Online determination of ammonium in PM2.5 aerosol by a continual aerosol sampler and parallel size-segregated determination of ammonium and anions using a cascade impactor

Ammonium in urban fine aerosol (Particulate Matter, [PM2.5]) was analysed in parallel using two different methods. The first method, a continuous Condensation-Growth Unit − Aerosol Counterflow Two-Jets Unit sampler combined with a fluorescence detector (FLD), allowed the ultrasensitive (limit of detection = 1.04 × 10–9 µg L, 3 S/N) and fast (1 s time resolution) online determination of NH4+, while the second method was based on a sampling of aerosols on a high-flow cascade impactor (HFI) with 7 size fractions. The aerosols collected by the HFI were analysed offline for ammonium using the FLD and for seven water‐soluble anions using ion chromatography. Comparison of the results from both methods allowed quantification of NH4+ losses of semi-volatile NH4+ salts in aerosols collected on the HFI. The measurement of mass size distribution was used to distinguish the sources of ions in the fine PM. Nitrate and sulphate were the most abundant anions, accounting for 80.4% and 76.1% of total ion concentration and 7.93% and 10.2% of aerosol mass during summer and winter campaigns, respectively. Sampling of atmospheric aerosol during 2-week campaigns in summer 2021 and winter 2022 provided seasonal variations of analysed ions in urban PM2.5 aerosols in Brno, Czech Republic.