Surface temperatures drive strong seasonality in urban reactive carbon emissions

Urban air quality is affected by a complex mix of volatile organic compound (VOC) sources, including fossil-fuel combustion, volatile chemical products (VCPs), cooking, and vegetation. Prior studies have identified gaps in emissions inventories and a need to better understand the seasonal mechanisms controlling these sources. Here, we combine high-resolution proton-transfer reaction mass spectrometry (PTRMS) with the eddy covariance method to directly quantify VOC fluxes at an urban/suburban site in New York during summer and winter. The emissions are strongly seasonal: over twice as many individual VOCs undergo surface-atmosphere exchange during summer, and the resulting mass-based and OH reactivity-weighted fluxes are 2-3.5x higher at this time. We find that temperature-dependent processes predominate during summer, with VCPs accounting for ~50% of the emitted VOC-C mass flux and ~30% of the emitted OH reactivity. Ethanol alone accounts for ~25% of the total mass fluxes. Biogenic and residential sources are also substantial, contributing 28% of the emitted OH reactivity. During winter, temperature-dependent emissions are reduced and traffic becomes the largest VOC source. An updated inventory agrees with summer observations to within 25%, but overestimates winter fluxes by >2×. The winter discrepancy arises from overestimated VCP and cooking emissions and from missing temperature-dependent volatilization in the inventory framework. Results highlight the need to account for seasonal and temperature-dependent urban VOC emissions to support air quality and mitigation assessment in the context of global change.