Abstract
Surface ozone concentrations have been increasing in many regions of China for the past few years, in contrast to policy-driven declines in other key air pollutants such as particulate matter. While the central role of meteorology in modulating ozone pollution is widely acknowledged, its quantitative contribution remains highly uncertain. Here, we use a data-driven machine learning approach to assess the impacts of meteorology on surface ozone variations in China for the years 2015 to 2019, considering the months of highest ozone pollution from April to October. To quantify the importance of various meteorological driver variables, we apply non-linear random forest regression (RFR) and linear ridge regression (RR) to learn relationships between meteorological variability and surface ozone in China, and contrast the results to those obtained with the widely used multiple linear regression (MLR) and stepwise MLR. We show that RFR outperforms the three linear methods when predicting ozone using only local meteorological predictor variables. This implies the importance of non-linear relationships between local meteorological factors and ozone, which are not captured by linear regression algorithms. In addition, we find that including non-local meteorological predictors can further improve the modelling skill of RR, particularly for Southern China, highlighting the importance of large-scale meteorological phenomena for ozone pollution in that region. Overall, RFR and RR are in close agreement concerning the leading meteorological drivers behind regional ozone pollution. For example, we find that temperature variations are the dominant meteorological driver for ozone pollution in Northern China (e.g., Beijing Tianjin Hebei region), whereas variations in relative humidity are the most important factor in Southern China (e.g., Pearl River Delta). Variability in surface solar radiation modulates photochemistry but was not considered as such in previous controlling factor analyses, and is found to be the most important predictor in the Yangtze River Delta and Sichuan Basin regions. In general, our analysis underlines that hot and dry weather conditions with high sunlight intensity are strongly related to high ozone pollution across China. This further validates our novel approach to quantify the central role of meteorology. By contrasting our meteorological ozone predictions with ozone measurements between 2015 and 2019, we estimate that almost half of the observed ozone trends across China might have been caused by meteorological variabilities on average. We highlight that these insights are of particular importance given possible increases in the frequency and intensity of weather extremes such as heatwaves under climate change.
Highlights
Surface ozone is an air pollutant that can induce severe harm to both human health and ecosystems (Lefohn et al, 2018; Lelieveld et al 2015)
The averaged R2 across all ERA5 grid locations within BTH, Yangtze River Delta (YRD), Pearl River Delta (PRD), and Sichuan Basin is 0.46, 0.56, 0.53 and 0.57 respectively for random forest regression (RFR), which are all higher than the equivalent R2 for RR (BTH: 0.41, YRD: 0.48, PRD: 0.47, Sichuan Basin: 0.53)
We find that the R2 in RFR-2D for PRD region is still slightly less than the R2 using RR-2D (0.76) and the predictions from RR-2D are closer to observations with less deviations at both high and low ozone value predictions, suggesting that RR is better at handling the dimensionality increase of predictors, which slightly outweighs the importance of non-linearity in high dimensions
Summary
Surface ozone is an air pollutant that can induce severe harm to both human health and ecosystems (Lefohn et al, 2018; Lelieveld et al 2015). Despite decreasing trends in NOx and CO, summertime surface ozone concentrations have been increasing from 2013 to 2019 at a rate of about 1.9 ppb yr-1 on average across China, with a faster rate of 3.3 ppb yr-1 in the North China Plain (Li et al., 2020) It is well-known that the effectiveness of ozone production is strongly dependent on the atmospheric chemical regime (e.g., Squire et al, 2015, Archibald et al, 2020), in which ozone production is mainly controlled by the abundance of NOx or VOCs. Many urban and industrial regions in China have been identified and categorized as being within the VOC-limited regime (Ou et al, 2016; Wang et al, 2017).
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