Pollution characteristics of volatile organic compounds above subtropical forest canopy in Lingnan and the influence of regional anthropogenic emissions

Volatile organic compounds (VOC) in the atmosphere have significant impacts on air quality, climate change, and human health. Atmospheric VOC concentrations in subtropical forests of southern China are affected by regional anthropogenic source emissions. To quantitatively explore the impacts of regional anthropogenic source emissions on forest atmosphere, VOC and ozone concentrations at different vertical levels above the canopy were collected by drone-based samplers in the afternoon and evening in Dinghushan (DHS) and Chebaling (CBL) Nature Reserves in August and September 2019 and were analyzed offline. Moreover, WRF-GC model simulations and scenario analysis were conducted to quantitatively evaluate the impact of regional anthropogenic source emissions on the atmosphere in forest areas. The results showed that the biogenic VOC (BVOC) concentrations were low and anthropogenic VOC (AVOC) concentrations were relatively high at both sites. Compared with DHS, the AVOC concentrations in CBL were lower and the BVOC concentrations were higher, which could be attributed to less anthropogenic influence by regional transport. The ratios of (MVK+MACR)/isoprene at both sites were high, indicating a rapid atmospheric conversion. No significant difference in AVOC concentrations at different sampling heights were found at both sites. BVOC concentrations in CBL were also similar for both of the sampling heights. In DHS, the concentrations of isoprene and α-pinene were significantly different for 25 and 100 m sampling heights, which may be explained by vertical eddy diffusion. Moreover, the surface concentrations of air pollutants in DHS were well simulated by WRF-GC model. Under the scenario of no anthropogenic emissions, the simulated daily average concentrations of isoprene increased by 4 times and that of ozone decreased by 3 times in DHS compared with the default setting. This result suggested that because of the influence of anthropogenic emissions in southern China, the conversion of BVOC to its oxidation products was accelerated to promote the formation of biogenic secondary organic aerosol (SOA). Meanwhile, the ozone concentrations were greater than the plant tolerance threshold, for which long-term exposure would cause vegetation damage in the forested area.