Quantitative study on the interference of OH radicalmeasurement in the air pollution complex

Abstract

<p indent="0mm">OH radical is the most important gas-phase oxidant in the troposphere, which oxidizes the primary pollutants to secondary pollutants and causes the regional secondary pollution. Hence, it is crucial to acquire comprehensive knowledge of the radical chemistry to mitigate the secondary air pollution. However, the well-established photochemistry is insufficient to describe the chemical processes in the complex air mixture. For instance, the model severely underpredicts OH radical concentration in forest areas. <italic>In situ</italic> measurement of OH radical is an efficient way to quantify the atmospheric oxidation capacity and improve our understanding of atmospheric chemical mechanisms. While due to its high reactivity and low concentration, measurement of OH radical is one of the most challenging tasks in the atmospheric research. Laser-induced fluorescence (LIF) is the most widely used technique to measure OH due to its high sensitivity and selectivity. However, studies have shown that under high volatile organic compounds (VOCs) conditions, LIF technique based on traditional spectral modulation methods may suffer from interference, whose origin has not yet been known. In this study, based on the Peking University Laser-Induced Fluorescence system (PKU-LIF), a chemical modulation method was developed, to separate interference by quantifying the chemical background signals, through chemical removing of the ambient OH prior to be sampled into the detection cell, thereby enabling accurate measurement of OH radical. During the winter campaign in Beijing in 2016, simultaneous OH radical measurement based on both the chemical modulation method and the spectral modulation method was achieved for the first time, and two sets of OH radical concentrations were obtained. It was found that, the two sets of OH radical concentrations agreed within the combined uncertainties of 31% (correlation slope: 0.88±0.01), indicating that there was no significant interference in the measurement of OH based on the traditional spectral modulation method during this winter campaign. It is worth nothing that whether the LIF measurement of OH radicals will suffer from interference and the degree of interference highly depend on the specific instrument configurations and the environmental conditions. Therefore, when applying LIF technique to measure the concentration of OH radical under complex environments in the future, the chemical modulation method is required to achieve accurate OH measurement.