Tropospheric NO 2 measurements using a three-wavelength optical parametric oscillator differential absorption lidar

Su, Jia; McCormick, M. Patrick; Johnson, Matthew S.; Sullivan, John T.; Newchurch, Michael J.; Berkoff, Timothy A.; Kuang, Shi; Gronoff, Guillaume P.

The conventional two-wavelength differential absorption lidar (DIAL) has measured air pollutants such as nitrogen dioxide (NOinline-formula2). 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 NOinline-formula2. We proposed a new technique to obtain more accurate measurements of NOinline-formula2 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 NOinline-formula2 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 NOinline-formula2 absorption cross-sections and reduce aerosol interference. NOinline-formula2 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 NOinline-formula2 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 NOinline-formula2 profiles, we compared the NOinline-formula2 profiles to simulated data from the Weather Research and Forecasting Chemistry (WRF-Chem) model. This comparison suggests that the NOinline-formula2 profiles retrieved with the three-wavelength DIAL technique have similar vertical structure and magnitudes typically within inline-formula±0.1 ppb compared to modeled profiles.

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Su, Jia / McCormick, M. Patrick / Johnson, Matthew S. / et al: Tropospheric NO2 measurements using a three-wavelength optical parametric oscillator differential absorption lidar. 2021. Copernicus Publications.

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