We report an open-path incoherent broadband cavity-enhanced absorption spectroscopy (OP-IBBCEAS) technique for in situ simultaneous optical monitoring of NO
2, NO
3, and H
2O in a reaction chamber. The measurement precision values (1
σ) are 2.9 ppbv and 2.9 pptv for NO
2 and NO
3 in 2 s, respectively, and the measurement uncertainties are 6% for NO
2 and 14% for NO
3. Intercomparison of measured concentrations of NO
2 and NO
3 by open-path and extractive IBBCEAS was carried out in the SAES-ARC reaction chamber during the reaction of NO
2 with O
3. The measurement accuracy of OP-IBBCEAS is verified by an NO
2 intercomparison and the NO
3 transmission efficiency of the extractive IBBCEAS is determined by comparison against the in situ NO
3 measurement. The relationship between H
2O 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 H
2O 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 H
2O structure. An inappropriate H
2O absorption cross section can cause an overestimation of NO
3 concentration of about 28% in a humid atmosphere (H
2O = 1.8%). This spectroscopic correction provides an approach to obtain accurate NO
3 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 NO
2 and NO
3, respectively.
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