Removal of methyl chloroform in a coastal salt marsh of eastern China

The atmospheric burden of methyl chloroform (CH3CCl3) is still considerable due to its long atmospheric lifetime, although CH3CCl3 emissions have declined considerably since it was included into the Montreal Protocol. Moreover, CH3CCl3 emissions are used to estimate hydroxyl radical (OH) levels, trends, and hemispheric distributions, and thus the mass balance of the trace gas in the atmosphere is critical for characterizing OH concentrations. Salt marshes may be a potential sink for CH3CCl3 due to its anoxic environment and abundant organic matter in sediments. In this study, seasonal dynamics of CH3CCl3 fluxes were measured using static flux chambers from April 2004 to January 2005, along an elevational gradient of a coastal salt marsh in eastern China. To estimate the contribution of higher plants to the gas flux, plant aboveground biomass was experimentally harvested and the flux difference between the treatment and the intact was examined. In addition, the flux was analyzed in relation to soil and weather conditions.Along the elevational gradient, the salt marsh generally acted as a net sink of CH3CCl3 in the growing season (from April to October). The flux of CH3CCl3 ranged between −3.38 and −32.03 nmol m−2 d−1 (positive for emission and negative for consumption), and the maximum negative rate occurred at the cordgrass marsh. However, the measurements made during inundation indicated that the mudflat was a net source of CH3CCl3. In the non-growing season (from November to March), the vegetated marsh was a minor source of CH3CCl3 when soil was frozen, the emission rate ranging from 3.43 to 7.77 nmol m−2 d−1. However, the mudflat was a minor sink of CH3CCl3 whether it was frozen or not in the non-growing season. Overall, the coastal salt marsh in eastern China was a large sink for the gas, because the magnitude of consumption rate was lager than that of emission, and because the duration of the growing season was longer than that of the non-growing season. Plant aboveground biomass had a great effect on the flux. Comparative analysis showed that the direction and magnitude of the effect of higher plants on the flux of CH3CCl3 depended on timing of sampling vegetation type. In the growing season the plant biomass decreased the gas flux and acted as a large sink of the gas, whereas it presented as a minor source in the non-growing season. However, the mechanism underlying plant uptake process is not clear. The CH3CCl3 flux was positively related to the dissolved salt concentration and organic matter content in soil, as well as light intensity, but it was negatively related to soil temperature, sulfate concentrations, and initial ambient atmospheric concentrations of CH3CCl3. Our observations have important implications for estimation of the tropospheric lifetime of CH3CCl3 and global OH concentration from the global budget concentration of CH3CCl3.