⁻¹</span>, 0.5 <span class="inline-formula">±</span> 0.2 m<span class="inline-formula">²</span> g<span class="inline-formula">⁻¹</span>) than in the winter period (2.8 <span class="inline-formula">±</span> 1.9 Mm<span class="inline-formula">⁻¹</span>, 1.1 <span class="inline-formula">±</span> 0.3 m<span class="inline-formula">²</span> g<span class="inline-formula">⁻¹</span>). Using a parallel factor (PARAFAC) analysis to identify chromophores, two different groups of highly oxygenated humic-like substances (HO-HULIS) dominated in summer and contributed 96 <span class="inline-formula">±</span> 6 % of the total fluorescence intensity. In contrast, less-oxygenated HULIS (LO-HULIS) dominated the total fluorescence intensity in winter with 57 <span class="inline-formula">±</span> 12 %, followed by HO-HULIS with 31 <span class="inline-formula">±</span> 18 %. Positive matrix factorization (PMF) analysis of organic compounds detected in real time by an online aerosol mass spectrometer (AMS) led to five characteristic organic compound classes. The statistical analysis of PARAFAC components and PMF factors showed that LO-HULIS chromophores were most likely emitted from biomass burning in winter. HO-HULIS chromophores could be low-volatility oxygenated organic aerosol from regional transport and oxidation of biogenic volatile organic compounds (VOCs) in summer.</p> <p>Five nitro-aromatic compounds (NACs) were identified by a chemical ionization mass spectrometer (C<span class="inline-formula">₇</span>H<span class="inline-formula">₇</span>O<span class="inline-formula">₃</span>N, C<span class="inline-formula">₇</span>H<span class="inline-formula">₇</span>O<span class="inline-formula">₄</span>N, C<span class="inline-formula">₆</span>H<span class="inline-formula">₅</span>O<span class="inline-formula">₅</span>N, C<span class="inline-formula">₆</span>H<span class="inline-formula">₅</span>O<span class="inline-formula">₄</span>N, and C<span class="inline-formula">₆</span>H<span class="inline-formula">₅</span>O<span class="inline-formula">₃</span>N), which contributed 0.03 <span class="inline-formula">±</span> 0.01 % to the total organic mass but can explain 0.3 <span class="inline-formula">±</span> 0.1 % of the total absorption of MS-BrC at 365 nm in winter. Furthermore, we identified 316 potential brown carbon molecules which accounted for 2.5 <span class="inline-formula">±</span> 0.6 % of the organic aerosol mass. Using an average mass absorption efficiency (MAE<span class="inline-formula">₃₆₅</span>) of 9.5 m<span class="inline-formula">²</span>g<span class="inline-formula">⁻¹</span> for these compounds, we can estimate their mean light absorption to be 1.2 <span class="inline-formula">±</span> 0.2 Mm<span class="inline-formula">⁻¹</span>, accounting for 32 <span class="inline-formula">±</span> 15 % of the total absorption of MS-BrC at 365 nm. This indicates that a small fraction of brown carbon molecules dominates the overall absorption. The potential BrC molecules assigned to the LO-HULIS component had a higher average molecular weight (265 <span class="inline-formula">±</span> 2 Da) and more nitrogen-containing molecules (62 <span class="inline-formula">±</span> 1 %) than the molecules assigned to the HO-HULIS components. Our analysis shows that the LO-HULIS, with a high contribution of nitrogen-containing molecules originating from biomass burning, dominates aerosol fluorescence in winter, and HO-HULIS, with fewer nitrogen-containing molecules as low-volatility oxygenated organic aerosol from regional transport and oxidation of biogenic volatile organic compounds (VOC), dominates in summer.</p>