Characteristics of VOCs during haze and non-haze days in Beijing, China: Concentration, chemical degradation and regional transport impact

Abstract

This paper focused on VOCs (referring to C2C12 hydrocarbons) in Beijing and conducted an integrated comparison study on VOCs concentration, composition, chemical degradation and regional transport impact between haze and non-haze days in July and December of 2015, through VOCs monitoring and WRF-Chem modeling simulation. Our monitoring results show that in July VOCs was 20.1% higher in haze days (24.5 ± 6.6 ppbv) than in non-haze days (20.4 ± 7.8 ppbv), but VOCs:CO in haze days was 13.4% lower than the non-haze level. The relative change in VOCs:CO from non-haze days to haze days responded negatively to VOCs reaction ability with OH radical (KOH). Combining VOCs:CO decline during 8:00–14:00 with KOH, OH level was estimated to increase from 0.75 × 106 molec/cm3 in non-haze days to 1.17 × 106 molec/cm3 in haze days. WRF-Chem simulations calculate the regional transport contribution to Beijing VOCs, averagely −0.5 ppbv in haze days and 6.3 ppbv in non-haze days in July. These analyses implied a significant VOCs chemical degradation and a weak VOCs regional transport in summer haze days. In December, VOCs reached 91.0 ± 37.3 ppbv in haze days and 21.4 ± 14.1 ppbv in non-haze days, and VOCs:CO still dropped by 21.2% lower in haze days. However, the VOCs:CO decrease was mostly contributed by alkanes, acetylene and C4C5 alkenes. The relative change in VOCs:CO from non-haze days to haze days seemingly responded positively to KOH values and VOCs chemical degradation was never explicitly found in both haze and non-haze days. Model simulations obtain a higher regional transport contribution to VOCs in haze days (4.4–31.0 ppbv) than in non-haze days (0.5–6.5 ppbv). The winds transport also explained the difference in VOCs composition and VOCs:CO ratio between haze and non-haze days of winter, for prevalent south winds in haze days transporting air masses of South Beijing and South Hebei that were more polluted and characterized by higher proportions of alkenes, aromatics and CO.