Improvement of Comparative Reactivity Method (CRM) for measuring OH reactivity in various environments

Abstract

OH reactivity (OHR), defined as the OH loss rate through various reactions with atmospheric chemical compounds, is a useful tool to characterize the oxidation capacity of the atmosphere, linked to the potential for secondary air pollutants formation. Due to the limitations in directly monitoring OH radical, comparative reactivity method (CRM) has been developed over the past two decades to quantify OHR; CRM gauges OHR by measuring changes in the concentration of reference species, mostly pyrrole. However, the inherent limitations of the CRM approach have limited its use in a wide range of environments since its development. The major limitations of CRM are (i) unavoidable operation under pseudo-first order violation condition for ensuring the measurement sensitivity, (ii) interferences of ambient chemical species such as NO, NO2, O3, and H2O and (iii) unstable performance of reactor due to changes in the flow of gas entering and leaving the reactor. Firstly, to overcome these limitations, this work upgraded the original CRM to operate the reactor under reduced pressure (13.85 - 13.90 psi) and named as UP-CRM (Under Pressure-CRM). This improvement not only increased the homeostasis within the reactor by maintaining constant flow dynamics to reduce fluctuations in the pyrrole concentration, but also minimized the potential losses of ambient pollutants due to the surfaces of devices located along the in-let upstream. Under this stable measurement condition, quantitative corrections for the limitations of (i) and (ii) were conducted for preparing the UP-CRM utilization in a wide range of environments. As a result of correction experiments, it is observed that pseudo-first order MS/EN 20221028 Gahyun Lee (이가현). Improvement of Comparative Reactivity Method (CRM) for measuring OH reactivity in various environments (다양한 환경에서의 OH 반응성 측정을 위한 비교반응도법 개선). School of Earth Sciences and Environmental Engineering. 2024. 42p. Prof. Kyung-Eun Minvi violation and ambient chemical interference of NO, NO2, and H2O corrections have significant impacts on OHR quantification but interference of O3 is found to be negligible. The development of the stable reactor operation scheme along with the systematic interference correction procedures in this study will enable more accurate OHR measurements in various environments, especially urban sites where OHR measurements through CRM has not been frequently performed due to the high chemical interferences.