Quantifying the Role of Hydroxyl-Driven Loss in Recent Methane Trends

Abstract:

The past two decades have seen the growth of atmospheric methane concentrations pause, resume, then accelerate. Variations in the rate of methane loss, primarily through reaction with the hydroxyl radical, have been posited as an explanation. We use a novel adjoint modelling technique to estimate the sensitivity of the hydroxyl-driven loss of methane to precursor emissions of NOx, CO, and to the actinic UV flux that produces hydroxyl. This information, combined with timely estimates of changes to precursor emissions from a tropospheric chemical reanalysis, yields observation-constrained, spatially-attributable changes to the methane sink. Early work suggests that changes to the sink are small compared to the change in methane abundance over the 2006 to 2015 period. This small change reflects a balance of increased methane loss from NOx emissions increases at low latitudes and decreased methane loss from decreased actinic flux due to ozone column recovery.