Seasonal changes of sources, volatility, and aging of organic aerosols in eastern Europe reflected in the stable isotopic composition

The stable carbon isotope ¹³C has the potential to give insights into sources and processing of organic aerosol. We use a method to measure d¹³C in OC desorbed from filter samples at three different temperature steps: 200 °C, 350°C and 650°C. The results give a rough indication of aerosol volatility, as more volatile compounds usually desorb at lower temperatures.

We demonstrate with an extensive source study that in Lithuania and likely other Eastern European regions, the main anthropogenic primary sources for organic carbon (OC) have distinct isotopic signatures. d¹³C values of vehicular emissions show the most negative values around - 29 ‰, emissions from combustion of the most common wood types are more enriched with values around -26 to -27 ‰, and coal burning is around -25‰. For source samples, d¹³C values at the three desorption temperature steps usually do not differ more than 1 ‰.

In the ambient samples, OC had more negative δ¹³C values in summer than in winter, which can be explained by the contribution of biomass/coal burning sources in winter. At the urban site δ¹³C of OC did not change much with increasing desorption temperature in winter, which is typical for primary sources. In the summer δ¹³C of OC was clearly more negative for lower desorption temperatures at all three sites. This is likely due to the influence of secondary organic aerosol formation in summer, which should have depleted (more negative) isotopic signature and contributes strongly to the more volatile fraction.A higher fraction of more refractory OC in summer compared to winter-time suggests active photochemical processing of the primary organic aerosol as an important process at all three sites.

This is consistent with our laboratory studies, where we age source samples in a small reactor under UV light. Photolysis causes mainly mass loss in OC that desorbs at 200 °C. At the same time, ¹³C becomes more enriched in OC desorbed at the higher temperature steps, leading to a bigger difference in d¹³C between OC200 and OC350, as observed in the ambient atmosphere.

In summary, analyzing stable isotopes of OC at different desorption temperature steps gives a powerful tool for diagnosing aging processes.