Resetting Tropospheric OH and the CH4 Lifetime with UV H2O Absorption

Water vapor (H₂O) absorption in the ultraviolet region 290-350 nm is weak, barely larger than Rayleigh scattering cross sections. Given the large amount of H₂O in the lower tropical atmosphere, however, use of new data on H₂O absorption notably decreases solar ultraviolet radiation by 8-12%. Inclusion of new laboratory measurements in photolysis models reduces near-surface photolysis of O₃ to O(¹D) and hence production of OH radicals, the primary sink for the greenhouse gas CH₄. This process is missing from current models. If this new data is included in photolysis calculations, we find the that primary source of OH, reaction of O(¹D) plus H₂O, is reduced by about 10% in the boundary layer. To first order, the CH₄ lifetime calculated in global chemistry models would lengthen by 5-10%, but the impacts will be more complex because the reduced photolytic rates apply across most of the species involved in the O₃ and OH budgets. Given the complexity, a multi-model assessment is needed.