ABSTRACT
Although the air quality in China has been greatly improved in recent years, the air pollution remains severe. The annual mean PM2.5 concentrations have not met the second grade of the National Ambient Air Quality Standards in China and are still much higher than the guideline value of the World Health Organization. Thus, the PM2.5 concentration needs to be further reduced. Secondary organic aerosol (SOA) is an important component of PM2.5 and has an important impact on air quality, global climate change, and human health. Therefore, understanding the formation mechanism of SOA is an important basis to control SOA and further reduce PM2.5. As an important precursor of SOA, volatile organic compounds (VOCs) can be oxidized by oxidants such as ·OH, NO3[KG-*2/3]·, Cl·, and O3 to generate low volatile organic compounds and further to form SOA through gas-particle partitioning, homogeneous nucleation, aqueous phase reaction, and heterogeneous reaction processes. The formation of SOA can be affected by many factors, such as the types and initial concentrations of VOCs, VOCs/NOx ratios, relative humidity (RH), temperature (T), seed aerosols, oxidants, aqueous phase process, and photochemical process. The observed SOA concentration is always underestimated by air quality models because a comprehensive understanding of the complexity of SOA chemical composition and formation mechanisms is still lacking, especially that under the highly complex air pollution conditions in China. Therefore, the formation mechanism and influencing factors of SOA under highly complex air pollution conditions have become an important concern in the field of atmospheric sciences. Recently, much laboratory work has focused on the formation of SOA under complex conditions. The research progress of SOA formation from different anthropogenic VOCs are reviewed here, and the methods used and the impact of different influencing factors on SOA formation are introduced. Finally, the key scientific issues that exist in the research of the SOA mechanism at present are put forward, and the future research direction is projected.
ABSTRACT
Volatile organic compounds (VOCs) are important precursors of O3 and secondary organic aerosols (SOAs). In order to fully understand the impacts of VOCs from motor vehicles on urban air quality, the current state-of-the-art research results on source apportionment are systematically introduced. The contribution of VOCs from motor vehicles to secondary pollutants in China is further discussed. It was found that motor vehicle exhaust is the largest source of urban atmospheric VOCs, and the average contribution ratio is up to 36.8%, wherein motorcycles and light gasoline vehicles are the main emission sources. The results showed that VOCs from vehicles play an important role in O3 and SOA formation in urban areas, whereas with the improvement of emission standards and operating conditions, the motor vehicle emission factors and ozone formation potentials (OFPs) are obviously reduced. Moreover, the composition profile of the exhaust is mainly composed of aromatic hydrocarbons and alkenes, which contribute more to secondary pollution. This work can aid in providing data and theoretical support for the control of VOCs from motor vehicles in the future.
