Formation mechanism of HCHO pollution in the suburban Yangtze River Delta region, China: A box model study and policy implementations

Abstract

High levels of formaldehyde (HCHO) have been frequently detected during spring-summertime in the densely populated and rapidly developing Yangtze River Delta (YRD) region, China. However, the detailed formation mechanism of HCHO pollution remains unclear. Based on a comprehensive field campaign conducted at Dianshan Lake (DSL) station, a supersite located in the suburban YRD region, the influence of VOCs on HCHO formation, as well as the role of HCHO in the ROx radical recycling was analyzed. Results indicate that the maximum atmospheric oxidation capability (AOC) during HCHO pollution episodes varies from 0.65 × 108 to 1.29 × 108 molecules cm−3 s−1, and high AOC value favors the secondary formation of HCHO. Meanwhile, the photolysis of HCHO accounts for 21.2% of the primary ROx, suggesting the significant role of HCHO in ROx recycling. Isoprene was identified to be a key precursor of HCHO, and the RO from isoprene contributed ∼15% to the total daytime HCHO production rate (PHCHO). Sensitivity analysis reveal that when isoprene, xylene or ethylene was cut off, the simulated HCHO concentration could be reduced by 50%, 15%, or 13%, respectively. Based on the isopleth of HCHO under different VOCs/NOx reduction ratios, we found that the VOCs/NOx reduction ratio should be above 1.03 to mitigate the HCHO pollution. Reactive VOCs (such as isoprene, xylene and alkenes) should be reduced to suppress HCHO pollution in the YRD region.