Characteristics and source apportionment of atmospheric volatile organic compounds in Beijing, China

This paper focused on VOCs and their source apportionment in urban Beijing. Our monitoring measured 52 VOCs in July 2014 and January 2015. The concentration of VOCs was in the range of 14.5~95.2 ppb in July and 2.1~93.1 ppb in January, with the top five compounds of toluene (10.7%), ethane (6.9%), ethylene (6.3%), n-butane (5.7%), and propane (5.6%) in July and ethylene (14.7%), n-butane (14.2%), ethane (9.6%), propylene (8.0%), toluene (7.9%), and benzene (6.9%) in January. The ratio of VOCs to CO reached 0.059 in July and 0.022 in January on average. These differences implied a potential seasonal difference in the VOC source contribution. Then, we conducted a source apportionment study based on 21 major VOCs and CO by using probabilistic matrix factorization (PMF) receptor model. According to the similarity between the PMF analysis profiles and the known source profiles, combustion sources, petrochemical industry sources, solvent utilization sources, and gasoline evaporation sources were identified. The correlation coefficient (R) between the PMF analysis profile and the source profile reached 0.68~0.87 in July and 0.53~0.92 in January. The better apportionment performance in July was mainly due to the use of intensive VOC observations at a 3-h resolution. When we conducted another PMF source apportionment for July based on 12-h resolved concentration input, the R values decreased to 0.47~0.73. Thus, the PMF model depends heavily on the sample number of concentration inputs, and intensive observation is more propitious. Our PMF apportionment results showed that combustion sources, petrochemical industry, solvent utilization, gasoline evaporation, and other sources contributed ambient VOCs in Beijing urban areas of 13.7 ppb, 5.1 ppb, 7.7 ppb, 12.8 ppb, and 3.3 ppb in July and 13.2 ppb, 2.0 ppb, 5.7 ppb, 6.6 ppb, and 1.0 ppb in January, respectively, on a monthly average. These apportionment results match well with the 2013 VOC emission inventory calculated by this study, but also presented significant seasonal differences in the petrochemical industry and gasoline evaporation, in which VOC emissions strongly respond to environmental temperature.