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.
Highlights
Nitrogen dioxide (NO2) plays a critical role in the tropospheric chemistry and is one of the reactive gases collectively referred to as “nitrogen oxides” (NOx = nitric oxide and nitrogen dioxide, NO + NO2) (U.S EPA, 2020)
Aerosols are abundant within the planetary boundary layer (PBL) and can cause significant retrieval errors in a two-wavelength differential absorption lidar (DIAL) technique to measure NO2. To better understand this aerosol problem and produce a more accurate NO2 profile measurement, we described a new technique using a three-wavelength DIAL technique based on the intrinsic capabilities of using a multi-wavelength optical parametric oscillator (OPO) laser system
Hampton University (HU) lidar 438 nm, 439.5 nm and 441 nm elastic signals measured at 21:00 LT on 13 May 2020 and 22:00 LT on 27 July 2020 are shown in Fig. 13a and c, respectively
Summary
Nitrogen dioxide (NO2) plays a critical role in the tropospheric chemistry and is one of the reactive gases collectively referred to as “nitrogen oxides” (NOx = nitric oxide and nitrogen dioxide, NO + NO2) (U.S EPA, 2020). The sources of NOx emissions include transportation (on-road vehicles, airplanes, trains, ships), wood burning, industrial and chemical processes, activities for oil and gas development, soil emissions, lightning, and wildfires (see Nitrogen Oxides Emissions indicator) (U.S EPA, 2020). Scientific evidence indicates that short-term NO2 exposure, ranging from 30 min to 24 h, can cause the exacerbation of asthma symptoms, in some cases resulting in hospitalization (Berglund, et al, 1993). Long-term NO2 exposure is likely to have a causal relationship with respiratory effects, based on evidence for the development of asthma (U.S EPA, 2016). Local or global NO2 monitoring is essential for understanding atmospheric chemistry as well as for humanhealth and environmental management and control
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