Surface-air exchanges of HS and SO 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 (HS) and sulfur dioxide (SO), 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 HS and SO budget. In this study, the surface-air exchange fluxes of HS and SO 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 HS emission, might be caused by the relatively high pH value and low sulfate concentration in the soil. Although HS showed emission in the daytime of spring and summer, an overall HS flux of -0.04 kg S ha yr was observed throughout the year. Meanwhile, the urban wetland presented a net sink of SO, with a deposition flux of 0.14 kg S ha yr. The negative peaks of SO flux corresponded to the suddenly elevated SO concentration in the ambient air especially in spring and winter. Through linear fitting of SO flux and concentration, the concept of SO "compensation point" was proposed. The compensation point is the concentration level at which the observed SO 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 HS but could act as a significant sink of SO with high SO concentration in the urban region.