Sampling of Airborne Particles

A two-stage stacked filter unit (SFU) sampler [32] that operated at a flow rate of 17 standard liters per min (slpm) was employed for the collection of airborne particles. The sampling times for the samplers varied between 2 and 3 days, resulting in an average air sampling volume of 57 m³. In total, 40 size-resolved samples were collected. Blank samples were obtained by having no air drawn through the SFU during the length of the sampling period.

The SFU contained a coarse (8 μm pore size) filter and a fine (0.4 μm pore size) Whatman® Nuclepore™ polycarbonate filter, respectively. The filters were bought from Merck Millipore Company (Billerica, MA, USA). The coarse filter had a 50% collection efficiency at 2-μm equivalent aerodynamic diameter, EAD [33], and thus collected particles within the size range 2–10 μm EAD, whereas the fine filter collected particles < 2 μm EAD. The SFU was operated downstream of a cyclone inlet at ambient conditions, which removed particles (and cloud elements) above 10 μm EAD. In order to minimize contamination of the filter substrates (ambient and blank), the samplers were changed in a glove box (free from particles, sulfur dioxide, and ammonia) both before and after sampling. Further, to avoid high chemical blank values, all filter substrates were cleaned by ethanol and ultrapure water and then dried before use. All filter substrates were stored at – 80 °C prior to the determinations of TAAs, including both FAAs and PAAs.