| 09 Sep 2026
An incoherent broadband cavity-enhanced absorption spectrometer for simultaneous NO3 and N2O5 measurements, integrated into a NOy instrument suite: characterization and first field deployment
Abstract. Reactive nitrogen species (NOy) are involved in the formation of secondary pollutants, thus following their interconversions is crucial for understanding their impact on air quality. While nitric oxide (NO) and nitrogen dioxide (NO2), are important contributors to total NOy, further constituents such as the nitrate radical (NO3) and dinitrogen pentoxide (N2O5), formed from NO2 oxidation, are key intermediates in nocturnal oxidation processes. Even though NO3 and N2O5 affect the NOy budget in the following day, their role is poorly constrained in isolated measurements. We present the “NOyBOx”, a new instrumental framework combining measurements of NO, NO2 and NOy from commercial analyzers with a custom-built incoherent broadband cavity-enhanced absorption spectrometer (IBBCEAS) that measures NO3 directly and N2O5 via a thermal dissociation inlet (TDI) and channel differencing. We describe the design of the IBB-CEAS subsystem, characterize the TDI, cavity optics, transmission losses, and, using a synthetic spectra simulation approach, the bias of ambient NO2 and water vapor on our concentration retrieval procedure. Under optimal laboratory conditions we find a limit of detection (LOD) of 1.5 and 2.2 pptv (1σ Allan deviation, 15 s) for NO3 and N2O5, respectively. Under field conditions, interference by water vapor and NO2 worsens these LODs to 3.1 pptv with a total measurement uncertainty (TMU) of 12 % for NO3, and to 4.4 pptv with a mean TMU of 21 % for N2O5. Exemplary results from its first deployment during a field campaign in a temperate forest atop a 40 m tall tower are discussed.
Competing interests: At least one of the (co-)authors serves as editor for the special issue to which this paper belongs.
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