Evaluation of air quality in Chengdu, Sichuan Basin, China: are China's air quality standards sufficient yet?

Air quality evaluation is important in order to inform the public about the risk level of air pollution to human health. To better assess air quality, China released its new national ambient air quality standards (NAAQS-2012) and the new method to classify air quality level (AQL) in 2012. In this study, we examined the performance of China's NAAQS-2012 and AQL classification method through applying them, the World Health Organization (WHO) guidelines, and the US AQL classification method to evaluate air quality in Chengdu, the largest city in southwestern China. The results show that annual mean concentrations of PM10, PM2.5, SO2, NO2, and O-3 at the seven urban sites were in the ranges of 138-161, 87-98, 18-32, 54-70, and 42-57 mu g/m(3), respectively, and the annual mean concentrations of CO were in the range of 1.09-1.28 mg/m(3). Chengdu is located in one of the four largest regions affected by haze in China, and PM10 and PM2.5 were the top air pollutants, with annual concentrations over 2 times of their standards in NAAQ S-2012 and over 7 times of the WHO guidelines. Annual mean concentrations of the pollutants were much lower at the background site (LYS) than at the urban sites, but the annual mean concentrations of PM10 and PM2.5 at LYS were 3.5 and 5.7 times of the WHO guidelines, respectively. These suggest that severe air pollution in Chengdu was largely associated with local emissions but also related to regional air pollution. The compliance rates of PM10, PM2.5, SO2, and O-3 met China's NAAQ S-2012 standards four times more frequently than they met the WHO guidelines, as NAAQS-2012 uses the loosest interim target (IT) standards of WHO for these four pollutants. Air pollution in Chengdu was estimated and stated to be less severe using China's classification than using the US classification, as China uses weaker concentration breakpoints and benign descriptions of AQL. Furthermore, China's AQL classification method does not capture the cumulative effects of multiple pollutants, and the risk assessment is mainly based on the exposure-response relationship between air pollutant and human health quantified in the North America and West Europe; these can bring some uncertainties into evaluating the risk to human health in China. In summary, although China greatly improved its NAAQS and AQL classification method in 2012, further improvements are still needed.