Contribution of bacteria-like particles to PM2.5 aerosol in urban and rural environments

We report highly time-resolved estimates of airborne bacteria-like particle concentrations in ambient aerosol using an Aerodyne aerosol mass spectrometer (AMS). AMS measurements with a newly developed PM 2.5 and the standard (PM 1 ) aerodynamic lens were performed at an urban background site (Zurich) and at a rural site (Payerne) in Switzerland. Positive matrix factorization using the multilinear engine (ME-2) implementation was used to estimate the contribution of bacteria-like particles to non-refractory organic aerosol. The success of the method was evaluated by a size-resolved analysis of the organic mass and the analysis of single particle mass spectra, which were detected with a light scattering system integrated into the AMS. Use of the PM 2.5 aerodynamic lens increased measured bacteria-like concentrations, supporting the analysis method. However, at all sites, the low concentrations of this component suggest that airborne bacteria constitute a minor fraction of non-refractory PM 2.5 organic aerosol mass. Estimated average mass concentrations were below 0.1 μg/m 3 and relative contributions were lower than 2% at both sites. During rainfall periods, concentrations of the bacteria-like component increased considerably reaching a short-time maximum of approximately 2 μg/m 3 at the Payerne site in summer.