An incoherent broadband cavity-enhanced absorption spectrometer for simultaneous NO 3 and N 2 O 5 measurements, integrated into a NO y instrument suite: characterization and first field deployment

Reactive nitrogen species (NO y ) 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 (NO 2 ) are important contributors to total NO y , further constituents such as the nitrate radical (NO 3 ) and dinitrogen pentoxide (N 2 O 5 ), formed from NO 2 oxidation, are key intermediates in nocturnal oxidation processes. Even though NO 3 and N 2 O 5 affect the NO y 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, NO 2 and NO y from commercial analyzers with a custom-built incoherent broadband cavity-enhanced absorption spectrometer (IBBCEAS) that measures NO 3 directly and N 2 O 5 via a thermal dissociation inlet (TDI) and channel differencing. We describe the design of the IBBCEAS subsystem, characterize the TDI, cavity optics, transmission losses, and, using a synthetic spectra simulation approach, the bias of ambient NO 2 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 (1 σ Allan deviation, 15 s) for NO 3 and N 2 O 5 , respectively. Under field conditions, interference by water vapor and NO 2 worsens these LODs to 3.1 pptv with a total measurement uncertainty (TMU) of 12 % for NO 3 , and to 4.4 pptv with a mean TMU of 21 % for N 2 O 5 . Exemplary results from its first deployment during a field campaign in a temperate forest atop a 40 m tall tower are discussed.