A Comprehensive Characterization of Empirical Parameterizations for OH Exposure in the Aerodyne Potential Aerosol Mass Oxidation Flow Reactor (PAM-OFR)

Abstract. The oxidation flow reactor (OFR) has been widely used to simulate secondary organic aerosol (SOA) formation in laboratory and field studies. The extent of hydroxyl radical (OH) oxidation (or OH exposure, OH_exp), normally expressed as the product of OH concentration and residence time in the OFR, is important in assessing the oxidation chemistry in SOA formation. Several models have been developed to quantify the OH_exp in OFRs, and empirical equations have been proposed to parameterize OH_exp. Practically, the empirical equations and the associated parameters are derived under atmospheric relevant conditions (i.e., external OH reactivity) with limited variations of calibration conditions, such as residence time, water vapor mixing ratio, O₃ concentration, etc. Whether the equations or parameters derived under limited sets of calibration conditions can accurately predict the OH_exp under dynamically changing experimental conditions with large variations (i.e., extremely high external OH reactivity) in real applications remains uncertain. In this study, we conducted 62 sets of experiments (416 data points) under a wide range of experimental conditions to evaluate the scope of the application of the empirical equations to estimate OH_exp. Sensitivity tests were also conducted to obtain a minimum number of data points that is necessary for generating the fitting parameters. We showed that, for the OFR185 mode (185-nm lamps with internal O₃ generation), except for external OH reactivity, the parameters obtained within a narrow range of calibration conditions can be extended to estimate the OH_exp when the experiments are in wider ranges of conditions. For example, for water vapor mixing ratios, the parameters obtained within a narrow range (0.49–0.99 %) can be extended to estimate the OH_exp under the entire range of water vapor mixing ratios (0.49–2.76 %) studied. However, the parameters obtained when the external OH reactivity is below 23 s^-1 could not be used to reproduce the OH_exp under the entire range of external OH reactivity (4–204 s^-1). For the OFR254 mode (254-nm lamps with external O₃ generation), all parameters obtained within a narrow range of conditions can be used to estimate OH_exp accurately when experimental conditions are extended, but too-low lamp voltages should be avoided. Regardless of OFR185 or OFR254 mode, at least 20–30 data points from SO₂ or CO decay with varying conditions are required to fit a set of empirical parameters that can accurately estimate OH_exp. Caution should be exercised to use fitted parameters from low external OH reactivity to high ones, for instance, those from direct emissions such as vehicular exhaust and biomass burning.