Season and city shape urban bioaerosol composition beyond vegetation and socioeconomic gradients

Urban vegetation varies with socio-economic gradients, as lower-income neighborhoods often host sparser and less diverse green spaces. This disparity may affect respiratory health by influencing exposure to bioaerosols. Understanding the characteristics of this aerobiome could help anticipate risks related to allergies and other respiratory conditions. Here, we hypothesized that urban vegetation cover and socio-economic status shape urban bioaerosol dynamics. We sampled bioaerosols at 65 sites across three Canadian cities of varying population size and density using an active air sampler over four months, and characterized their bacterial, fungal, and plant particles composition using amplicon sequencing. Seasonal alpha diversity varied significantly for fungi and plant particles. Based on beta diversity, sampling period alone explained up to 40% of plant particle, 29% of fungal, and 11% of bacterial bioaerosol composition variation. In contrast, vegetation cover explained only a minor portion of the variance in bioaerosol composition, and median household income, almost none. These findings provide a critical baseline for understanding the urban aerobiome and highlight the need to study how vegetation identity and diversity, rather than cover alone, may shape bioaerosol dynamics in cities. As cities grow and urban greening initiatives expand, demystifying the aerobiome dynamics becomes an urgent public health priority. • Seasonality drives urban bioaerosol composition more than vegetation or income. • Plant particles, fungi, and bacteria show distinct seasonal variation across cities. • Vegetation cover and socio-economic status explain minor bioaerosol variation. • Findings inform urban greening strategies and respiratory health risk assessment. • Provide a baseline for aerobiome studies in global urbanization and climate change