Dissolved organic carbon (DOC) and select aldehydes in cloud and fog water: the role of the aqueous phase in impacting trace gas budgets 1Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, USA
21 May 2013
2Chemical Sciences Division, NOAA Earth System Research Laboratory, Boulder, Colorado, USA
3Arizona State University, Department of Chemistry and Biochemistry, Arizona, USA
4Air Quality Research Division, Atmospheric Science and Technology Directorate, Science and Technology Branch, Environment Canada, Downsview, Canada
5Cain Department of Chemical Engineering, Louisiana State University, Baton Rouge, LA, USA
Received: 29 Oct 2012 – Published in Atmos. Chem. Phys. Discuss.: 19 Dec 2012 Abstract. Cloud and fog droplets efficiently scavenge and process water-soluble
compounds and, thus, modify the chemical composition of the gas and particle
phases. The concentrations of dissolved organic carbon (DOC) in the aqueous
phase reach concentrations on the order of ~ 10 mgC L−1
which is typically on the same order of magnitude as the sum of inorganic
anions. Aldehydes and carboxylic acids typically comprise a large fraction
of DOC because of their high solubility. The dissolution of species in the
aqueous phase can lead to (i) the removal of species from the gas phase
preventing their processing by gas phase reactions (e.g., photolysis of
aldehydes) and (ii) the formation of unique products that do not have any
efficient gas phase sources (e.g., dicarboxylic acids).
Revised: 09 Apr 2013 – Accepted: 20 Apr 2013 – Published: 21 May 2013
We present measurements of DOC and select aldehydes in fog water at high
elevation 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 the
micromolar range and comprised ≤ 2% each individually of the DOC.
Comparison of the DOC and aldehyde concentrations to those at other
locations shows good agreement and reveals highest levels for both in
anthropogenically impacted regions. Based on this overview, we conclude that
the fraction of organic carbon (dissolved and insoluble inclusions) in the
aqueous phase of clouds or fogs, respectively, comprises 2–~ 40%
of total organic carbon. Higher values are observed to be associated
with aged air masses where organics are expected to be more highly oxidised
and, thus, more soluble. Accordingly, the aqueous/gas partitioning ratio
expressed here as an effective Henry's law constant for DOC
(KH*DOC) increases by an order of magnitude from
7 × 103 M atm−1 to 7 × 104 M atm−1 during the
ageing of air masses.
The measurements are accompanied by photochemical box model simulations.
These simulations are used to contrast two scenarios, i.e., an
anthropogenically vs. a more biogenically impacted one as being
representative for Davis and Whistler, respectively. Since the simplicity of
the box model prevents a fully quantitative prediction of the observed
aldehyde concentrations, we rather use the model results to compare trends
in aldehyde partitioning and ratios. They suggest that the scavenging of
aldehydes by the aqueous phase can reduce HO2 gas phase levels
significantly by two orders of magnitude due to a weaker net source of
HO2 production from aldehyde photolysis in the gas phase. Despite the
high solubility of dicarbonyl compounds (glyoxal, methylglyoxal), their
impact on the HO2 budget by scavenging is < 10% of that of
formaldehyde. The overview of DOC and aldehyde measurements presented here
reveals that clouds and fogs can be efficient sinks for organics, with
increasing importance in aged air masses. Even though aldehydes,
specifically formaldehyde, only comprise ~ 1% of DOC, their
scavenging and processing in the aqueous phase might translate into
significant effects in the oxidation capacity of the atmosphere.
Citation: Ervens, B., Wang, Y., Eagar, J., Leaitch, W. R., Macdonald, A. M., Valsaraj, K. T., and Herckes, P.: Dissolved organic carbon (DOC) and select aldehydes in cloud and fog water: the role of the aqueous phase in impacting trace gas budgets, Atmos. Chem. Phys., 13, 5117-5135, doi:10.5194/acp-13-5117-2013, 2013.