Field observations of intermediate volatile organic compounds during haze events in three cities of northern China: Characteristics, sources and contributions to SOA formation

Abstract

Intermediate volatile organic compounds (IVOCs) are the most important precursor of secondary organic aerosols (SOA) in the atmosphere, and there remains a deficiency of in-depth research regarding their concentrations, compositions, sources, and contribution to SOA. In this study, IVOCs and PM_2.5 samples with high-temporal-resolution were collected during five winter haze episodes (defined as ZB-P1, ZB-P2, HEB-P1, HEB-P2, and ZK-P1) in three cities in northern China (Harbin, Zibo, and Zhoukou). Among the three sampling sites, Harbin (HEB) recorded the highest IVOCs (73.1 ± 20.9 μg/m³) and PM_2.5 (193.5 ± 62.7 μg/m³), followed by Zhoukou (ZK) 57.5 ± 40.5 μg/m³ and 153.3 ± 78.4 μg/m³, and the lowest was Zibo (ZB) 24.4 ± 11.1 μg/m³ and 130.5 ± 53.2 μg/m³. During all haze episodes, IVOC_UCM was the main component of IVOCs, but the second dominant components varied among the cities. IVOC_n-alkanes was the dominant component in ZB, While IVOC_PAHs in HEB, and IVOC_b-alkanes in ZK. IVOCs exhibited a statistically significant inverse correlation with PM_2.5 and SOA_OC/EC (R = −0.48). It was found that IVOC_PAHs exhibited the highest reactivity among the four categories and might contribute more significantly to SOA. For some n-alkanes and PAHs, Dodecane (n-C12), Heptadecane (n-C17), and Acenaphthene (ACE) were more reactive species. Using the PMF model and accounting for the photochemical loss of IVOCs, four sources of IVOCs were identified: gasoline exhaust, biomass/coal combustion, diesel-related emissions, and other sources (industrial sources and fireworks emissions). Biomass/coal combustion dominated (54.6 % and 58.1 %, observed and initial) in HEB, while industrial sources were predominant in ZB (63 % and 62.7 %), and fireworks were the main source in ZK (67.5 % and 68.2 %). Our results highlight that biomass/coal combustion, industrial sources and vehicle emissions remain the primary control objects. We also suggest that there is a need to strengthen the emission control strategy for PAHs in IVOCs.