Sources of formaldehyde and their contributions to photochemical O3 formation at an urban site in the Pearl River Delta, southern China

- Secondary formation was the greatest contributor to ambient HCHO, followed by vehicular exhaust and solvent usage.
- Alkenes was the most important group contributing to the secondary formation of HCHO.
- trans-2-Butene had the largest contribution to secondary HCHO formation, followed by i-butene, cis-2-butene and propene.
- Secondary formation contributed about 2.8 ppbv and 1.1 × 107 molecule/cm3 to the production of O3 and HOx radical.

Two models (the Positive Matrix Factorization (PMF) model and a photochemical box model with Master Chemical Mechanism (PBM-MCM)) were applied to analyze the formaldehyde (HCHO) data collected in July 2006 at an urban site (GPEMC) in the Pearl River Delta (PRD), southern China. Three major HCHO sources (secondary formation, vehicular exhaust, and solvent usage) were identified and they were found to contribute in average 53%, 31% and 16% respectively to the total HCHO loading at GPEMC. Alkenes was the most important group contributing to the secondary formation of HCHO, followed by aromatics and alkanes. Among them, trans-2-butene had the largest contribution to secondary HCHO formation, with the average percentage of 16 ± 4%, followed by i-butene, cis-2-butene, propene, isoprene and m,p-xylene. Secondary HCHO and HCHO emitted from vehicular emissions contributed comparably to ground-based measured O3 and HOx radical at GPEMC, higher than that from solvent usage (1.3 ± 0.1 ppbv and (4.1 ± 0.3) × 106 molecule/cm3 for O3 and HOx radical). Our results highlight the importance of secondary HCHO formation for both photochemical formation of ozone and the oxidative capacity of the atmosphere in this region. It is hence critical for policy makers to propose strategies for controlling VOCs from vehicular emissions in order to reduce secondary HCHO formation. Our results also have important implication for improving the understanding of the source apportionments of HCHO and their contributions to photochemical pollution in the PRD region in China.