Relaxed Eddy Accumulation based Flux Measurement of Atmospheric Inorganic Acidic Species over Cropland under the Long-Term Exposure to Chemical Industry Emissions in a Chinese Megacity

Abstract

Industrial emissions in China's densely populated regions turn surrounding lands into high-deposition-load hotspots, alter physicochemical properties of soils and vegetation, and lead to complex sink-source transitions of land surface. The lack of local flux data impedes integrated air-soil pollution control. We developed a Relaxed Eddy Accumulation (REA) system capable of simultaneous flux measurements of eight inorganic species (HNO3, HONO, SO2, HCl, nitrate, nitrite, sulfate, chloride). System characterization showed detection limits of 6.1 × 10-4-2.4 × 10-1 μg m-2 s-1 and flux precisions of 5.4 %-32.3 %, with uncertainties dominated by mass analysis, lag time error, and sonic-temperature-derived proportionality coefficient β. Flux measurements conducted in winter at a vegetable cropland adjacent to chemical industry facilities revealed that HONO and nitrate fluxes at this site were 1-2 orders of magnitude higher than those reported in the literature. Bidirectional fluxes of the species indicate the cropland acts as both source and sink; but winter averages showed net emission fluxes only for HONO and nitrite (mean daily 2.49 and 0.53 mg m-2 d-1). Gross upward emission fluxes of HNO3 and HONO were 1.1 ± 0.9 and 0.4 ± 0.4 μg m2- s-1, respectively, with HNO3 emissions enhanced by turbulence and HONO promoted at low temperatures. Such emissions are expected to enhance atmospheric nitrate aerosol formation and atmospheric oxidative capacity. These results provide critical observational constraints for acidic species exchange parameterization in industrial-influenced regions, advancing understanding of reactive nitrogen cycling and supporting air pollution control and agro-ecological protection strategies.