Nitrate Photolysis: An Overlooked Factor in Atmospheric Brown Carbon Photobleaching

Brown carbon (BrC) aerosols significantly contribute to global warming, yet their atmospheric aging remains poorly constrained. BrC photobleaching is considered a key process that reduces its radiative forcing. However, as the mass fraction of nitrate, which is also photochemically active, in aerosols increases, its impact on BrC photobleaching remains highly controversial. In this work, we demonstrated that nitrate photolysis effectively mitigates BrC photobleaching through a previously overlooked compensation mechanism. Combining high-resolution mass spectrometry, multivariate statistics, and quantum chemical calculations, we reveal that nitrate photolysis continuously generates light-absorbing N-containing compounds, predominantly nitrogen-aromatic conjugates, that offset absorption losses from photobleaching. Globally, neglecting this effect leads to a 1.0%-3.7% underestimation of BrC's direct radiative effect, escalating to 2.4%-8.5% in nitrate-rich regions such as East Asia. Our findings establish nitrate photolysis as a crucial moderator of BrC's atmospheric lifetime and warming potency, necessitating a reconsideration of BrC's climatic role in anthropogenically influenced regions.