A ground-based system for measuring tropospheric OH radical based on laserinduced fluorescence (AIOFM-LIF) was developed in this work. In this system, ambient air is expanded through a 0.4 mm nozzle to low pressure in a detection chamber, where OH radical is irradiated by the 308 nm laser pulse at a repetition rate of 8.5 kHz. Then, the resultant fluorescence corresponding to the A2S (0) X2. (0) i +. =.. = transition at 308 nm is detected using gated photon counting. The AIOFM-LIF system was integrated into a mobile observing platform for the field observation following the series of laboratory characterization. A portable standard OH radical source by water photolysis-ozone actinometry was established and optimized for accurate system calibration. The factors affecting the system sensitivity were quantified. It was shown that the ultimate system sensitivity is 9.9 x 10(-8) cps (molecules cm(-3))-1 mw-1; the minimum detection limits are (1.84 +/- 0.26) x 10(5) cm(-3) and (3.69 +/- 0.52) x 10(5) cm(-3) at night and noon, respectively; and the whole error of AIOFM-LIF system is about 16%. Then, the system was deployed in Shenzhen, China, during the " A comprehensive STudy of the Ozone foRmation Mechanism in Shenzhen" (STORM) campaign. Valid OH radical concentrations for 31 days were obtained, and the peak of the daily average concentration was 6.6 x 10(6) cm(-3) around 12: 00. And a high correlation (R-2 = 0.77) between OH and j(O1D) was also observed in this field campaign. The relationship between OH concentration and NOx was attentively discussed. The deployment of AIOFM-LIF system in STORM campaign has demonstrated its capability of measuring tropospheric OH radical with high sensitivity and accuracy in a polluted environment. (c) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

An inexpensive, compact instrument for sensitive measurement of nocturnal nitrogen oxides NO₃ and N₂O₅ in ambient air at high time resolution has been described. The instrument measures NO₃ and N₂O₅ which is converted into the NO₃ radical through thermal decomposition by optical extinction using a diode laser at 662.08 nm in two separate detection channels. The minimum detection limits (1σ) for the NO₃ radical and N₂O₅ are estimated to be 2.3 pptv and 3.1 pptv in an average time of 2.5 s, with the accessible effective absorption path length generally exceeding 30 km, which is sufficient for quantifying NO₃ radical and N₂O₅ concentrations under moderately polluted conditions. The total uncertainties of the NO₃ and N₂O₅ measurements are 8% and 15% respectively, which are mainly dominated by the uncertainty of NO₃ across section calculated for 353 K in this system. In addition, the dependence of the instrument’s sensitivity and accuracy on a variety of conditions was presented in winter of 2016 and in summer of 2017 during two China-UK joint campaigns. Distinct N₂O₅ vertical profiles were observed at night in winter. The equilibrium among observed NO₂, NO₃ and N₂O₅ based on the equilibrium constants during summer time also provides confirmation of the measurement accuracy of the instrument.

Nitric oxide (NO) and nitrogen dioxide (NO₂) are relevant to air quality due to their roles in tropospheric ozone (O₃) production. In China, NO_x emissions are very high and NO_x emissions exhausted from on-road vehicles make up 20 % of total NO_x emissions. In order to detect the NO and NO₂ emissions on road, a dual-channel cavity ring-down spectroscopy (CRDS) system for NO₂ and NO detection has been developed. In the system, NO is converted to NO₂ by its reaction with excess O₃ in the NO_x channel, such that NO can be determined through the difference between two channels. The detection limits of NO₂ and NO_x for the system are estimated to be about 0.030 (1σ, 1 s) and 0.040 ppb (1σ, 1 s), respectively. Considering the error sources of NO₂ absorption cross section and R_L determination, the total uncertainty of NO₂ measurements is about 5%. The performance of the system was validated against a chemiluminescence (CL) analyser (42i, Thermo Scientific, Inc.) by measuring the NO₂ standard mixtures. The measurement results of NO₂ showed a linear correction factor (R²) of 0.99 in a slope of 1.031±0.006, with an offset of ($-\mathrm{0.940}\pm \mathrm{0.323}$ ) ppb. An intercomparison between the system and a cavity-enhanced absorption spectroscopy (CEAS) instrument was also conducted separately for NO₂ measurement in an ambient environment. Least-squares analysis showed that the slope and intercept of the regression line are 1.042±0.002 and ($-\mathrm{0.393}\pm \mathrm{0.040}$ ) ppb, respectively, with a linear correlation factor of R²=0.99. Another intercomparison conducted between the system and the CL analyser for NO detection also showed a good agreement within their uncertainties, with an absolute shift of (0.352±0.013) ppb, a slope of 0.957±0.007 and a correlation coefficient of R²=0.99. The system was deployed on the measurements of on-road vehicle emission plumes in Hefei, and the different emission characteristics were observed in the different areas of the city. The successful deployment of the system has demonstrated that the instrument can provide a new method for retrieving fast variations in NO and NO₂ plumes.