Improved understanding of the atmospheric methyl bromide and methyl chloride budgets

Methyl bromide (CH3Br) is an important ozone depleting substance. It contributes about 34 % of the total stratospheric bromine. As anthropogenic emissions of CH3Br decrease due to the phase out, the atmospheric CH3Br has been declining. This creates a great opportunity to re-examine the current understanding of the atmospheric CH3Br budget. The current CH3Br budget given by WMO (2011) shows that a significant missing source is needed to balance sources and sinks. Unlike CH3Br, atmospheric methyl chloride (CH3Cl) does not have significant anthropogenic emissions and is not being phased out. Atmospheric CH3Cl and CH3Br share many common natural sources (e.g. ocean, biomass burning, salt marshes, fungi, wetlands, mangroves, rice paddies etc.) and common sinks (e.g. hydroxyl radicals, ocean, loss to stratosphere, and soil). In this study, we use the observed atmospheric temporal trend in CH3Cl, along with its known sources and sinks, to quantify the strength and the seasonal and inter-annual variability of the total emission from all the terrestrial sources. Then, using the same approach, we calculate the strength and the seasonal and inter-annual variability of the total terrestrial emission of CH3Br. Model results show a high correlation between the total terrestrial emissions of CH3Cl and CH3Br in both hemispheres.