VOC source apportionment in Shanghai based on the identification of two-stage photochemical ages

The positive matrix factorization (PMF) model is widely used for volatile organic compound (VOC) source apportionment, yet its assumption of chemical stability conflicts with reactive transport processes. To address this, we developed a two-stage approach that quantifies photochemical aging by separately identifying distinct atmospheric processing phases. Analyzing one year of VOC measurements in Shanghai, we resolved five factors, including diesel exhaust, industry/solvents, and aged VOCs. By correlating factor contributions with OH radical reaction rate constants (k_OH), we determined transport times for primary emissions (stage I) and the ultimate aging state (stage II). Correcting for these stages revealed that conventional ozone formation potential (OFP) estimates account for only 15% and 42% of the actual potential at QP and PD, respectively, when accounting for photochemical loss. This framework provides a more accurate representation of initial emission profiles, demonstrating that pre-receptor processes are critical for characterizing the true impact of VOC sources on urban air quality.