Surface-air exchanges of H₂S and SO₂ in an urban wetland in eastern China

Wetlands are widely recognized as hot spots for the emission or deposition of biogenic sulfur gases, including hydrogen sulfur (H₂S) and sulfur dioxide (SO₂), which significantly affect air quality and climate change. With the expansion of urban wetlands, it is critical to know the roles that urban wetlands played in atmospheric H₂S and SO₂ budget. In this study, the surface-air exchange fluxes of H₂S and SO₂ were measured by the Dynamic Flux Chamber (DFC) method in a typical urban wetland in eastern China from Sep 2022 to July 2023. It was found that the urban wetland did not have the expected high H₂S emission, might be caused by the relatively high pH value and low sulfate concentration in the soil. Although H₂S showed emission in the daytime of spring and summer, an overall H₂S flux of -0.04 kg S ha^-1 yr^-1 was observed throughout the year. Meanwhile, the urban wetland presented a net sink of SO₂, with a deposition flux of 0.14 kg S ha^-1 yr^-1. The negative peaks of SO₂ flux corresponded to the suddenly elevated SO₂ concentration in the ambient air especially in spring and winter. Through linear fitting of SO₂ flux and concentration, the concept of SO₂ "compensation point" was proposed. The compensation point is the concentration level at which the observed SO₂ flux equals zero. The "compensation point" changed with the season and was related to temperature and humidity. The "compensation point" in summer and autumn were larger, being 2.37 ppb and 1.40 ppb, respectively, while they were 1.07 ppb and 0.86 ppb in spring and winter respectively. Our results suggest that the urban wetland expansion may have little risk of increasing air H₂S but could act as a significant sink of SO₂ with high SO₂ concentration in the urban region.