Convective storms alter bioaerosol populations through cold pools and precipitation.

Meteorology can alter bioaerosol properties, potentially enhancing their impact on public health and cloud microphysics. The BioAerosols and Convective Storms (BACS) study was conducted over May-June 2022 and 2023 in Northern Colorado and examines how convective storm processes such as precipitation and cold pools affect bioaerosol concentrations and properties, including pollen, fungal spores and bacterial endotoxin. The two seasons were vastly different climatologically, with drought-like conditions and greater endotoxin concentrations during 2022 and near record rainfall with higher fungal spore concentrations during 2023. Online (fluorescence) and offline (chemical tracer) measurements were used to characterize bioaerosols, alongside collocated measurements of ice-nucleating particles (INPs). Precipitation events generally increased supermicron fluorescent particle concentrations which consisted primarily of fungal spores, as determined from fungal spore counts, chemical tracers, and fluorescent particle types. Storm-generated cold pools had more varied impacts on bioaerosols, sometimes causing depletion and other times enrichment, with peak fluorescent particle concentrations correlating significantly with cold pool strength (r s = 0.79, p < 0.05, n = 12), indicating that stronger cold pools produce greater increases in local bioaerosol concentrations. Biological INP concentrations in air active at warmer than -15 °C from 1-10 µm in size were enhanced by roughly one order of magnitude in samples collected during convective storms compared to pre-rain samples. Contributions of fungal spores to the enhanced INPs were supported by a significant correlation between large (2.5-10 µm) heat-labile INP concentrations active at -15 °C with mannitol, a fungal spore tracer (r = 0.91, n = 8, p < 0.01). This study found convective storms can greatly increase boundary-layer concentrations of fungal spores and warm-temperature biological INPs, leading to high exposure risks for sensitized populations and the potential for bioaerosols to influence cloud processes.