An exploratory study on the aerosol height retrieval from OMI measurements of the 477 nm O2–O2 spectral band, using a Neural Network approach

Short summary

We have developed artificial neural network algorithms to retrieve aerosol layer height from satellite OMI observations of the 477 nm O₂–O₂spectral band. Based on 3-year (2005–2007) cloud-free scenes over north-east Asia, the results show uncertainties of 260–800 m when aerosol optical thickness is larger than 1. These algorithms also enable aerosol optical thickness retrievals by exploring the OMI continuum reflectance. These results may be used for future trace gas retrievals from TROPOMI.

Key figures

O2-O2_SCD_Aerosols — Continuum reflectance (475 nm) and O2-O2 slant column density (477 nm) as a function of aerosol optical thickness (550 nm) and aerosol layer pressure assuming surface albedo = 0.07, aerosol Angstrom coefficient = 1.5, and aerosol single scattering albedo = 0.95.

Diagram of Multilayer Perceptron (MLP) Neural Network (NN) architecture designed for Aerosol Layer Pressure (ALP) and aerosol optical thickness retrieval algorithms from the OMI O2-O2 477 nm spectal band. Each circle represents a specific processor (named neuron) including either an input, output or an activation sigmoid function. A weighted sum is performed before the transport through the activation function.

Box-whisker plots of retrieved Aerosol Layer Pressure (ALP) biases as a function of true aerosol optical thickness over the whole learning data set.

Spatial averages of aerosol optical thickness (550 nm), over North-East Asia. Statistics are computed over 3 years (2005-2007) for the 3 seasons autumn, winter and spring. The retrievals, from OMI, are obtained either with OMLER or MODIS black sky albedo, assuming aerosol single scattering albedo (SSA) 0.9 or 0.95.

Comparison of the average ALH retrievals obtained over scenes with collocated MODIS aerosol optical thickness > 1.0, with the LIVAS ALH climatology database. The retrievals are achieved over north-eats Asia for cloud-free scenes, over 3 years 2005-2007 and for the 4 seasons. OMI and MODIS black sky surface albedos are alternatively considered. The NN OMI ALH algorithms (see Sect. 5.1.). A temperature correction is applied to the OMI O2-O2 SCD prior to the retrievals (*cf.* Sect.6.1).

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.