Volatile organic compounds in urban environments: composition, source apportionment, and chemical transformation

Abstract

Volatile organic compounds (VOCs) contribute significantly to the formation of ozone (O3) and secondary organic aerosol (SOA) in the troposphere. Besides their influence on tropospheric chemistry, VOCs also affect air quality and climate both directly and indirectly. Biogenic VOC (BVOCs) emissions are the predominant source on a global scale, while in the urban areas anthropogenic VOCs are more important. Among them, traffic and solid fuel combustion are important emission sources. After emission, they react with various oxidants, yielding oxygenated VOCs (OVOCs). Due to the high variabilities of VOC sources and reactivity, the determination of VOC sources remains challenging. Due to limitations in the measurement techniques, a large fraction of OVOCs in the urban areas is still poorly understood. In this dissertation, we used a proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToF-MS) in different urban environments to investigate the VOC emission sources and to obtain a deeper understanding of their chemical evolution with support from the positive matrix factorization (PMF) receptor model. Intensive campaigns were carried out at two sites in Delhi, India, as well as an urban site in Beijing, China, and an urban site in Zurich, Switzerland. The measurement campaigns in Delhi aimed at characterizing for the first time a wide range of VOCs in this severely polluted environment. At both the urban and suburban site, high nocturnal concentrations of VOCs were observed with strong day-night variations. Primary emissions were predominant at both sites, mostly from traffic and solid fuel combustion. Large amount of aromatics, furans and phenols were emitted and were significantly elevated at night. The secondary factors, which were dominated by oxygenated VOCs were substantially higher during daytime. A comparison between the two sites showed that primary sources were significant at both sites, however, OVOC concentrations were higher at the suburban site. While the urban OVOCs were tightly associated with aging of local primary emissions, the suburban OVOCs represented both local oxidation and regional aging. The measurement campaign in Beijing was performed to investigate VOC evolution during winter and the potential roles of VOCs to SOA formation under haze conditions. Ten VOC families were classified, which showed clear variation of the relative composition as a function of the VOC concentration. The haze periods were driven by very high concentrations of aromatics originating from traffic and solid fuel combustion. Trajectory analysis indicated that both accumulation and regional influences were important for high VOC concentrations during haze periods. The OVOC factor was predominant during the clean periods, but decreased significantly during the haze periods. Meanwhile, oxygenated organic aerosol (OOA) concentration were enhanced during high VOC concentration periods, indicating the importance of OVOCs to secondary organic aerosol formation. The Zurich campaign focused on the chemical evolution of VOCs and their oxidation products in a relatively clean urban environment in summer. Traffic was the major anthropogenic source, with an additional local source related to activities such as cigarette smoke. Three OVOC factors were identified representing oxidation processes of different oxidants and precursors. Two OVOC factors featured strong contributions from aromatic oxidation and were separated by day-night chemistry. The third OVOC factor was characterized by strong biogenic influences and exhibited similar temporal variation as biogenic SOA. A comparison with the biogenic SOA composition showed similarities in important organic family time series in both the gas and aerosol phase.