Variations in Global Tropospheric OH Over the Last Several Decades

The oxidizing capacity of the troposphere is controlled primarily by the abundance of hydroxyl radical (OH). The global mean concentration of tropospheric OH, [OH]^TROP (the burden of OH in the global troposphere appropriate for calculating the lifetime of CH₄) inferred from measurements of methyl chloroform has remained relatively constant during the past several decades despite rising levels of CH₄ that should have led to a decline. Here we examine other factors that have affected [OH]^TROP such as the changing values of stratospheric ozone, rising tropospheric H₂O, varying burden of NO_x (=NO+NO₂), rising temperatures, and widening of the climatological tropics due to expansion of the Hadley cell. Our analysis suggests the positive trends in [OH]^TROP due to H₂O, NO_x, overhead O₃, and tropical expansion are large enough (D[OH]^TROP = +0.95±0.18% decade⁻¹) to counter almost all of the expected decrease in [OH]^TROP due to rising CH₄ (D[OH]^TROP = −1.01±0.05% decade⁻¹) over the period 1980 to 2015, while variations in temperature contribute almost no trend (D[OH]^TROP = −0.02±0.02% decade⁻¹) in [OH]^TROP. The approximated impact of Hadley cell expansion on [OH]^TROP is also a small, but not insignificant factor partially responsible for the steadiness of tropospheric oxidizing capacity over the past several decades, which free-running models likely do not capture.