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 (delta C-13) values are systematically used to study phase interactions in bulk aerosol and gaseous carbonaceous samples. Seasonal variations in the delta C-13 of total carbon (TC; delta C-13(TC)) and water-soluble organic carbon (WSOC; delta C-13(WSOC)) in fine aerosol particles (PM2.5) as well as in the total carbon of part of the gas phase (TCgas; delta C-13(TCgas)) 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 C-13 enrichment was found in WSOC, followed by particulate TC, whereas the highest C-13 depletion was found in gas-eous TC. We observed a clear seasonal pattern for all delta C-13, with the highest values in winter (avg. delta C-13(TC) = -25.5 +/- 0.8 parts per thousand, delta C-13(WSOC) = -25.0 +/- 0.7 parts per thousand, delta C-13(TCgas) = -27.7 +/- 0.5 parts per thousand) and the lowest values in summer (avg. delta C-13(TC) = -27.2 +/- 0.5 parts per thousand, delta C-13(WSOC) = -26.4 +/- 0.3 parts per thousand, delta C-13(TCgas) = -28.9 +/- 0.3 parts per thousand). 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 (Delta delta C-13) between delta C-13(TC) (aerosol) and delta C-13(TCgas) (gas phase) was similar during the seasons (year avg. 1.97 +/- 0.50 parts per thousand). Moreover, Delta delta C-13 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 parts per thousand). During the entire period, TCgas and WSOC were the most C-13-depleted and most 13C-enriched fractions, respectively, and al-though the resulting difference Delta(delta C-13(WSOC) - delta C-13(TCgas)) was significant, it was almost invariant throughout the year (2.67 +/- 0.44 parts per thousand). 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. (C) 2021 The Authors. Published by Elsevier B.V.

Automated summary

This study systematically investigated the phase interactions between aerosol and gaseous carbon samples using seasonally resolved stable carbon isotope ratio values. It was found that despite different carbon sources in aerosols, the isotope differences remained similar between seasons.