Development of a UV broadband cavity enhanced absorption spectrometer for simultaneous detection of NO2, HCHO, and HONO from point sources

Abstract

The recent emergence of interest in nitrogen dioxide (NO2), formaldehyde (HCHO), and nitrous acid (HONO) as air pollutants has led to a heightened awareness of their potential risks to human health and the production of secondary atmospheric pollutants. The necessity for accurate measurements of these species is becoming increasingly evident in the context of monitoring the implementation of regulations, particularly at point sources such as tail pipe and industry. Thus, this study presents the development of a broadband cavity enhanced absorption spectrometer for the simultaneous detection of NO2, HCHO, and HONO. With the most up-to-date techniques in light source and high reflection mirror set in UV region, (340 nm centered ultraviolet-light emitting diode and wavelength coverage of 330.6 nm to 351.4 nm mirror set with maximum reflectivity of 99.965% at 345.1 nm), the effective path length of light of this instrument reaches up 1.51 km which corresponding to the minimum detectable extinction of 4.994×10-9 cm-1 (2σ) in 3.3 min which is suitable for monitoring point source emissions. By taking advantage of a specially designed optic cage system as well as the capability of data retrieval processes using broadband absorption features of subject species, this instrument has the special strength of robust performances in severe environments, such as temperature and pressure fluctuations, high humidity, or external vibrations. Based on the result of the first airborne deployment in March 2024, this instrument successfully detected NO2, HCHO, and HONO with the precision (3σ) of 0.885 ppbv for NO2 and 6.92 ppbv for HCHO in 3.3 minutes, and 1.16 ppbv for HONO in 4 minutes, and uncertainties of 6.3%, 7.0%, and 10.0% for NO2, HCHO, and HONO respectively. Though this instrument needs the validation with higher concentration range similar to point source emissions, it still demonstrates the potential for calibration-free simultaneous detection of NO2, HCHO, and HONO at concentrations relevant to ambient levels, with the improvement in absorption cross-section spectra and optic components.