Abstract
Abstract. Nitrated and oxygenated polycyclic aromatic hydrocarbons (N/OPAHs) are emitted in combustion processes and formed in polluted air. Their environmental cycling through wet deposition has hardly been studied. Fresh snow samples at urban and rural sites in central Europe, as well as surface snow from a remote site in Svalbard, were analysed for 17 NPAHs, 8 OPAHs, and 11 nitrated mono-aromatic hydrocarbons (NMAHs), of which most N/OPAHs as well as nitrocatechols, nitrosalicylic acids, and 4-nitroguaiacol are studied for the first time in precipitation. In order to better understand the scavenging mechanisms, the particulate mass fractions (θ) at 273 K were predicted using a multi-phase gas-particle partitioning model based on polyparameter linear free energy relationships. ∑NPAH concentrations were 1.2–17.6 and 8.8–19.1 ng L−1 at urban and rural sites, whereas ∑OPAHs were 79.8–955.2 and 343.3–1757.4 ng L−1 at these sites, respectively. 9,10-anthraquinone was predominant in snow aqueous and particulate phases. NPAHs were only found in the particulate phase with 9-nitroanthracene being predominant followed by 2-nitrofluoranthene. Among NMAHs, 4-nitrophenol showed the highest abundance in both phases. The levels found for nitrophenols were in the same range or lower than those reported in the 1980s and 1990s. The lowest levels of ∑N/OPAHs and ∑NMAHs were found at the remote site (3.5 and 390.5 ng L−1, respectively). N/OPAHs preferentially partitioned in snow particulate phase in accordance with predicted θ, whereas NMAHs were predominant in the aqueous phase, regardless of θ. It is concluded that the phase distribution of non-polar N/OPAHs in snow is determined by their gas-particle partitioning prior to snow scavenging, whereas that for polar particulate phase substances, i.e. NMAHs, is determined by an interplay between gas-particle partitioning in the aerosol and dissolution during in- or below-cloud scavenging.
Highlights
Nitrated and oxygenated polycyclic aromatic hydrocarbons (N/OPAHs) are formed primarily by oxidation of parent PAHs during combustion of fossil fuels as well as biomass burning, and secondarily through reactions of PAHs with atmospheric oxidants, such as O3, OH and NOx (Walgraeve et al, 2010; Bandowe and Meusel, 2017)
Our sample processing was performed in such way to minimize the analyte phase change prior to analysis; i.e. the samples were thawed at room temperature in the lab and, immediately after thawing, when the samples were near freezing point, the meltwater was passed through a preassembled filtration–extraction set-up (Fig. S1 in the Supplement), which allowed the simultaneous separation of meltwater particulate phase and extraction of aqueous phase
Our gas-particle partitioning (GPP) model suggests that at near-zero temperatures the targeted NPAHs would be completely sorbed to the particulate phase in the atmosphere, with the exception 1-NNAP, 2-NNAP, 5-NACE, and 2-NFLU, which would partition between gas and particulate phases
Summary
Nitrated and oxygenated polycyclic aromatic hydrocarbons (N/OPAHs) are formed primarily by oxidation of parent PAHs during combustion of fossil fuels as well as biomass burning, and secondarily through reactions of PAHs with atmospheric oxidants, such as O3, OH and NOx (Walgraeve et al, 2010; Bandowe and Meusel, 2017). Benzanthrone (OBAT) does not have a stable parent PAH precursor with the same number of rings in the atmosphere (Kojima et al, 2010) This substance, together with benz(a)fluorenone (BaOFLN) and benz(b)fluorenone (BbOFLN), was associated with primary combustion sources (Albinet et al, 2007; Karavalakis et al, 2010; Shen et al, 2013b; Huang et al, 2014; Souza et al, 2014; Tomaz et al, 2016; Vicente et al, 2016). The particle scavenging is relevant for SVOCs that show higher affinity towards the particulate phase (Bidleman, 1988; Shahpoury et al, 2015) This is affected by SVOC gas-particle partitioning (GPP) in the aerosol, a process controlled mainly by the substance molecular structure, PM chemical composition, and ambient temperature (Shahpoury et al, 2016). For calculating θ , our method took into account the interaction of atmospheric SVOCs with PM liquid organic and polymeric phases, as well as the interaction with PM black carbon and salts, while disregarding the partitioning into PM aqueous phase, particle–liquid interactions, and liquid–liquid phase separation within PM (Sect. 2.6)
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