Peking University Cold Aerosol Flow Tube (PKU-CAT): Development and Characterization for Studying Wintertime Secondary Organic Aerosol Formation.

Secondary organic aerosol (SOA) is an important contributor to atmospheric particulate matter. Temperature has been suggested as a key factor impacting SOA yields, yet laboratory data are sparse, particularly for technically challenging subzero temperature ranges. To address this issue, we developed the Peking University Cold Aerosol Flow Tube (PKU-CAT). The PKU-CAT can be operated for the temperature range of -20 to 30 °C, covering most ground surface temperatures, particularly those encountered in urban environments. The PKU-CAT features a 1.5 m long quartz tube, with an inner diameter of 19 cm and a total volume of 42.5 L. It employs light-emitting diodes (LEDs) with a peak wavelength of 367 nm to photolyze isopropyl nitrite for OH radical generation, achieving an atmospheric equivalent OH exposure time of approximately 1 day for the whole operational temperature range. Relative humidity (RH) in the PKU-CAT can be controlled up to 80%, even below 0 °C. The system is equipped with a scanning mobility particle analyzer (SMPS) that allows isothermal sampling of particles. The residence time distribution and gas- and particle-transmission efficiencies of the PKU-CAT are comparable to those of existing quasi-laminar oxidation flow reactors (OFRs) operated at room temperature. Photooxidation experiments of α-pinene were conducted for characterizing SOA formation. At 20 °C, the SOA yields obtained with the PKU-CAT ranged from 13 to 21% for organic mass concentration of 17-57 μg m