The contribution of VOCs to formaldehyde and its role in radical formation in an urban area

Formaldehyde (HCHO) is a key oxygenated volatile organic compound (VOC) influencing radical levels and ozone (O3) formation in the urban atmosphere. Airborne measurements from the Texas Air Quality Study (TexAQS) in 2000 and 2006 have shown that highly reactive VOCs (HRVOCs) emissions and consequently O3 concentrations significantly declined significantly between the two periods. High concentrations (maximal: 40 ppb) of HCHO were formed as an intermediate product of HRVOCs oxidation in the Houston Ship Channel. This study is focused on the change of HCHO as an important reaction intermediate product due to the HRVOC emission reductions and its impact on radical formation in Houston area. The relative contribution of various VOCs (HRVOC, other anthropogenic VOCs, isoprene, and methane) to HCHO formation is quantified by a chemical method assuming that HCHO is formed as a direct (first-generation) product of VOC oxidation by OH. We evaluate the change of contributions of these VOCs to HCHO from 2000 to 2006. Changes of HCHO flux, indicative of the secondary formation of HCHO, as industrial plumes transported downwind in the Houston Ship Channel are computed with the observed HCHO concentration and wind measurement. By applying an observation-constrained box model (Dynamically Simple Model of Atmospheric Chemical Complexity), we evaluate the contribution of HCHO to radicals during high O3 episodes and assess the change of its contribution to radicals after HRVOC reductions.