CH₄ Boosts the Atmospheric Nitrate-to-HONO Formation by Photoinduced ^•CH₃ Radicals

The impact of rising CH₄ concentrations has become a key global focus in air pollution and climate change, as its photoconversion generates highly reactive methyl radicals with translational potential. However, the overlooked role of ^•CH₃ radicals on HONO formation has led to the underestimated contribution of greenhouse gases on atmospheric oxidative capacity. Here, we propose a CH₄-promoted nitrate photolysis route where the nitrate-to-HONO is significantly promoted, varying from 46.98% to 95.75% as the relative humidity increased. In situ spectroscopic and theoretical analyses reveal that the ^•CH₃ radicals activate the NO₃^- molecule to facilitate N-O bond cleavage and promote H₂O dissociation to supply hydrogen atoms for HONO formation, with pivotal ^•OH radicals from H₂O further enhancing this effect under high RH conditions. We also roughly estimated the enhancement of the atmospheric ^•OH radical budget by CH₄ and confirmed its environmental significance through demonstrations on the surfaces of atmospheric fine particulate matter and industrial photoactive materials. This work underscores the interactive relationship between CH₄ and atmospheric oxidative capacity and provides direct evidence for the observed enhancement of atmospheric oxidative capacity contributed by greenhouse gases.