Dissolved organic carbon (DOC) and select aldehydes in cloud and fog water: the role of the aqueous phase in impacting trace gas budgets

摘要

Cloud and fog droplets efficiently scavenge and process water-solublecompounds and, thus, modify the chemical composition of the gas and particlephases. The concentrations of dissolved organic carbon (DOC) in the aqueousphase reach concentrations on the order of ~ 10 mgC L?1which is typically on the same order of magnitude as the sum of inorganicanions. Aldehydes and carboxylic acids typically comprise a large fractionof DOC because of their high solubility. The dissolution of species in theaqueous phase can lead to (i) the removal of species from the gas phasepreventing their processing by gas phase reactions (e.g., photolysis ofaldehydes) and (ii) the formation of unique products that do not have anyefficient gas phase sources (e.g., dicarboxylic acids).We present measurements of DOC and select aldehydes in fog water at highelevation and intercepted clouds at a biogenically-impacted location(Whistler, Canada) and in fog water in a more polluted area (Davis, CA).Concentrations of formaldehyde, glyoxal and methylglyoxal were in themicromolar range and comprised ≤ 2% each individually of the DOC.Comparison of the DOC and aldehyde concentrations to those at otherlocations shows good agreement and reveals highest levels for both inanthropogenically impacted regions. Based on this overview, we conclude thatthe fraction of organic carbon (dissolved and insoluble inclusions) in theaqueous phase of clouds or fogs, respectively, comprises 2–~ 40%of total organic carbon. Higher values are observed to be associatedwith aged air masses where organics are expected to be more highly oxidisedand, thus, more soluble. Accordingly, the aqueous/gas partitioning ratioexpressed here as an effective Henry's law constant for DOC(K_H*DOC) increases by an order of magnitude from7 × 103 M atm?1 to 7 × 104 M atm?1 during theageing of air masses.The measurements are accompanied by photochemical box model simulations.These simulations are used to contrast two scenarios, i.e., ananthropogenically vs. a more biogenically impacted one as beingrepresentative for Davis and Whistler, respectively. Since the simplicity ofthe box model prevents a fully quantitative prediction of the observedaldehyde concentrations, we rather use the model results to compare trendsin aldehyde partitioning and ratios. They suggest that the scavenging ofaldehydes by the aqueous phase can reduce HO₂ gas phase levelssignificantly by two orders of magnitude due to a weaker net source ofHO₂ production from aldehyde photolysis in the gas phase. Despite thehigh solubility of dicarbonyl compounds (glyoxal, methylglyoxal), theirimpact on the HO₂ budget by scavenging is < 10% of that offormaldehyde. The overview of DOC and aldehyde measurements presented herereveals that clouds and fogs can be efficient sinks for organics, withincreasing importance in aged air masses. Even though aldehydes,specifically formaldehyde, only comprise ~ 1% of DOC, theirscavenging and processing in the aqueous phase might translate intosignificant effects in the oxidation capacity of the atmosphere.