Effect of isoprene emissions from major forests on ozone formation in the city of Shanghai, China

Abstract. Ambient surface level concentrations of isoprene (C₅H₈) were measured in the major forest regions located south of Shanghai, China. Because there is a large coverage of broad-leaved trees in this region, high concentrations of isoprene were measured, ranging from 1 to 6 ppbv. A regional dynamical/chemical model (WRF-Chem) is applied for studying the effect of such high concentrations of isoprene on the ozone production in the city of Shanghai. The evaluation of the model shows that the calculated isoprene concentrations agree with the measured concentrations when the measured isoprene concentrations are lower than 3 ppb, but underestimate the measurements when the measured values are higher than 3 ppb. Isoprene was underestimated only at sampling sites near large bamboo plantations, a high isoprene source, indicating the need to include geospatially resolved bamboo distributions in the biogenic emission model. The assessment of the impact of isoprene on ozone formation suggests that the concentrations of peroxy radicals (RO₂) are significantly enhanced due to the oxidation of isoprene, with a maximum of 30 ppt. However, the enhancement of RO₂ is confined to the forested regions. Because the concentrations of NO_x were low in the forest regions, the ozone production due to the oxidation of isoprene (C₅H₈ + OH → → RO₂ + NO → → O₃) is low (less than 2–3 ppb h⁻¹). The calculation further suggests that the oxidation of isoprene leads to the enhancement of carbonyls (such as formaldehyde and acetaldehyde) in the regions downwind of the forests, due to continuous oxidation of isoprene in the forest air. As a result, the concentrations of HO₂ radical are enhanced, resulting from the photo-disassociation of formaldehyde and acetaldehyde. Because the enhancement of HO₂ radical occurs in regions downwind of the forests, the enhancement of ozone production (6–8 ppb h⁻¹) is higher than in the forest region, causing by higher anthropogenic emissions of NO_x. This study suggests that the biogenic emissions in the major forests to the south of Shanghai have important impacts on the levels of ozone in the city, mainly due to the carbonyls produced by the continuous oxidation of isoprene in the forest air.