An oxidation flow reactor for simulating and accelerating secondary aerosol formation in aerosol liquid water and cloud droplets

Abstract. Liquid water in cloud droplets and aqueous aerosols serves as an important reaction medium for the formation of secondary aerosol through aqueous-phase reactions (aqSA). Large uncertainties remain in estimates of the production and chemical evolution of aqSA in the dilute solutions found in cloud droplets and the concentrated solutions found in aerosol liquid water, which is partly due to the lack of available measurement tools and techniques. A new oxidation flow reactor (OFR), the Accelerated Production and Processing of Aerosols (APPA) reactor, was developed to measure secondary aerosol formed through gas- and aqueous-phase reactions, both for laboratory gas mixtures containing one or more precursors and for ambient air. For simulating in-cloud processes, droplets formed on monodisperse seed particles are introduced into the top of the reactor and the relative humidity (RH) inside it is controlled to 100 %. Similar measurements made with the RH in the reactor <100 % provide contrasts for aerosol formation with no liquid water and with varying amounts of aerosol liquid water. The reactor was characterized through a series of experiments and used to form secondary aerosol from known concentrations of an organic precursor and from ambient air. The transmission efficiency of O3 and CO2 for all RH and of SO2 for low RH exceeds 90 %, while it falls to about 70 % for SO2 at 100 % RH. Particle transmission efficiency increases with increasing particle diameter from 0.67 for 0.050 μm particles to 0.98 at 0.20 μm, while that of the ~3.3 μm droplets formed on seed particles is greater than 80 %. The residence time distributions of both gases and particles are narrow relative to other OFRs and lack the tails at long residence time expected with laminar flow. Initial cloud processing experiments focused on the well-studied oxidation of dissolved SO2 by O3, with observed growth of seed particles resulting from the added sulfuric acid agreeing well with estimates based on the relevant set of aqueous phase reactions. The OH exposure (OHexp) for low RH, high RH, and in-cloud conditions was determined experimentally from the loss of SO2 and benzene, and simulated from the KinSim chemical kinetics solver with inputs of measured 254 nm UV intensity profile through the reactor and loss of O3 due to photolysis. The aerosol yield for benzene at high OHexp ranged from 18 % at low RH with dry seed particles present in the reactor to 59 % with cloud droplets present. Measurement of the composition of the secondary aerosol formed from ambient air using an aerosol mass spectrometer showed that the oxygen to carbon ratio (O : C) of the organic component increased with increasing RH (and liquid water content).