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Airborne bacteria that nucleate ice at relatively warm temperatures (>−10°C) can interact with cloud water droplets, affecting the formation of ice in clouds and the residency time of the cells in the atmosphere. We sampled 65 precipitation events in southeastern Louisiana over 2 years to examine the effect of season, meteorological conditions, storm type, and ecoregion source on the concentration and type of ice-nucleating particles (INPs) deposited. INPs sensitive to heat treatment were inferred to be biological in origin, and the highest concentrations of biological INPs (∼16,000 INPs liter(−1) active at ≥–10°C) were observed in snow and sleet samples from wintertime nimbostratus clouds with cloud top temperatures as warm as –7°C. Statistical analysis revealed three temperature classes of biological INPs (INPs active from −5 to –10°C, −11 to –12°C, and −13 to –14°C) and one temperature class of INPs that were sensitive to lysozyme (i.e., bacterial INPs, active from −5 to –10°C). Significant correlations between the INP data and abundances of taxa in the Bacteroidetes, Firmicutes, and unclassified bacterial divisions implied that certain members of these phyla may possess the ice nucleation phenotype. The interrelation between the INP classes and fluorescent dissolved organic matter, major ion concentrations (Na(+), Cl(−), SO(4)(2−), and NO(3)(−)), and backward air mass trajectories indicated that the highest concentrations of INPs were sourced from high-latitude North American and Asian continental environments, whereas the lowest values were observed when air was sourced from marine ecoregions. The intra- and extracontinental regions identified as sources of biological INPs in precipitation deposited in the southeastern United States suggests that these bioaerosols can disperse and affect meteorological conditions thousands of kilometers from their terrestrial points of origin. IMPORTANCE The particles most effective at inducing the freezing of water in the atmosphere are microbiological in origin; however, information on the species harboring this phenotype, their environmental distribution, and ecological sources are very limited. Analysis of precipitation collected over 2 years in Louisiana showed that INPs active at the warmest temperatures were sourced from terrestrial ecosystems and displayed behaviors that implicated specific bacterial taxa as the source of the ice nucleation activity. The abundance of biological INPs was highest in precipitation from winter storms and implied that their in-cloud concentrations were sufficient to affect the formation of ice and precipitation in nimbostratus clouds.

Aldehydes and ketones, ubiquitous air, cloud, and fog water pollutants, are precursors to secondary organic aerosol formation and photochemical smog. Traditional aldehyde and ketone determination methods involve the addition of 2,4-dinitrophenylhydrazine (DNPH) as a derivatization agent, but many require a large sample volume or a lengthy extraction/concentration process. For fog water, where the sample size is inherently small, a DNPH derivatization method, based on U.S. EPA Method 8315A, was developed to combat this issue. In this method, a manual injection online concentration system in conjunction with HPLC was used, eliminating all liquid-liquid extraction and concentration steps and reducing the required sample volume. Hence, concentration and separation were combined in a single step. Using this injection method shortened the procedure time and also lowered the limit of detection to the nanomolar range. In this study, fourteen fog water samples, collected from October 2012 through April 2014 in Baton Rouge, LA, were analyzed for the concentration of aldehydes and ketones in order to test the feasibility of this method. Dissolved organic content (DOC), ionic concentration, and pH were measured. Formaldehyde, acetaldehyde, acrolein, butyraldehyde, benzaldehyde, and acetone were quantified. The DOC of the collected fog samples varied between 6.2 and 262.2 mgC/L. The wide range of organic content in the fog water samples corresponds to a diverse sample set, highlighted by the large variation of observed acetone concentration (under 5 nM to 1.05 mM). However, formaldehyde had a relatively stable concentration between each event (0.5 to 4.5 μM).

Atmospheric aerosols contain a significant quantity of water in the form of both bulk water and thin water films. These provide very high surface areas and adsorptive surfaces for organic compounds of low solubility and low vapor pressures. They also provide highly active sites for oxidation reactions with hydroxyl, singlet oxygen, and ozone species in the gaseous atmosphere. Thus, organic compounds are transformed readily by the reaction at the air–water interface of fog/cloud droplets and in the thin water film in atmospheric aerosols. Mathematical models suggest that the reactions in fog and cloud droplets are not limited by gas phase diffusion or mass accommodation at the surface; rather, they primarily depend on the partition constant and reaction rates at the surface. Laboratory data on both the latter parameters are available in the literature for a variety of organic compounds and they are also supported by field data. Considerable data already exist on the aqueous processing of organic species that are highly water soluble in atmospheric aerosols. However, our works suggest that aqueous processing of organic chemicals (possessing low solubility and low vapor pressure) at the interface should also be considered in fate models for secondary organic aerosols in the atmosphere.