Contributions of nitrated aromatic compounds to the light absorptionof water-soluble and particulate brown carbon in differentatmospheric environments in Germany and China

Abstract. The relative contribution of eight nitrated aromatic compounds (NACs, nitrophenols + 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. 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 – 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. 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: at pH 10, the aqueous light absorption coefficient Abs370 and the mass absorption efficiency (MAE370) at 370 nm were a factor of 1.6 and 1.4 larger than at pH 2, respectively. In general, Abs370 ranged from 0.21–21.8 Mm−1 under acidic conditions and 0.63–27.2 Mm−1 under alkaline conditions, over all campaigns. The observed MAE370 was in a range of 0.10–1.79 m2 g−1 and 0.24–2.57 m2 g−1 for acidic and alkaline conditions, respectively. For MAE370 and Abs370, the observed values were higher in winter than in summer, in agreement with other studies. Furthermore, it was found that the MAE370 values in winter in Germany exceeded those of the Chinese summer background stations (average of 0.85±0.24 m2 g−1 compared to 0.47±0.15 m2 g−1). The lowest MAE was observed for the Waldstein (summer) campaign (average of 0.17±0.03 m2 g−1), 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. Regarding the particulate light absorption at 370 nm, a mean particulate light absorption coefficient babs,370 of 54 Mm−1 and 6.0 Mm−1 was determined for the TROPOS (winter) and Waldstein (summer) campaigns, respectively, with average contributions of particulate brown carbon to babs,370 of 46 % at TROPOS (winter) and 15 % at Waldstein (summer). The absorption Ångström exponent of the ambient aerosol during the campaigns at TROPOS (winter) and Waldstein (summer) was found to be 1.5±0.1 and 1.2±0.3, respectively. 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). The mean contribution of NACs to the aqueous extract light absorption over all campaigns ranged from 0.10 %–1.25 % under acidic conditions and 0.13 %–3.71 % 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±0.06 % (acidic conditions) and 0.13±0.09 % (alkaline conditions) during the Waldstein (summer) campaign and 0.25±0.21 % (acidic conditions) and 1.13±1.03 % (alkaline conditions) during the TROPOS (winter) campaign. A correlation of NAC concentrations with Abs370 was observed for the BB-influenced campaigns at TROPOS (winter) and Melpitz (winter). 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.

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