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

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 mg C L<sup>−1</sup> 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). <br><br> We present measurements of DOC and select aldehydes in fog water at high elevation and intercepted clouds in 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 comprises 1–~40% of total organic carbon. Higher values are observed to be associated with aged air masses where organics are expected to be more highly oxidized and thus more soluble. Accordingly, the aqueous/gas partitioning ratio expressed here as an effective Henry's law constant for DOC (<i>K</i><sub>H</sub><sup>*DOC</sup>) increases by an order of magnitude from 7×10<sup>3</sup> M atm<sup>−1</sup> to 7×10<sup>4</sup> M atm<sup>−1</sup> during the ageing of air masses. <br><br> The measurements are accompanied by photochemical box model simulations. They suggest that the scavenging of aldehydes by the aqueous phase can reduce HO<sub>2</sub> gas phase levels by two orders of magnitude due to a weaker net source of HO<sub>2</sub> production from aldehyde photolysis in the gas phase. Despite the high solubility of dialdehydes (glyoxal, methylglyoxal), their impact on the HO<sub>2</sub> 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 on the oxidation capacity of the atmosphere

Radiation fog chemical composition and its temporal trend over an eight year period

“…Recent examples of field-based radiation fog research include the documentation of fog composition and identification of pollutant sources Błas et al, 2010;Raja et al, 2008), the effect of droplet size on composition and the production of inorganic aerosol mass (Fahey et al, 2005;Moore et al, 2004a;2004b;Reilly et al, 2001), the removal of trace species from the atmosphere Burkard et al, 2003;Collett et al, 2001), scavenging efficiencies , organic nitrogen speciation and processing Zang and Anastasio, 2003a;2003b;McGregor and Anastasio, 2001), organic carbon speciation and processing Ervens et al, 2013;Ehrenhauser et al, 2012;Raja et al, 2009;Collett et al, 2008;Fuzzi et al, 2002), and secondary particle formation (Kaul et al, 2011;Dall'Osto et al, 2009). Nearly all radiation fog field campaigns are short term, intensive studies with samples obtained during a limited number of events.…”

Short Sampling Periods: A New Setup to Evaluate the Change in the Chemical Composition of Fog at High Time Resolution

“…The Caltech design (initially developed at the California Institute of Technology) includes a fog water collection unit with a mesh or strings, and a fan unit which moves the air through the collection unit. This design has been used since the 1990s (Collett, Oberholzer and Staehelin, 1993;Demoz, Collett and Daube, 1996), with some slight design modifications, such as samplers for organic materials that are made of stainless steel (Collett et al, 2002;2008;Herckes, Leenheer and Collett, 2007;Ervens et al, 2013;Kim et al, 2019), and others for inorganic materials made of PEEK and Teflon (Degefie et al, 2015;Simon et al, 2016;Nieberding et al, 2018). Some even include multi-stage designs for droplet size resolved sampling (Moore et al, 2002;Straub and Collett, 2002).…”

A rapid scan vacuum FTIR method for determining diffusion coefficients in viscous and glassy aerosol particles

“…20,21 The organic-rich phase of atmospheric aerosols can be classified as a liquid ( < 10 2 Pa·s; where is the dynamic viscosity), a semi-solid (10 2 Pa·s < <10 12 Pa·s) or a solid ( > 10 12 Pa·s), depending on ambient conditions and chemical composition. [22][23][24][25] Indeed, Virtanen et al showed that secondary organic aerosol (SOA) could exist in a glassy phase, while other studies suggest that aerosol particles could be characterised as gels or rubbers.…”

“…[8][9][10] Hygroscopicity is an important property of aerosols, impacting on aerosol particle size and composition, influencing aerosol phase chemical reactions, and affecting optical properties. 11,12 Many thermodynamic models have been developed to estimate the equilibrium partitioning of water and semi-volatile organic components between the gas and condensed phases in atmospheric aerosols. [13][14][15][16][17][18] However, an assumption that thermodynamic equilibrium is always achieved may not be correct when predicting the properties of viscous or amorphous aerosols due to slow mass transport rates arising from slow bulk-phase diffusion.…”