Tropospheric NO<sub>2</sub> Measurements Using a Three-wavelength Optical Parametric Oscillator Differential Absorption Lidar

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 of 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 nm and 439.5 nm) and three-wavelength DIAL (438 nm, 439.5 nm and 441 nm) techniques. The retrieval uncertainties of aerosol extinction using the three-wavelength DIAL technique are reduced to less than 2 % of 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 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 retrievals to simulated data from 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, of modeled profiles.