How Airborne Formaldehyde Studies Contribute to an Improved Understanding of Atmospheric Chemical Processing and Transformations

Alan Fried

NCAR/EOL

     The chemistry and distribution of the reactive trace gas formaldehyde (CH₂O) has been of interest to atmospheric scientists for nearly three decades. This gas is emitted directly into the atmosphere by incomplete fossil fuel combustion as well as from secondary processes involving hydrocarbon oxidation. As the oxidation of most hydrocarbons produce CH₂O as an intermediate, this gas represents an important test species in evaluating our understanding of the mechanistic details of tropospheric photochemistry. Under remote background conditions, such as in or over the marine boundary layer, measurement-model comparisons of CH₂O have resulted in large discrepancies in many past studies, revealing large gaps in our knowledge of CH₂O sources and sinks as well as persistent questions about measurement accuracy.  Prior to the present studies, such discrepancies have raised questions regarding our fundamental knowledge of hydrocarbon oxidation mechanisms, even in such simplified background atmospheric regimes where only methane chemistry is involved. This has even led to speculation regarding the need for additional oxidation mechanisms. 

     In the mid to upper troposphere, decomposition of CH₂O is an important source of reactive hydrogen radicals (HO₂) where other radical production sources diminish in importance. Since HO₂ reactions produce ozone in this region, understanding the concentrations of trace gases like CH₂O and the peroxides in the upper troposphere is essential for a comprehensive knowledge of upper tropospheric ozone production mechanisms. Like measurement-model comparisons of HO₂ in the upper atmosphere, model studies of trace gases like CH₂O have in past studies underpredicted observations by factors of two or more, particularly during convection. Large uncertainties in the degree to which precipitation scavenging of soluble gases like CH₂O takes place during convection further complicates this situation. Hence, there are major gaps in our knowledge of ozone production in the upper troposphere, in part reflecting a paucity of high quality measurements of precursors for ozone and HO₂ like CH₂O.

     To address these and other important scientific questions, NCAR scientists have embarked on a long term effort spanning more than 15 years in measuring CH₂O throughout the atmosphere on a variety of airborne platforms with both high measurement sensitivity and accuracy. The present talk will highlight results from a select set of recent campaigns, emphasizing how the resulting measurement-model analyses are contributing to an improved understanding of hydrocarbon oxidation mechanisms. This talk will also highlight present gaps in our understanding of such processes and discusses a proposed future study that will help to address these issues.