A missing sink for gas‐phase glyoxal in Mexico City: Formation of secondary organic aerosol

The sources of secondary organic aerosol (SOA) are highly uncertain. Direct measurements of gas‐phase glyoxal in Mexico City are compared to experimentally constrained model predictions. Observed glyoxal concentrations are found significantly below those predicted. Additional glyoxal sources are likely and would increase these differences; an additional glyoxal sink must be operative. The model‐measurement differences are fully resolved by a sink parameterized from aerosol parameters as either (1) irreversible uptake to aerosol surface area (uptake coefficient γ ≈ 0.0037); reversible partitioning to (2) aerosol liquid water (effective Henry's law coefficient H eff ≈ 4 × 10 9 M atm −1 ), or (3) the oxygenated organic aerosol phase (activity coefficient ζ ≈ 6 × 10 −9 ); (4) a combination of the above. The missing sink has the potential to determine 70–95% of the atmospheric lifetime of glyoxal. The glyoxal imbalance corresponds to several μ g m −3 of equivalent SOA mass, and can explain at least 15% of the SOA formation in Mexico City.