Seasonal changes in stable carbon isotopic composition in the bulk aerosol and gas phases at a suburban site in Prague.

Isotope fractionation between the gas and aerosol phases is an important phenomenon for studying atmospheric processes. Here, for the first time, seasonally resolved stable carbon isotope ratio (δ13C) values are systematically used to study phase interactions in bulk aerosol and gaseous carbonaceous samples. Seasonal variations in the δ13C of total carbon (TC; δ13CTC) and water-soluble organic carbon (WSOC; δ13CWSOC) in fine aerosol particles (PM2.5) as well as in the total carbon of part of the gas phase (TCgas; δ13CTCgas) were studied at a suburban site in Prague, Czech Republic, Central Europe. Year-round samples were collected for the main and backup filters from 14 April 2016 to 1 May 2017 every 6 days with a 48 h sampling period (n = 66). During all seasons, the highest 13C enrichment was found in WSOC, followed by particulate TC, whereas the highest 13C depletion was found in gaseous TC. We observed a clear seasonal pattern for all δ13C, with the highest values in winter (avg. δ13CTC = -25.5 ± 0.8‰, δ13CWSOC = -25.0 ± 0.7‰, δ13CTCgas = -27.7 ± 0.5‰) and the lowest values in summer (avg. δ13CTC = -27.2 ± 0.5‰, δ13CWSOC = -26.4 ± 0.3‰, δ13CTCgas = -28.9 ± 0.3‰). This study supports the existence of different aerosol sources at the site during the year. Despite the different seasonal compositions of carbonaceous aerosols, the isotope difference (Δδ13C) between δ13CTC (aerosol) and δ13CTCgas (gas phase) was similar during the seasons (year avg. 1.97 ± 0.50‰). Moreover, Δδ13C between WSOC and TC in PM2.5 showed a difference between spring and winter, but in general, these values were also similar year-round (year avg. 0.71 ± 0.37‰). During the entire period, TCgas and WSOC were the most 13C-depleted and most 13C-enriched fractions, respectively, and although the resulting difference Δ(δ13CWSOC - δ13CTCgas) was significant, it was almost invariant throughout the year (2.67 ± 0.44‰). The present study suggests that the stable carbon isotopic fractionation between the bulk aerosol and gas phases is probably not entirely dependent on the chemical composition of individual carbonaceous compounds from different sources.