Linking emission sources to secondary carbon monoxide formation: A chemical reaction chain sensitivity-based analysis

Secondary carbon monoxide (CO) formation is a poorly constrained yet potentially critical component of urban atmospheric oxidation, integrating the effects of emission sources and multigenerational chemical oxidation. Here, we developed a quantitative analysis framework to investigate the complete linkage from emissions through chemical processing to secondary CO formation. Simulations revealed distinct diurnal variability, with pronounced daytime formation (0.05-4.4 ppb h^-1) and negligible production at night (<0.01 ppb h^-1), highlighting the dominant role of photochemical oxidation. Pathway analysis identified formaldehyde as the central intermediate for daytime CO formation. Its photolysis and reaction with OH radical contributed to more than half (51.1%) of total daytime production. Species-based relative incremental reactivity (RIR) analysis consistently identified isoprene was the main precursor species for secondary CO. Source-resolved RIR results revealed biogenic emissions dominated daytime CO formation sensitivity (73.1%). These findings demonstrate that secondary CO formation is governed by dynamic source-chemistry interactions that vary with oxidation regimes. They provide new constraints for CO source apportionment and mechanistic insights into secondary CO formation.