Discrepancies in ozone levels and temporal variations between urban and rural North China Plain

Abstract

Effective emission reductions of some primary pollutants have brought down aerosol loadings but led to increasing relative importance of secondary pollutants, as was indicated by the rising O3 levels during warm seasons within urban and suburban areas of China, which has received much attention in recent years, especially in the North China Plain (NCP). This has raised serious concerns on its agricultural impacts, which were mainly evaluated based upon O3 model simulations or urban/suburban measurements due to a lack in long-term rural observations. In this study, we present highly valuable continuous O3 observations at a rural NCP site during 2013–2019. Compared to nearby urban/suburban sites, which experienced increased O3 levels, rural observations exhibited decreasing O3 mole fractions. While O3 mole fractions and AOT40 widely increased at urban/suburban NCP sites from 2013 to 2019, O3 observations in the rural NCP site (GC) revealed decreases, especially during summer and autumn with greater rates for AOT40. A reassessment of O3 agricultural impacts in the NCP region was performed using rural observations, resulting in wheat, maize and soybean averaged relative yield losses of 37 ± 14, 8 ± 4 and 30 ± 13% yr–1, respectively. O3 impacts on crop yields and resulting economic losses did not increase as was suggested by previous estimations based on urban/suburban O3 data. Our analyses indicated high overestimations (i.e., average relative differences in estimated crop production loss reaching 53%, 112% and 75%, respectively, for wheat, maize, and soybean). Despite alleviated O3 agricultural impacts, the total economic cost loss in Hebei province still took up 0.89% of the gross domestic production (3.47 × 1012 USD) in Hebei province. Since the China National Environmental Monitoring Center mainly aims at monitoring O3 levels in populated areas, observation sites representative of agricultural regions are lacking across China. The current study highlights the urgent necessity for the establishment of rural O3 observation networks and encourages extensive field experiments on exposure–response relationships of different crops varieties to O3 for more accurate agricultural impact evaluations. Additionally, explorations into the underlying mechanisms behind the reversed O3 temporal variation between rural and urban areas should be conducted for future development of pollution control strategies.