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).

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 (K_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.

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...

Dust storms known as ‗haboobs' occur in Tempe, AZ during the North American monsoon season. This work presents a catalog of haboob occurrence over the time period 2005 to 2014. A classification method based on meteorological and air quality measurements is described. The major factors that distinguish haboobs events from other dust events and from background conditions are event minimum visibility, maximum wind or gust speed, and maximum PM 10 (particulate matter with aerodynamic diameters of 10 µm or less) concentration. We identified from 3 to 20 haboob events per year over the period from 2005 to 2014. The calculated annual TSP (total suspended particulate) dry deposition ranged from a low of 259 kg ha-1 in 2010 to a high of 2950 kg ha-1 in 2011 with a mean of 950 kg ha-1 yr-1. The deposition of large particles (PM >10) is greater than the deposition of PM 10. The TSP dry deposition during haboobs is estimated to contribute 74% of the total particulate mass deposited in Tempe.

The importance of the atmospheric aqueous phase of fogs and clouds, for the processing and removal of polycyclic aromatic hydrocarbons (PAHs) is not well known. A multiphase model was developed to determine the fate and lifetime of PAHs in fogs and clouds for a limited set of daytime conditions. The model describes partitioning between three phases (aqueous, liquid organic, and gas), experimental and estimated (photo)oxidation rates. Using a limited set of microphysical and chemical input conditions, the loss rates of PAHs in the complex threephase system are explored. At 25°C, PAHs with two, three and four rings were predicted to be primarily in the gas phase (fraction in the gas phase x g > 90%) while five-and six-ring PAHs partitioned significantly into droplets with aqueous phase fractions of 1 to 6% and liquid organic phase fractions of 31 to 91%, respectively. The predicted atmospheric chemical lifetimes of PAHs in the presence of fog or cloud droplets (< 8 hours) were significantly shorter than literature predictions of PAH lifetimes due to wet and dry deposition (1 to 14 days and 5 to 15 months, respectively) and shorter than or equal to predicted lifetimes due to chemical reactions in the gas and organic particulate phases (1 to 300 hours). Even though PAH solubilities are  4 ×10-2 g L-1 , the results of the current study show that often the condensed phase of fog and cloud droplets cannot be neglected as a PAH sink. When fogs and clouds are present, PAHs are found in the aqueous phase (e.g., BAP: ×10-7 g L-1 ; Herckes et al., 2002) despite the low water solubilities of PAHs (1.4 ×10-7 to 3.2 ×10-2 g L-1 ; Pearlman et al., 1984). Capel et al. (1991) proposed three mechanisms to account for PAH presence in fog water: dissolved organic compounds that act as co-solvents for PAHs, organic compounds acting as surfactants at the droplet surface or in colloids, and PAHs bound to scavenged particles. It was found that filtering fog water isolated most PAHs (Leuenberger et al., 1988) which lead Capel et al. (1991) to conclude that scavenged particles were the largest reservoir of PAHs in fog. Nonetheless, Capel et al. (1990) found that the surface tension in fog water is lower than in pure water, which is a manifestation of surfactant (surface film) and/or cosolvent behavior. Valsaraj (2004; 2009) demonstrated PAH adsorption to water surfaces and to surfactant-like organic matter on water surfaces (Donaldson and Valsaraj, 2010; Chen et al., 2011). There have been few atmospheric PAH multiphase (i.e., more than two phases) studies. Lei and Wania (2004) employed partitioning ratios to predict PAH distribution in clouds. Ehrenhauser et al. (2012) compared the observed PAH distribution in fog with simple predictions using partitioning ratios. While the phase distribution and reactions within a three-phase system are not well known for PAHs, partitioning ratios have been measured and estimated for octanol

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^&minus;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 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 &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 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>_H^*DOC) increases by an order of magnitude from 7&times;10³ M atm^&minus;1 to 7&times;10⁴ M atm^&minus;1 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 HO₂ gas phase levels 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 dialdehydes (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 on the oxidation capacity of the atmosphere