A dual-channel incoherent broadband cavity-enhanced absorption spectrometer for sensitive atmospheric NOx measurements

Abstract

We describe and characterize a dual-channel incoherent broadband cavity-enhanced absorption spectrometer (DC-IBBCEAS) for the sensitive measurements of NO_x: the sum of nitrogen monoxide (NO) and nitrogen dioxide (NO₂) in the atmosphere. The instrument employs two 1 m long optical cavities, with air being extracted from a common sampling line. The first channel (cavity-1) spans 340–380 nm and the second channel (cavity-2) spans the 405–460 nm spectral range, both measuring NO₂ simultaneously. High absorption cross-sections of NO₂ in both channels are effectively utilized for its sensitive quantification. NO is quantified by titrating it with ozone to NO₂ in channel-2, where the difference of NO₂ measured from that in channel-1 corresponds to the NO concentration in the sampled air. The instrument offers 1-ppb detection sensitivity for both NO and NO₂ with a maximum possible uncertainty of ∼9%. The use of close yet different spectral regions in the two channels readily extended measurements to a broader range without compromising its sensitivity to NO_x quantification. This would extend the DC-IBBCEAS applicability to simultaneously monitor interfering species with significant absorption cross-sections in the region in either channel, such as glyoxal (CHOCHO), methylglyoxal (CH₃COCHO), and nitrous acid (HONO).