Exploring the potential for spatio-temporal mapping of VOC-OH reactions from column measurements of CH2O and NO2

Abstract:

Reactions of OH with volatile organic compounds (VOC) such as CH₄ and isoprene produce formaldehyde (CH₂O). The concentration of OH and the chemistry of peroxy radicals, a reactive intermediate of VOC + OH reactions, depend strongly on the concentration of NO_x. Here, we investigate the influence of NO_x on the formation of CH₂O in an isoprene-rich atmosphere (Martin Lake Power Plant, NE Texas) and in a “background” atmosphere (Navajo Power Plant, N Arizona) using conceptual models and the WRF-Chem regional chemistry-transport model alongside satellite-based (Aura-OMI) and flight-based (ARCTAS) measurements. In the conceptual model, the enhancement of CH₂O in an NO₂ plume is large and depends on the magnitude of the OH enhancement, the lifetime of the parent VOC, the concentration of intermediate oxidation products, and the impact of NOx on the branching ratios of peroxy radicals. Preliminary analysis of WRF-Chem results supports these findings. For a large point source of NO_x in a low NO_x-background, the enhancement of the CH₂O concentration in the NO_x plume is more than two times that of the surrounding region in both the isoprene-rich and the “background” WRF-Chem simulations. Furthermore, the spatial correlation of OH and CH₂O in these simulated plumes suggests that simultaneous measurement of CH₂O and NO₂ offers the potential to better constrain the processes affecting the reaction of VOC with OH, and thus the factors controlling O₃ production and the NO_x lifetime. The precision of UV/Visible spectrometers planned for future geostationary missions, such as TEMPO, suggest that the routine measurement of these relationships will be possible.