Abstract
With the objective of better understanding the sources and dynamics of carbonaceous fractions of the aerosols present in the atmosphere of Montreal, we implemented here an online wet oxidation/isotope ratio mass spectrometry (IRMS) method to simultaneously measure both water-soluble organic carbon (WSOC) content and the corresponding δ13C of aerosol samples collected at four monitoring stations over a 1-year period representing distinct types of environmental conditions (i.e., background, road traffic, industrial, and downtown). We coupled these data with the corresponding concentrations of other carbon fractions: total carbon (TC), elemental carbon plus organic carbon (EC + OC), and carbonates. Results show that TC (6.64 ± 2.88 μg m–3), EC + OC (4.98 ± 2.23 μg m–3), and carbonates (1.71 ± 1.09 μg m–3) were characterized by lower concentrations in winter and higher ones between spring and early autumn, with all fractions expectedly showing significantly lower concentrations for aerosols collected at the background station. We observed a seasonal dependence of the δ13CEC+OC (−25.31 ± 0.94‰) with the EC + OC/total suspended particles (TSP) ratio: (i) an increase of the ratio during late spring, summer and early autumn associated to road traffic emissions characterized by a δ13C of ∼−25‰ and (ii) lower ratios during the winter months indicating the influence of two distinct emission sources, a first one with a δ13C ∼−27‰, suggesting the local influence of combined biomass burning from residential heating and of fossil fuel combustion, and a second one with a δ13C ∼−21‰, likely related to more regional emissions. WSOC (1.14 ± 0.67 μg m–3) presented a similar seasonal pattern for all monitoring stations, with low concentrations in winter, early spring and late autumn that rapidly increased until summer. Our results indicate that this seasonality is controlled by higher anthropogenic contributions from southern Canada and northeastern United States regions and probably from biogenic emissions during the warm months. Moreover, δ13CWSOC (−25.08 ± 1.47‰) showed a 13C-depletion in summer, indicating higher fossil fuel and biogenic contributions, whereas the higher isotope compositions observed in winter may result from the photochemical aging of regional aerosols. Ultimately, we identified the influence of local industrial emissions late in 2013 as well as the impact of aerosol emissions associated to the Lac-Mégantic rail disaster that occurred on July 6, ∼200 km east of Montreal.
Highlights
It has clearly been established that atmospheric particulate matter (PM) affects our climate (Wurzler et al, 2000), the ecosystems (Grantz et al, 2003; Okin et al, 2011), and human health (Le Tertre et al, 2002; Pope and Dockery, 2006; Batterman et al, 2014), especially in urban areas where it is present in significant concentrations (Zhang et al, 2015)
While primary OC particles are mainly generated by combustion and/or biogenic sources, secondary organic carbon (SOC) particles are mainly produced through atmospheric reactions involving gaseous organic precursors (Jones and Harrison, 2005; Górka et al, 2014; Sudheer et al, 2016)
The analysis of aerosols samples from different monitoring stations disseminated in Montreal and collected during 2013, showed that the dynamics of carbonaceous aerosols, and their water-soluble organic carbon (WSOC) fraction, is complex and appear to be controlled by different emission sources, atmospheric processes and weather conditions
Summary
It has clearly been established that atmospheric particulate matter (PM) affects our climate (Wurzler et al, 2000), the ecosystems (Grantz et al, 2003; Okin et al, 2011), and human health (Le Tertre et al, 2002; Pope and Dockery, 2006; Batterman et al, 2014), especially in urban areas where it is present in significant concentrations (Zhang et al, 2015). The carbonaceous material encompasses a wide variety of natural and anthropogenic species with different chemical compositions and structures, and represents between 2 and 5% of the global aerosol emissions (Smithson, 2002). The OC fraction are primary and secondary contaminants (with both natural and anthropogenic origins) and are formed by condensation of volatile organic compounds (VOCs) (Richter and Howard, 2000; Seinfeld and Pankow, 2003; Weber et al, 2007). The water-soluble organic carbon (WSOC) fraction can represent up to 80% of the total OC (Wang et al, 2010; Kirillova et al, 2014a; Bikkina et al, 2017; Zhang et al, 2019). Once dissolved within cloud droplets, WSOC modifies the cloud chemistry (Decesari et al, 2000)
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