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Sensitivity of Ambient Atmospheric Formaldehyde and Ozone to Precursor Species and Source Types Across the United States
Luecken, D., S. Napelenok, M. Strum, R. Scheffe, AND S. Phillips. Sensitivity of Ambient Atmospheric Formaldehyde and Ozone to Precursor Species and Source Types Across the United States. ENVIRONMENTAL SCIENCE & TECHNOLOGY. American Chemical Society, Washington, DC, 52(8):4668–4675, (2018).
Formaldehyde (HCHO) is an important air pollutant from both an atmospheric chemistry and human health standpoint. This study uses an instrumented photochemical Air Quality Model, CMAQ-DDM, to identify the sensitivity of HCHO concentrations across the United States (U.S.) to major source types and hydrocarbon speciation. In July, biogenic sources of hydrocarbons contribute the most (92% of total hydrocarbon sensitivity), split between isoprene and other alkenes. Among anthropogenic sources, mobile sources of hydrocarbons and nitrogen oxides (NOx) dominate. In January, HCHO is more sensitive to anthropogenic hydrocarbons than biogenic sources, especially mobile sources and residential wood combustion (36% of national hydrocarbon sensitivity). While ozone (O3) is three times more sensitive to NOx than hydrocarbons across most areas of the U.S., HCHO is six times more sensitive to hydrocarbons than NOx, largely due to sensitivity to biogenic precursors and the importance of low-NOx chemistry. In winter, both HCHO and O3 show negative sensitivity to NOx (increases with the removal of NOx), although O3 increases are larger. Relative sensitivities do not change substantially across different regions of the country.
Our results showing that HCHO has much smaller sensitivity to NOx than VOCs indicates the importance of low NOx chemistry on HAP formation. As condensed chemical mechanisms, such as CB05 and updated versions, are used to develop emission control strategies, these sensitivities must be confirmed so that impacts of NOx changes are neither over- nor under-estimated. Multipollutant control strategies aim to develop emission reductions targeted at one important compound but with co-benefits for reducing other harmful pollutants. While both O3 and HCHO are formed under similar conditions (daylight, high actinic flux, high VOC and NOx), they show strikingly different sensitivities, although directionally similar. Strategies to reduce O3 will not necessarily be optimal for HCHO, although they will have some positive impact. The sources with the most similar relative contribution to anthropogenic sensitivity are mobile NOx in July and mobile VOC in both months. Uncertainties in mechanism condensation can cause uncertainties in HCHO concentrations and predictions of multipollutant behavior. AQMs have difficulties in reproducing point measurements of explicit VOC compounds, partially due to the condensations necessary to represent numerous VOCs and the limited temporal and spatial extent of observations. New techniques for chemical mechanism creation and condensation should consider a more multi-chemical perspective.40 Because of the importance of HCHO as both a component of atmospheric chemistry and a HAP, acquiring a better understanding of both emissions and chemistry, including how to adequately represent HCHO formation in a condensed format, is essential.
Record Details:Record Type: DOCUMENT (JOURNAL/PEER REVIEWED JOURNAL)
Organization:U.S. ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF RESEARCH AND DEVELOPMENT
NATIONAL EXPOSURE RESEARCH LABORATORY
COMPUTATIONAL EXPOSURE DIVISION
ATMOSPHERIC MODEL DEVELOPMENT BRANCH