Quantifying background PM_2.5 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/m³, while the corresponding grid-averaged values calculated were 6.7, 4.6, and 4.9 μg/m³, 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.

Graphical abstract

Keywords

Background value / PM_2.5 concentration / Source contribution / Hainan Province / CMAQ model

Highlight

	● Local emissions (49.5%) and regional transport (29.3%) dominate Hainan’s PM2.5.
	● Annual mean PM2.5 background levels in Hainan are low (4.2–6.9 μg/m3).
	● Estimated PM2.5 background values can be decreased by meteorological fluctuations.
	● Hainan has high potential to achieve world-leading air quality targets by 2035.

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