Biological Ice-Nucleating Particles Deposited Year-Round in Subtropical Precipitation

Abstract

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−, SO42−, and NO3−), 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.

Scavenging of Sub-Micron to Micron-Sized Microbial Aerosols during Simulated Rainfall

“…In addition to improving understanding of bioaerosol removal by descending rain drops in the atmosphere, information about the scavenging process can be used to assess the fraction of particles in rain that originated from the source cloud. We used data on microbial cell concentrations from Louisiana rain [30] and the scavenging behaviors documented in this study to infer that the majority (55% to 73%) of 1 µm microbes in low to moderate rainfall intensity events originated from within the cloud (Table 2). When compared to low intensity rainfall, moderately intense rainfall produces larger drop sizes, which tend to be more efficient at scavenging below-cloud aerosols (Figure 4).…”

“…Various studies have leveraged the scavenging gap in attempts to estimate the concentration of particles and microbial cells in cloud water based on precipitation data [19,30,46]. If, however, the contribution of below-cloud scavenging is much larger than currently appreciated, then the cloud contribution of particles and cells will be overestimated [19,46] and assumptions inherent to aerosol transport models [14,47] may be flawed.…”

“…To assess the contribution of scavenging to the microbes deposited in rain at the surface, the fraction of cells scavenged and that originated from the precipitating cloud was estimated using select events from a two-year record of rainfall data from Baton Rouge, Louisiana [30]. Following Marshall and Palmer's [31] drop-size distributions derived from rain rates, 13 rain events were chosen for the scavenging calculations, representing end members of rain rates (mean 2.8 mm h −1 , n = 7; and 14.5 mm h −1 , n = 6) that would be expected to contain drop sizes relevant to this study (average equivalent diameter of 3.0 and 3.6 mm).…”

“…Following Marshall and Palmer's [31] drop-size distributions derived from rain rates, 13 rain events were chosen for the scavenging calculations, representing end members of rain rates (mean 2.8 mm h −1 , n = 7; and 14.5 mm h −1 , n = 6) that would be expected to contain drop sizes relevant to this study (average equivalent diameter of 3.0 and 3.6 mm). The cloud-base heights for both subsets of storms ranged from 100 to 1160 m above ground level, and the cell concentrations in the rain samples ranged from 8.45 × 10 4 to 1.3 × 10 6 cells L −1 [30]. Using Equation 2, the number of 1 µm bioaerosols (in cells m −3 ) for each rain event was estimated based on the 24-h average PM 10 records for Baton Rouge (US Environmental Protection Agency daily records, https://www.epa.gov/outdoor-air-quality-data).…”

“…The efficiency at which 3.6 mm drops scavenged particles in the 0.4 to 2 µm size range was~twofold higher than the 3.0 mm drops. To explore the contribution of scavenged bioaerosols to cell concentrations in precipitation, we used select events from a published precipitation dataset collected in Louisiana [30]. We assumed the majority of bacterial-sized aerosols were 1 µ m in diameter and used the corresponding modelled scavenging efficiencies for 3.0-and 3.6-mm drops to infer source contributions from scavenging versus the precipitating cloud ( Table 2).…”