Developing a novel hybrid model for the estimation of surface 8 h ozone (O3) across the remote Tibetan Plateau during 2005-2018

We developed a two-stage model called the random-forest-generalised additive model (RF-GAM), based on satellite data, meteorological factors, and other geographical covariates, to predict the surface 8 h O-3 concentrations across the remote Tibetan Plateau. The 10-fold cross-validation result suggested that RF-GAM showed excellent performance, with the highest R-2 value (0.76) and lowest root-mean-square error (RMSE) (14.41 mu g m(-3)), compared with other seven machine-learning models. The predictive performance of RF-GAM showed significant seasonal discrepancy, with the highest R-2 value observed in summer (0.74), followed by winter (0.69) and autumn (0.67), and the lowest one in spring (0.64). Additionally, the unlearning ground-observed O-3 data collected from open-access websites were applied to test the transferring ability of the novel model and confirmed that the model was robust in predicting the surface 8 h O-3 concentration during other periods (R-2=0.67, RMSE = 25.68 mu g m(-3)). RF-GAM was then used to predict the daily 8 h O-3 level over the Tibetan Plateau during 2005-2018 for the first time. It was found that the estimated O-3 concentration displayed a slow increase, from 64.74 +/- 8.30 mu g m(-3) to 66.45 +/- 8.67 mu g m(-3) from 2005 to 2015, whereas it decreased from the peak to 65.87 +/- 8.52 mu g m(-3) during 2015-2018. Besides this, the estimated 8 h O-3 concentrations exhibited notable spatial variation, with the highest values in some cities of the northern Tibetan Plateau, such as Huangnan (73.48 +/- 4.53 mu g m(-3)) and Hainan (72.24 +/- 5.34 mu g m(-3)), followed by the cities in the central region, including Lhasa (65.99 +/- 7.24 mu g m(-3)) and Shigatse (65.15 +/- 6.14 mu g m(-3)), and the lowest O-3 concentration occurred in a city of the southeastern Tibetan Plateau called Aba (55.17 +/- 12.77 mu g m(-3)). Based on the 8 h O-3 critical value (100 mu g m(-3)) provided by the World Health Organization (WHO), we further estimated the annual mean nonattainment days over the Tibetan Plateau. It should be noted that most of the cities on the Tibetan Plateau had excellent air quality, while several cities (e.g. Huangnan, Haidong, and Guoluo) still suffered from more than 40 nonattainment days each year, which should be given more attention in order to alleviate local O-3 pollution. The results shown herein confirm that the novel hybrid model improves the prediction accuracy and can be applied to assess the potential health risk, particularly in remote regions with few monitoring sites.