Contributions of nitrated aromatic compounds to the light absorption of water-soluble and particulate brown carbon in different atmospheric environments in Germany and China

Monique Teich, Dominik van Pinxteren, Michael Wang, Simonas Kecorius, Zhibin Wang, Thomas Müller, Griša Močnik, Hartmut Herrmann
2017 Atmospheric Chemistry and Physics  
<p><strong>Abstract.</strong> The relative contributions of eight nitrated aromatic compounds (NACs: nitrophenols and nitrated salicylic acids) to the light absorption of aqueous particle extracts and particulate brown carbon were determined from aerosol particle samples collected in Germany and China.</p><p class="p">High-volume filter samples were collected during six campaigns, performed at five locations in two seasons: (I) two campaigns with strong influence of biomass-burning (BB) aerosol
more » ... urning (BB) aerosol at the TROPOS institute (winter, 2014, urban background, Leipzig, Germany) and the Melpitz research site (winter, 2014, rural background); (II) two campaigns with strong influence from biogenic emissions at Melpitz (summer, 2014) and the forest site Waldstein (summer, 2014, Fichtelgebirge, Germany); and (III) two CAREBeijing-NCP campaigns at Xianghe (summer, 2013, anthropogenic polluted background) and Wangdu (summer, 2014, anthropogenic polluted background with a distinct BB episode), both in the North China Plain. <br><br> The filter samples were analyzed for NAC concentrations and the light absorption of aqueous filter extracts was determined. Light absorption properties of particulate brown carbon were derived from a seven-wavelength aethalometer during the campaigns at TROPOS (winter) and Waldstein (summer). The light absorption of the aqueous filter extracts was found to be pH dependent, with larger values at higher pH. In general, the aqueous light absorption coefficient (Abs<sub>370</sub>) ranged from 0.21 to 21.8<span class="thinspace"></span>Mm<sup>−1</sup> under acidic conditions and 0.63 to 27.2<span class="thinspace"></span>Mm<sup>−1</sup> under alkaline conditions, over all campaigns. The observed mass absorption efficiency (MAE<sub>370</sub>) was in a range of 0.10–1.79<span class="thinspace"></span>m<sup>2</sup><span class="thinspace"></span>g<sup>−1</sup> and 0.24–2.57<span class="thinspace"></span>m<sup>2</sup><span class="thinspace"></span>g<sup>−1</sup> for acidic and alkaline conditions, respectively. For MAE<sub>370</sub> and Abs<sub>370</sub>, the observed values were higher in winter than in summer, in agreement with other studies. The lowest MAE was observed for the Waldstein (summer) campaign (average of 0.17<span class="thinspace"></span>±<span class="thinspace"></span>0.03<span class="thinspace"></span>m<sup>2</sup><span class="thinspace"></span>g<sup>−1</sup>), indicating that freshly emitted biogenic aerosols are only weakly absorbing. In contrast, a strong relationship was found between the light absorption properties and the concentrations of levoglucosan, corroborating findings from other studies. <br><br> Regarding the particulate light absorption at 370<span class="thinspace"></span>nm, a mean particulate light absorption coefficient <i>b</i><sub>abs, 370</sub> of 54<span class="thinspace"></span>Mm<sup>−1</sup> and 6.0<span class="thinspace"></span>Mm<sup>−1</sup> was determined for the TROPOS (winter) and Waldstein (summer) campaigns, respectively, with average contributions of particulate brown carbon to <i>b</i><sub>abs, 370</sub> of 46<span class="thinspace"></span>% at TROPOS (winter) and 15<span class="thinspace"></span>% at Waldstein (summer). Thus, the aethalometer measurements support the findings from aqueous filter extracts of only weakly absorbing biogenic aerosols in comparison to the more polluted and BB influenced aerosol at TROPOS (winter). <br><br> The mean contribution of NACs to the aqueous extract light absorption over all campaigns ranged from 0.10 to 1.25<span class="thinspace"></span>% under acidic conditions and 0.13 to 3.71<span class="thinspace"></span>% under alkaline conditions. The high variability among the measurement sites showed that the emission strengths of light-absorbing compounds and the composition of brown carbon were very different for each site. The mean contribution of NACs to the particulate brown carbon light absorption was 0.10<span class="thinspace"></span>±<span class="thinspace"></span>0.06<span class="thinspace"></span>% (acidic conditions) and 0.13<span class="thinspace"></span>±<span class="thinspace"></span>0.09<span class="thinspace"></span>% (alkaline conditions) during the Waldstein (summer) campaign and 0.25<span class="thinspace"></span>±<span class="thinspace"></span>0.21<span class="thinspace"></span>% (acidic conditions) and 1.13<span class="thinspace"></span>±<span class="thinspace"></span>1.03<span class="thinspace"></span>% (alkaline conditions) during the TROPOS (winter) campaign. <br><br> The average contribution of NACs to the aqueous extract light absorption over all campaigns was found to be 5 times higher than their mass contribution to water-soluble organic carbon indicating that even small amounts of light-absorbing compounds can have a disproportionately high impact on the light absorption properties of particles.</p>
doi:10.5194/acp-17-1653-2017 fatcat:twfsjwiimzf5jlmnm5ccndiqui