Carbon isotope for carbonyls in roadside air

Carbon isotopic dada of carbonyls are valuable for better insight into their sources and processes in ambient air. In the roadside air of Nanning, time series of delta 13C values were firstly investigated on 15-17 August 2019 for three dominant formaldehyde, acetaldehyde and acetone. The results showed that the three carbonyls have delta 13C values of -36.02%o to -31.18%o, -35.35%o to -32.01%o and -30.45%o to -29.09%o, respectively. The calculated delta 13C1/delta 13C2 ratio of formaldehyde/acetaldehyde was in a range from 0.97 to 1.03, which falls well in concentration range (1-2) of C1/C2 ratio in the urban areas, suggesting that such delta 13C1/delta 13C2 ratio was possibly an effective indicator for roughly identifying their sources in ambient air. Furthermore, a good correlation (R2 = 0.62-0.78) of the measured delta 13C values was observed among formaldehyde, acetaldehyde and acetone, likely implying their similar sources. The three carbonyls in the roadside air were enriched in the early afternoon by round 0.5-6%o in 13C compared to other time durations of sampling, which was likely due to positive photochemical production from hydrocarbons. Particularly, all measured delta 13C values (-36.5%o to -29.0%o) agreed with the forecasted delta 13C range (-43.0%o to -26.0%o) according to the 13C mass balance of carbonyls and their hydrocarbon precursors, indirectly confirming the positive production in the roadside air. Finally, the average contribution of such production to carbonyls was estimated as around 78-88% based on the stable carbon isotopic mixing model, which further provides a valuable isotopic evidence that ambient carbonyls in the roadside air are mainly dominated by the photochemical productions.

Automated summary

The study conducted in the roadside air of Nanning revealed that formaldehyde, acetaldehyde, and acetone in urban areas have similar sources and are mainly influenced by positive photochemical production from hydrocarbons, with enrichment observed in the early afternoon. In addition, the observed delta 13C values were in agreement with the forecasted range, confirming the positive production of carbonyls in the roadside air. The isotopic evidence provided further insight into the dominance of photochemical productions in ambient carbonyls.