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Development of a portable cavity ring down spectroscopy instrument for simultaneous, in situ measurement of NO3 and N2O5.

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.