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. China hosts a large number of industrial parks in densely populated areas, where pollutant emissions turn surrounding lands into high-deposition-load hotspots. Long-term exposure may alter physicochemical properties of soils and vegetation, leading to complex sink-source transitions of land surface. The lack of local flux data impedes integrated air-soil-water pollution control. We developed a Relaxed Eddy Accumulation 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 3.0 %–29.5 % with main uncertainty contributions from mass analysis and lag time inaccuracy. Flux measurement conducted 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 reports in the literature. Bidirectional fluxes of the species indicate the cropland acts as both source and sink; but winter average showed net emission fluxes only for HONO and nitrite (mean daily 53.0 and 11.5 μmol m^-2 d^-1). Gross upward emission fluxes of HNO₃ and HONO were 1.1 ± 0.9 μg m^-2 s^-1 and 0.4 ± 0.4 μg m^-2 s^-1, respectively, during the winter observation period. HNO₃ gross emissions were enhanced by elevated turbulence, while HONO emissions were promoted at lower ambient temperatures. This study provides observational constraints for acidic species flux parameterization over industrial-impacted croplands and informs air pollution control and agro-ecological protection strategies.