VOCs Source Apportionment in Shanghai Based on the Identification of the Two-stage Photochemical Ages

VOCs source apportionment is the basis for scientific and effective implementation of VOCs prevention and control strategies. Positive Matrix Factorization (PMF) model was used for VOCs source apportionment in this study. However, VOCs species are prone to undergo photochemical aging in the transport due to their high reactivity. This violates the basic assumption of PMF. Targeted on this problem, we conduct a novel method based on the two-stage photochemical ages and the modification of PMF uncertainty data to improve the accuracy of the source apportionment of VOCs.

In this study, the continuous online VOCs measurements were conducted at Qingpu (QP) and Pudong (PD) sites in Shanghai from July 2016 to June 2017. We selected different error coefficients to modify the conventional calculation of PMF uncertainty data based on the maximum incremental reactivity (MIR) of every VOCs species. According to the novel two-stage photochemical aging method, the PMF-resolved factors could be classified into relatively fresh factors and an aged factor with different photochemical ages. Five factors at QP and PD were identified, which were diesel exhaust, combustion, industry and solvent usage, LPG and gasoline and ship emissions, and aged emissions. The respective photochemical ages were 1.30 h, 1.53 h, 2.70 h, 1.99 h and 5.47 h at QP and 0.88 h, 0.60 h, 1.20 h, 1.51 h and 8.53 h at PD. Apart from LPG and gasoline and ship emissions, compared to the other site, the diurnal variations of other four factors were more significant at the site which had the longer photochemical age. For the LPG and gasoline and ship emissions, the discrepancy may resulted in the significant contribution of regional transport. When the dominant contributor was local emissions, the longer photochemical ages made the diurnal variation of factors more intense. With the consideration of photochemical aging of VOCs, the ozone formation potential (OFP) of every factor changed significantly. It indicated that the impact of photochemical aging should be considered in establishment of the priority of VOCs prevention and control strategies.

Keywords: VOCs source apportionment; two-stage photochemical aging; OFP

This study is funded by the National Natural Science Foundation of China (No. 91644221).