Temperature dependence and source apportionment of volatile organic compounds (VOCs) at an urban site on the north China plain

Abstract

Volatile organic compounds (VOCs) are largely affected by ambient temperature. However, responses of source-specific VOCs to ambient temperature are poorly understood. Based on one-year (2016.4–2017.3) observations for 57 VOC species (average: 33.4 ± 26 ppbv), and on the use of a k-means cluster technique and positive matrix factorization model, temperature dependence of VOCs and their potential sources were studied. The VOC reactivity was dominated by alkenes, though alkanes contributed the most to the VOCs both by volume and by carbon atoms. The cluster analysis suggested that real-world VOCs were influenced directly and indirectly by temperature. The responses of the VOCs in terms of volume and carbon atoms to temperature exhibited V-shape patterns, and the inflection points occurred when temperatures were around 15∼25∘C. Nevertheless, the VOC reactivity generally increased with increasing temperature, suggesting that temperature could be used as a proxy for VOC reactivity. The source apportionment study revealed that vehicle emissions (44.8%) contributed the most to ambient VOCs, and followed by natural gas (NG)/liquefied petroleum gas (LPG)/Combustion (24.9%), petrochemical industry (9.6%), other industrial process (8.4%), solvent use (7.4%), and biogenic emission (4.9%). Solvent use and biogenic emissions were the greatest contributors to VOC reactivity. Unexpectedly high propane/ethylene ratios were observed at low temperatures. This might be associated with domestic heating-related emissions from NG/LPG sources because their contributions generally increased with the decreasing temperature. By comparison with ∼25 ∘C, the VOC concentrations and ozone formation potentials contributed by vehicles were respectively 7- and 2.6- times greater at ∼0 ∘C (cold engine starts dominance), and were 50.4% and 57.5% greater at ∼35 ∘C (gasoline evaporation dominance). Local emission controls for VOCs were beneficial for alleviating atmospheric secondary pollution because the potential source-areas of VOCs were mostly distributed in the geographically flat North China Plain. However, alkanes-dominated VOC sources (e.g., vehicle emission, NG/LPG) could experience long-distance air transport bacause of their relatively long atmospheric lifetime. The results highlight that the temperature dependences of VOCs depend on the emission sources, and this is of great value in understanding the linkage between meteorology and air quality.