Abstract
Abstract. Formaldehyde (HCHO) is a radical source that plays an important role in urban atmospheric chemistry and ozone formation. The Mexico City Metropolitan Area (MCMA) is characterized by high anthropogenic emissions of HCHO (primary HCHO), which together with photochemical production of HCHO from hydrocarbon oxidation (secondary HCHO), lead to high ambient HCHO levels. The CAMx chemical transport model was employed to evaluate the impact of primary HCHO on its ambient concentration, on the ROx radical budget, and on ozone (O3) formation in the MCMA. Important radical sources, including HCHO, HONO, and O3-olefin reactions, were constrained by measurements from routine observations of the local ambient air monitoring network and the MCMA-2003 field campaign. Primary HCHO was found not only to contribute significantly to the ambient HCHO concentration, but also to enhance the radical budget and O3 production in the urban atmosphere of the MCMA. Overall in the urban area, total daytime radical production is enhanced by up to 10% and peak O3 concentration by up to 8%; moreover primary HCHO tends to make O3 both production rates and ambient concentration peak half an hour earlier. While primary HCHO contributes predominantly to the ambient HCHO concentration between nighttime and morning rush hours, significant influence on the radical budget and O3 production starts early in the morning, peaks at mid-morning and is sustained until early afternoon.
Highlights
Formaldehyde (HCHO) is the most abundant carbonyl compound in both urban areas and the remote troposphere (Lurmann and Main, 1992; Ho et al, 2002; Hellen et al, 2004)
The radical formation from the net HONO source (HONO photolysis minus OH+NO−→HONO) decreases by 5%. This decrease is due to the larger magnitude of the OH+NO−→HONO reaction over the photolysis in the early morning (07:00–09:00 a.m.) when primary HCHO is included, which leads to a marginal increase in HONO concentration, consistent with the HONO source apportionment during morning hours reported by Volkamer et al (2007)
Our preliminary CTM simulation results using the rate constant reported by Li et al (2008), which are in preparation for publication, show that the NO∗2+H2O radical source is negligible to O3 production in the polluted atmosphere over the Mexico City Metropolitan Area (MCMA) where photochemistry is characterized by complicated radical sources
Summary
Formaldehyde (HCHO) is the most abundant carbonyl compound in both urban areas and the remote troposphere (Lurmann and Main, 1992; Ho et al, 2002; Hellen et al, 2004) It can be directly emitted from incomplete combustion processes (primary HCHO), such as combustion engines and biomass burning, or produced by photooxidation of hydrocarbons (secondary HCHO). A detailed analysis of radical sources in the MCMA is presented by Volkamer et al (2007) using a box model featuring the Master Chemical Mechanism constrained by an extensive array of measurements. The objective is to examine the influence of primary HCHO on the odd hydrogen radical budget and O3 formation in the MCMA’s atmosphere
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