Detailed Carbon Isotope Study of PM2.5 Aerosols at Urban Background, Suburban Background and Regional Background Sites in Hungary

The aim of this study was to estimate and refine the potential sources of carbon in the atmospheric PM2.5 fraction aerosol at three sampling sites in Hungary. Quantification of total, organic and elemental carbon (TC, OC and EC, respectively), as well as radiocarbon (14C) and stable carbon isotope analyses were performed on exposed filters collected at an urban background site, a suburban background site of the capital of Hungary, Budapest from October 2017 to July 2018. Results were also collected from the rural regional background site of K-puszta. Compared to TC concentrations from other regions of Europe, the ratio of the lowest and highest values at all sites in Hungary are lower than these European locations, probably due to the specific meteorological conditions prevailing in the Carpathian Basin over the observation period. The concentration of OC was constantly higher than that of EC and a seasonal variation with higher values in the heating period (October–March) and lower values in the non-heating vegetation period (April–September) could be observed for both EC and OC fractions. Using 14C, the seasonal mean fraction of contemporary carbon (fC) within the TC varied between 0.50 and 0.78 at the sites, suggesting that modern sources were remarkable during the year, regardless of the heating or vegetation period. At the two urban sites, assuming constant industrial emission during the year, the fossil fuel combustion sources were responsible for the seasonal variation of EC, while modern carbon emissions from biomass-burning and biogenic sources influenced the OC concentration. The higher EC/TC ratios at these sites were associated with lower fC and δ13C values, which can be explained by soot emission from transportation. The notably high EC/TC ratios in the spring were likely caused by the reduced concentration of OC instead of increased EC concentrations. This could probably be caused by the ending of winter biomass burning, which emits a huge amount of OC into the atmosphere. On the contrary, the rural K-puszta site showed some differences relative to the sites in Budapest. No correlation could be revealed between the EC/TC ratio, fC and δ13C results, suggesting that the structure of sources was very stagnant and balanced in each season. In autumn, however, some less depleted values were observed, and agricultural corn-stalk burning after harvesting in the southern and eastern directions from Hungary can be suggested as the main source.