Development and application of a nitrogen oxides analyzer based on the Cavity Attenuated Phase Shift (CAPS) technique

Nitrogen oxides (NOx) are crucial in tropospheric photochemical ozone (O3) production and oxidation capacity. Currently, the widely used NOx measurement technique is chemiluminescence (CL) (CL-NOx), which tends to overestimate NO2 due to atmospheric oxidation products of NOx (i.e., NOz). We developed and characterized a NOx measurement system using the cavity attenuated phase shift (CAPS) technique (CAPS-NOx), which is free from interferences with nitrogen-containing species. The NOx measured by the CAPS- NOx and CL- NOx analyzers were compared. Results show that both analyzers showed consistent measurement results for NO, but the NO2 measured by the CAPS- NOx analyzer (NO2_CAPS) was mostly lower than that measured by the CL- NOx analyzer (NO2_CL), which led to the deviations in O3 formation sensitivity regime and Ox (= O3 + NO2) sources (i.e., regional background and photochemically produced Ox) determined by the ozone production efficiencies (OPE) calculated from NO2_CL and NO2_CAPS. Overall, OPE_CL exceeded OPE_CAPS by 18.9%, which shifted 3 out of 13 observation days from the VOCs-limited to the transition regime when judging using OPE_CL, as compared to calculations using OPE_CAPS. During the observation period, days dominated by regional background Ox accounted for 46% and 62% when determined using NO2_CL and NO2_CAPS, respectively. These findings suggest that the use of the CL- NOx analyzer tends to underestimate both the VOCs-limited regime and the regional background Ox dominated days. The newly built CAPS- NOx analyzer here can promote the accurate measurement of NO2, which is meaningful for diagnosing O3 formation regimes and Ox sources.