Direct and catalytic contribution of formaldehyde to particulate matter?

Formaldehyde (HCHO) is produced mainly via photochemical oxidation of volatile organic compounds as well as direct emissions mainly from combustion processes. HCHO has a high vapor pressure but as a result of the hydration of the aldehyde group, it has a Henry’s law constant that allows it to partition into cloud droplets. We present results of two different pathways through which HCHO may contribute to the mass of particulate matter: Formation of hydroxymethanesulfonate (HMS) from reaction of HCHO with dissolved sulfur dioxide (SO_2aq) and formation of sulfate by reaction of HCHO with hydrogen peroxide (H₂O₂) to form hydroxyl methyl hydroperoxide (HMHP), which in turn can oxidize SO_2aq to sulfate and reform HCHO. The former pathway contributes to both the carbon and sulfur component of particulate matter whereas the latter contributes to the sulfur particulate budget and suggests a catalytic role of formaldehyde.

We combine laboratory kinetics studies of these reactions with model simulations using GEOS-Chem. The model simulations are analyzed at regional and global scales under present day and simplified preindustrial conditions, in which all anthropogenic emissions are set to zero. The analysis suggests that, depending on conditions, these processes may have significant impact on the sulfur particulate matter budget, specifically the rate of particulate sulfur formation. The results also suggest that under conditions that favor HMS formation, HMS may be the most abundant single organic molecule contributing particulate matter carbon.