Tropospheric NO<sub>2</sub> measurements using a three-wavelength optical parametric oscillator differential absorption lidar

Abstract

Abstract. The conventional two-wavelength differential absorption lidar (DIAL) has measured air pollutants such as nitrogen dioxide (NO2). However, high concentrations of aerosol within the planetary boundary layer (PBL) can cause significant retrieval errors using only a two-wavelength DIAL technique to measure NO2. We proposed a new technique to obtain more accurate measurements of NO2 using a three-wavelength DIAL technique based on an optical parametric oscillator (OPO) laser. This study derives the three-wavelength DIAL retrieval equations necessary to retrieve vertical profiles of NO2 in the troposphere. Additionally, two rules to obtain the optimum choice of the three wavelengths applied in the retrieval are designed to help increase the differences in the NO2 absorption cross-sections and reduce aerosol interference. NO2 retrieval relative uncertainties caused by aerosol extinction, molecular extinction, absorption of gases other than the gas of interest and backscattering are calculated using two-wavelength DIAL (438 and 439.5 nm) and three-wavelength DIAL (438, 439.5 and 441 nm) techniques. The retrieval uncertainties in aerosol extinction using the three-wavelength DIAL technique are reduced to less than 2 % of those when using the two-wavelength DIAL technique. Moreover, the retrieval uncertainty analysis indicates that the three-wavelength DIAL technique can reduce more fluctuation caused by aerosol backscattering than the two-wavelength DIAL technique. This study presents NO2 concentration profiles which were obtained using the HU (Hampton University) three-wavelength OPO DIAL. As a first step to assess the accuracy of the HU lidar NO2 profiles, we compared the NO2 profiles to simulated data from the Weather Research and Forecasting Chemistry (WRF-Chem) model. This comparison suggests that the NO2 profiles retrieved with the three-wavelength DIAL technique have similar vertical structure and magnitudes typically within ±0.1 ppb compared to modeled profiles.

Review of "NitroNet – A deep-learning NO2 profile retrieval prototype for the TROPOMI satellite instrument"

“…LIDAR instruments) exist, these are not routinely deployed (see e.g. Sluis et al (2010); Bourgeois et al (2022); Lange et al (2023); Riess et al (2023); Volten et al (2009); Berkhout et al (2018); Su et al (2021)). Altogether, these measurements are valuable for the quantification of tropospheric vertical column densities, surface concentrations, and to some extent the tropospheric profile shapes.…”

Development of a compact 406 nm diode laser-based cavity-enhanced spectrometer for high-sensitive detection of NO2 levels in exhaust gas

“…The long-path techniques for measuring NO 2 can be broadly classified into free-space and cavity-based measurements. Free space measurement technique includes DOAS, tunable diode-laser absorption spectroscopy (TDLAS) [29] and differential absorption lidar (DIAL) [30].…”