Quantifying background PM2.5 concentrations in Hainan Province, China through model simulations constrained by multi-source observations

Abstract As a national ecological civilization pilot zone with unprecedented strategic importance in China, Hainan Province requires precise identification of PM 2.5 background concentrations to achieve world-leading air quality objectives. This study utilized the three-dimensional regional air quality modeling (WRF-CMAQ) and integrated multi-source ground observation data for model validation. To quantify the PM 2.5 background values for Hainan, representing concentrations largely influenced by natural emissions and regional transport, we determined the contributions from these sources. The results identified local anthropogenic emissions (49.5%) and regional transport (29.3%) as the dominant PM 2.5 sources in Hainan, with important natural contributions (19.0%). Based on the theoretical definition of background concentrations grounded in scientific research and the objective of providing practical policy insights for Hainan, the PM 2.5 background values were defined as the PM 2.5 concentration after (1) excluding local anthropogenic emissions; (2) excluding anthropogenic emissions across Hainan and its surrounding regions; and (3) excluding local emissions combined with projected 2035 emission scenarios for surrounding areas. Based on these three definition methods, the 18 cities-averaged PM 2.5 background values for Hainan were determined as 6.9, 4.2, and 5.2 µg/m 3 , while the corresponding grid-averaged values calculated were 6.7, 4.6, and 4.9 µg/m 3 , respectively. Accounting for meteorological fluctuations could yield lower estimated PM 2.5 concentrations for Hainan. Hainan’s PM 2.5 background values showed comparability with levels documented in clean island areas globally. Our study demonstrated that Hainan maintained a relatively low PM 2.5 background level, showing substantial emission reduction potential. Through implementation of effective control measures, Hainan is projected to achieve world-leading air quality by 2035.