Quantification of Urban Biogenic CO2 Fluxes Requires High Spatial Resolution Mapping of Vegetation Types

As cities set ambitious goals to reduce carbon emissions, there is a demonstrated need to separate out the biogenic CO2 fluxes from anthropogenic CO2 emissions. However, little is known about regional biogenic CO2 fluxes in urban areas because novel urban vegetation (e.g., lawns, trees along street and in parks, etc.) in developed land covers is often omitted in regional models. We set up a high-resolution (30m) biosphere model to estimate the regional biogenic CO2 fluxes in New York City and assess the importance of vegetation within developed land covers. Despite relatively low tree canopy cover in New York City (NYC), the regional biogenic CO2 fluxes are surprisingly large when vegetation within the developed land covers is included. Furthermore, the types of vegetation within the developed land covers are crucially important for estimating regional biogenic CO2 fluxes, demonstrated by a doubling in estimates of total biogenic CO2 flux when this vegetation is assumed to be grassland compared to forest. Using a Lagrangian Atmospheric Transport model driven by our modeled biogenic CO2 fluxes and an anthropogenic emission inventory, we find that the regional biogenic CO2 uptake could offset up to 40% of atmospheric CO2 enhancements attributed to anthropogenic emissions in summer afternoons and completely balance on-road traffic in one of the most congested cities in the US. Accurate characterization of biogenic carbon fluxes in cities are essential to the development of effective atmospheric monitoring tools. Future measurements should focus on constraining CO2 fluxes in urban lawns.