High-Precision and Low-Interference Measurement of Total OH Reactivity Based on the Comparative Reactivity Method (CRM)

Abstract The comparative reactivity method (CRM) had been widely applied for measuring the total OH reactivity (OHR) due to its portability and cost-effectiveness. However, conventional CRM systems suffer from large uncertainties under high-NOx conditions because of secondary OH• formation and long gas residence times. In this study, we developed a technically modified CRM system that featured a redesigned single-channel, short-residence flow tube (residence time = 1.2 s), which substantially reduced secondary OH• formation from NO−HO2• reactions and largely suppressed NO-RO2• interference while reducing pyrrole photolysis from 25% to 2%. The system was coupled with a Vocus Proton Transfer reaction time-of-Flight Mass Spectrometer (Vocus PTR-ToF-MS) for high time-resolution detection of pyrrole. Laboratory calibrations using various volatile organic compound (VOC) standards demonstrated excellent linearity (r2 >0.990) over a reactivity range of 0−120 s−1. Furthermore, we implemented real-time humidity correction and an optimized multi-component interference correction framework for NO, NO2, and O3, with the estimated uncertainties on the measured OHR being 5%, 5%, 10%, and 4% for humidity, NO, NO2, and O3, respectively. The total measurement uncertainty was estimated to be 15%. The modified CRM system achieved a detection limit of 2.1 s−1, and the field data were averaged to 5 min to match the temporal resolution of the LP-LIF measurements for subsequent intercomparison. Field measurements were conducted at an urban site in Beijing (high-NOx) and at Longquan Mountain (low-NOx), showing strong agreement with simultaneous laser flash photolysis-laser-induced fluorescence instrument (LP-LIF) measurements, with regression slopes of 1.064 (r2 = 0.939) in Beijing and 0.992 (r2 = 0.952) at Longquan Mountain. These results demonstrated that the modified CRM system provided high-precision, low-interference OHR measurement across diverse atmospheric environments, extending the applicability of the CRM to complex, high-NOx conditions.