Preprints
https://doi.org/10.5194/egusphere-2024-2721
https://doi.org/10.5194/egusphere-2024-2721
30 Sep 2024
 | 30 Sep 2024
Status: this preprint is open for discussion and under review for Atmospheric Measurement Techniques (AMT).

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

Qianying Liu, Dan Dan Huang, Andrew T. Lambe, Shengrong Lou, Lulu Zeng, Yuhang Wu, Congyan Huang, Shikang Tao, Xi Cheng, Qi Chen, Ka In Hoi, Hongli Wang, Kai Meng Mok, Cheng Huang, and Yong Jie Li

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, OHexp), 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 OHexp in OFRs, and empirical equations have been proposed to parameterize OHexp. 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 OHexp 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 OHexp. 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 OHexp 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 OHexp 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 OHexp 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 OHexp 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 OHexp. 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.

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Qianying Liu, Dan Dan Huang, Andrew T. Lambe, Shengrong Lou, Lulu Zeng, Yuhang Wu, Congyan Huang, Shikang Tao, Xi Cheng, Qi Chen, Ka In Hoi, Hongli Wang, Kai Meng Mok, Cheng Huang, and Yong Jie Li

Status: open (until 05 Nov 2024)

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Qianying Liu, Dan Dan Huang, Andrew T. Lambe, Shengrong Lou, Lulu Zeng, Yuhang Wu, Congyan Huang, Shikang Tao, Xi Cheng, Qi Chen, Ka In Hoi, Hongli Wang, Kai Meng Mok, Cheng Huang, and Yong Jie Li
Qianying Liu, Dan Dan Huang, Andrew T. Lambe, Shengrong Lou, Lulu Zeng, Yuhang Wu, Congyan Huang, Shikang Tao, Xi Cheng, Qi Chen, Ka In Hoi, Hongli Wang, Kai Meng Mok, Cheng Huang, and Yong Jie Li

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Short summary
We conducted experiments to evaluate the application of the empirical equations to estimate OHexp. For the OFR185, except for external OH reactivity, the parameters obtained within a narrow range of conditions can be extended to estimate the OHexp of wide ranges, which is also true for OFR254. 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 OHexp.