Natural methyl chloride (CH₃Cl) and methyl bromide (CH₃Br) emissions from coastal marsh ecosystems may constitute a significant proportion of stratospheric chlorine and bromine, which catalyze ozone depletion. Current inventories involve substantial uncertainties associated with up-scaling plot-scale footprints (i.e., ≤ 1m²).  Here we present net ecosystem flux measurements of methyl halides from a brackish tidal marsh on the west coast of the United States between April 2016 and June 2017 using the relaxed eddy accumulation method.  The measurement footprint encompasses a large part of the studied tidal marsh, including roughly 20 vascular plant species, open water and soil surfaces.  On the annual scale, ecosystem methyl halide emissions showed the strongest relationships to temperature and the growth cycle of halophyte vegetation, whereas on diurnal timescales, fluxes correlated the most with evapotranspiration. The maximum seasonal emissions occurred during the flowering season of Lepidium latifolium (perennial pepperweed), one of the most abundant halophytes on site.  The maximum hourly emissions of 111 µg CH₃Cl m^-2 hr^-1 and 38 µg CH₃Br m^-2 hr^-1 were observed during a heat wave in early June.  Annually integrated emissions were 135 mg m^-2 for CH₃Cl and 21 mg m^-2 for CH₃Br, scaling up to 621 kg and 96 kg over the entire marsh. We provide a global salt marsh emission inventory that takes into account the spatial distribution of salt marshes in different climate zones, yielding a global salt marsh source of 31 Gg yr^-1 CH₃Cl (range: 10 to 77) and 3 Gg yr^-1 CH₃Br (range: 1 to 8 ).

This dataset is the synthesis of a) high frequency (in situ) measurements of vertical wind speed and sonic temperature, and b) atmospheric concentrations of the methyl halides CH₃Cl and CH₃Br, measured by gas chromatography. The relaxed eddy accumulation method was used to calculate mass fluxes which are representative for large parts of the studied ecosystem. Additionally this dataset includes results from static flux chamber incubation experiments and auxiliary meteorological variables as listed in the readme.txt file.

Missing Values are represented by NaN.