Measurements of atmospheric HONO and NO<sub>2</sub> utilizing an open-path broadband cavity enhanced absorption spectroscopy based on an iterative algorithm

Abstract

Nitrous acid (HONO), as an important precursor of hydroxyl (OH) radical, has a low concentration, short lifetime, easy loss and high reactivity in the atmosphere. Thus, the high sensitivity detection of atmospheric HONO is a challenge. In this paper, we report an open-path broadband cavity enhanced absorption spectroscopy (OP-BBCEAS) system based on the iterative algorithm for simultaneous measurement of atmospheric HONO and NO<sub>2</sub>. In the conventional BBCEAS system, a pump is used to drive the ambient air into the optical cavity through the filter membrane for measurement, which can reduce the influence of aerosol particle extinction. However, the influence of wall loss and secondary formation caused by the optical cavity and sampling should be considered for reactive component measurements. The OP-BBCEAS with open-path configuration is adopted in this paper to avoid being influenced by wall effect. The effective absorption optical path is calculated by the iterative retrieval algorithm through multiple iterations, and the absorption of HONO and NO<sub>2</sub> are then quantified by the spectral fitting method of differential optical absorption spectroscopy, which removes the broadband change influence of the Mie scattering extinction by aerosol particles and the light intensity fluctuation. The atmospheric HONO and NO<sub>2</sub> with light (PM<sub>2.5</sub> < 75 μg/m<sup>3</sup>) and moderate (PM<sub>2.5</sub> > 75 μg/m<sup>3</sup>) aerosol loading are measured by the OP-BBCEAS system based on iterative algorithm, and compared with the counterparts by the conventional close-path BBCEAS system. The concentrations of HONO and NO<sub>2</sub> measured by the two BBCEAS systems are in good agreement (<i>R</i><sup>2</sup> > 0.99) for different PM<sub>2.5</sub> concentration levels, and the measurement differences of HONO and NO<sub>2</sub> concentrations (HONO ≤ 4.0%, NO<sub>2</sub> ≤ 6.5%) are less than the systematic measurement errors (HONO: 8.1%, NO<sub>2</sub>: 7.5%), which verifies the feasibility of iterative algorithm applied to OP-BBCEAS system for atmospheric measurement.