Surface temperatures drive strong seasonality in urban reactive carbon emissions.

Urban air quality is affected by diverse volatile organic compounds (VOCs), and prior studies reveal a need to understand their seasonal sources. We combine high-resolution mass spectrometry with eddy covariance to quantify VOC fluxes from an urban/suburban New York site during summer and winter. Emissions are strongly seasonal: During summer, twice as many VOCs undergo surface-atmosphere exchange, and molar and reactivity-based fluxes are 2 to 4× higher than in winter. Temperature-dependent emissions from volatile chemical products (VCPs), vegetation, and residences dominate during summer. Ethanol alone accounts for ∼25% of the total flux. During winter, temperature-dependent emissions are reduced and traffic sources dominate. An updated inventory agrees with summer observations within 40% but overestimates winter fluxes by >2.5×. The winter discrepancy reflects overestimated VCP/cooking emissions and missing temperature-dependent volatilization. Results highlight the need to account for seasonal and temperature-dependent urban emissions to support pollution and mitigation assessment in the context of global change.