Abstract
Abstract. This paper presents an exploratory study on the aerosol layer height (ALH) retrieval from the OMI 477 nm O2 − O2 spectral band. We have developed algorithms based on the multilayer perceptron (MLP) neural network (NN) approach and applied them to 3-year (2005–2007) OMI cloud-free scenes over north-east Asia, collocated with MODIS Aqua aerosol product. In addition to the importance of aerosol altitude for climate and air quality objectives, our long-term motivation is to evaluate the possibility of retrieving ALH for potential future improvements of trace gas retrievals (e.g. NO2, HCHO, SO2) from UV–visible air quality satellite measurements over scenes including high aerosol concentrations. This study presents a first step of this long-term objective and evaluates, from a statistic point of view, an ensemble of OMI ALH retrievals over a long time period of 3 years covering a large industrialized continental region. This ALH retrieval relies on the analysis of the O2 − O2 slant column density (SCD) and requires an accurate knowledge of the aerosol optical thickness, τ. Using MODIS Aqua τ(550 nm) as a prior information, absolute seasonal differences between the LIdar climatology of vertical Aerosol Structure for space-based lidar simulation (LIVAS) and average OMI ALH, over scenes with MODIS τ(550 nm) ≥ 1. 0, are in the range of 260–800 m (assuming single scattering albedo ω0 = 0. 95) and 180–310 m (assuming ω0 = 0. 9). OMI ALH retrievals depend on the assumed aerosol single scattering albedo (sensitivity up to 660 m) and the chosen surface albedo (variation less than 200 m between OMLER and MODIS black-sky albedo). Scenes with τ ≤ 0. 5 are expected to show too large biases due to the little impact of particles on the O2 − O2 SCD changes. In addition, NN algorithms also enable aerosol optical thickness retrieval by exploring the OMI reflectance in the continuum. Comparisons with collocated MODIS Aqua show agreements between −0. 02 ± 0. 45 and −0. 18 ± 0. 24, depending on the season. Improvements may be obtained from a better knowledge of the surface albedo and higher accuracy of the aerosol model. Following the previous work over ocean of Park et al.(2016), our study shows the first encouraging aerosol layer height retrieval results over land from satellite observations of the 477 nm O2 − O2 absorption spectral band.
Highlights
The ability to monitor air quality and climate from ultraviolet–visible (UV–vis) satellite spectral measurements requires accurate trace gas (e.g. NO2, SO2, HCHO, O3) and aerosol observations
The reason is threefold: (1) to maximize the probability of the selection of cloud-free Ozone Monitoring Instrument (OMI) observation pixels dominated by aerosol pollution; (2) to evaluate the retrieved OMI τ (550 nm) products by comparing with collocated MODIS τ (550 nm); and (3) to use the MODIS τ (550 nm) as input of the NNτ,NOs 2−O2 algorithm for retrieving the OMI aerosol layer pressure (ALP) product, assuming this is the most accurate τ information available for each collocated OMI observation pixel–MODIS aerosol grid cell
The aerosol height was here retrieved as aerosol layer pressure (ALP) and defined as the mid-pressure of an homogeneous scattering layer with a constant geometric thickness
Summary
The ability to monitor air quality and climate from ultraviolet–visible (UV–vis) satellite spectral measurements requires accurate trace gas (e.g. NO2, SO2, HCHO, O3) and aerosol observations. The importance of measuring vertical distribution of atmospheric aerosols on a global scale is threefold. Large uncertainties of aerosol optical properties limit our climate predictive capabilities (IPCC, 2007). In spite of more robust climate predictions in the last years, radiative forcing (RF) induced by aerosols is still the largest uncertainty to the total RF estimate (IPCC, 2014). The vertical distribution and relative location are determining factors of aerosol radiative forcing in the long-wave spectral range (Dufresne et al, 2002; Kaufman et al, 2002)
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