Intercomparison of NO3 under Humid Conditions with Open-Path and Extractive IBBCEAS in an Atmospheric Reaction Chamber

We report an open-path incoherent broadband cavity-enhanced absorption spectroscopy (OP-IBBCEAS) technique for in situ simultaneous optical monitoring of NO2, NO3, and H2O in a reaction chamber. The measurement precision values (1 sigma) are 2.9 ppbv and 2.9 pptv for NO2 and NO3 in 2 s, respectively, and the measurement uncertainties are 6% for NO2 and 14% for NO3. Intercomparison of measured concentrations of NO2 and NO3 by open-path and extractive IBBCEAS was carried out in the SAES-ARC reaction chamber during the reaction of NO2 with O-3. The measurement accuracy of OP-IBBCEAS is verified by an NO2 intercomparison and the NO3 transmission efficiency of the extractive IBBCEAS is determined by comparison against the in situ NO3 measurement. The relationship between H2O absorption cross section and its mixing ratio at 295 K and 1 atm was analysed. Due to the spectral resolution of IBBCEAS system, the strong and narrow absorption lines of H2O are unresolved and exhibit non-Beer-Lambert Law behaviour. Therefore, a correction method is used to obtain the effective absorption cross section for fitting the H2O structure. An inappropriate H2O absorption cross section can cause an overestimation of NO3 concentration of about 28% in a humid atmosphere (H2O = 1.8%). This spectroscopic correction provides an approach to obtain accurate NO3 concentrations for open-path optical configurations, for example in chamber experiments or field campaigns. The measurement precision values are improved by a factor of 3 to 4 after applying Kalam filtering, achieving sub-ppbv (0.8 ppbv) and sub-pptv (0.9 pptv) performance in 2 s for NO2 and NO3, respectively.