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

Abstract

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.