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. We report on the development, characterization, and fielddeployment of a fast-time-response sensor for measuring ozone (O3) andnitrogen dioxide (NO2) concentrations utilizing chemical ionizationtime-of-flight mass spectrometry (CI-ToFMS) with oxygen anion(O2-) reagent ion chemistry. Wedemonstrate that the oxygen anion chemical ionization mass spectrometer(Ox-CIMS) is highly sensitive to both O3 (180 counts s−1 pptv−1) and NO2 (97 counts s−1 pptv−1), corresponding todetection limits (3σ, 1 s averages) of 13 and 9.9 pptv,respectively. In both cases, the detection threshold is limited by themagnitude and variability in the background determination. The short-termprecision (1 s averages) is better than 0.3 % at 10 ppbv O3 and 4 %at 10 pptv NO2. We demonstrate that the sensitivity of the O3measurement to fluctuations in ambient water vapor and carbon dioxide isnegligible for typical conditions encountered in the troposphere. Theapplication of the Ox-CIMS to the measurement of O3 vertical fluxesover the coastal ocean, via eddy covariance (EC), was tested during the summer of2018 at Scripps Pier, La Jolla, CA. The observed mean ozone depositionvelocity (vd(O3)) was 0.013 cm s−1 with a campaign ensemblelimit of detection (LOD) of 0.0027 cm s−1 at the 95 % confidencelevel, from each 27 min sampling period LOD. The campaign mean and 1standard deviation range of O3 mixing ratios was 41.2±10.1 ppbv. Several fast ozone titration events from local NO emissions weresampled where unit conversion of O3 to NO2 was observed,highlighting instrument utility as a total odd-oxygen (Ox=O3+NO2) sensor. The demonstrated precision, sensitivity, and timeresolution of this instrument highlight its potential for directmeasurements of O3 ocean–atmosphere and biosphere–atmosphere exchangefrom both stationary and mobile sampling platforms.

atmospheric measurement techniques

Simultaneous detection of ozone and nitrogen dioxide by oxygen anion chemical ionization mass spectrometry: a fast-time-response sensor suitable for eddy covariance measurements