Rethinking In-Cloud Aqueous-Phase Nitrogen Chemistry: High-Molecular-Weight Carbonyls as Major Counterparts to Low-Molecular-Weight Dicarbonyls

Aqueous-phase reactions between carbonyls and reactive nitrogen species (e.g., ammonia/amine) are critical for forming secondary nitrogen-containing organic compounds (NOCs) and brown carbon (BrC). However, current atmospheric chemical paradigms center almost exclusively on low-molecular-weight carbonyls such as glyoxal, leaving the role of high-molecular-weight carbonyls (HMWCs, with 4 ≤ C ≤ 30) unexplored. Here, we utilize advanced nontargeted molecular fingerprinting (PFBHA derivatization and FT-ICR MS) to unveil an unexpected and highly diverse pool of 1,242 unique carbonyl formulas in cloud water. These HMWCs, predominantly distributed in the C8–C20 range, exhibit a disproportionately high signal intensity (up to ∼35%) despite representing only a small fraction (5.4–13.9%) of the total signal diversity. Using a mechanistic “precursor-product pair” framework, we demonstrate that these HMWCs contribute to nearly one-third of the total NOC diversity (including various light-absorbing N-heterocycles) via Maillard-like reactions. This contribution is strikingly comparable to that of well-studied low-molecular-weight dicarbonyls, i.e., glyoxal and methylglyoxal. We further show that this HMWC-driven pathway covaries with precursor availability and cloud water pH. These findings suggest that incorporating HMWC-mediated NOC formation into future atmospheric models may improve the representation of secondary BrC formation and its associated climate effects.