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 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). <br><br> 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 &le; 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 (<i>K</i>_H*^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. <br><br> 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, specifical...

Determination of aldehydes and acetone in fog water samples via online concentration and HPLC

“…The median concentration of the DOC content of 14.8 mgC·L −1 (average value: 37.2 mgC· L −1 ) was representative of other radiation fog waters Ervens et al 2013;Straub et al 2012). The fog event collected on 12/01/2012 exhibited a high amount of DOC content with 262.2 mgC·L −1 ; however, this value was consistent with previous values of DOC observed from the same sampling site (Raja et al 2005).…”

“…Although the majority of the samples in this study comprised less than 2 % of the total DOC, as typically found in other fog and cloud water analyses (Deguillaume et al 2014;Ervens et al 2013), the DOC values of these particular events were some of the highest measured in this study, with the maximum DOC (262 mg/L) observed on 12/01/2012. By summing up the identified carbon contributions in the study (all aldehydes and ketones), we determined that on 11/28/12, 12/01/2012, and 12/08/2012 acetone comprised 2.5, 14.4, and 22.4 %, respectively, of the total DOC.…”

“…Oyama, Japan (Matsumoto et al 2005), and several sites in California (Ervens et al 2013;Grosjean and Wright 1983;Steinberg and Kaplan 1984) is shown in Online Resource 5. Out of the fog sites compared in this study, the fog water collected in Po Valley, Italy had the highest concentration of aldehydes (Facchini -9 .…”

Structure–activity relationship for the estimation of OH-oxidation rate constants of carbonyl compounds in the aqueous phase

“…In these media, organic compounds are known to undergo oxidation by a number of radicals, among which OH radicals are the most reactive oxidants (Herrmann et al, 2010). This reactivity initiates chain reactions that are related to atmospherically important issues such as the oxidizing capacity of the atmosphere (Monod and Carlier, 1999;Monod et al, 2007;Poulain et al, 2010;Ervens et al, 2013), the fate of organic compounds (Blando and Turpin, 2000;Monod et al, 2005) and the formation of secondary organic aerosol (SOA) (Altieri et al, 2006;Carlton et al, 2007;Volkamer et al, 2009;Ervens and Volkamer, 2010;Tan et al, 2010Tan et al, , 2012Lim et al, 2010;Ervens et al, 2011).…”

Variability of dissolved organic carbon in precipitation during storms at the Shale Hills Critical Zone Observatory

“…Important atmospheric and climatic processes are mediated by organic compounds. Being highly reactive, atmospheric OC affects oxidative and trace element chemistry by changing the lifetimes of radical species (Arakaki et al, ; Ervens et al, ), and by complexing metals (Witt & Jickells, ). The light‐absorbing fractions of OC influence the amount of light reaching the land surface, which is directly related to climate (Kieber, Whitehead, Reid, Willey, & Seaton, ).…”