Abstract
The Ozone Mapping and Profiler Suite Limb Profiler (OMPS/LP) has been flying on the Suomi NPP satellite since October 2011. It is designed to produce ozone and aerosol vertical profiles at ~2 km vertical resolution over the entire sunlit globe. Aerosol extinction profiles are computed with Mie theory using radiances measured at 675 nm. The operational Version 1.0 (V1.0) aerosol extinction retrieval algorithm assumes a bimodal lognormal aerosol size distribution (ASD) whose parameters were derived by combining an in situ measurement of aerosol microphysics with the SAGE II aerosol extinction climatology. Internal analysis indicates that this bimodal lognormal ASD does not sufficiently explain the spectral dependence of LP measured radiances. In this paper we describe the derivation of an improved aerosol size distribution, designated Version 1.5 (V1.5), for the LP retrieval algorithm. The new ASD uses a gamma function distribution that is derived from Community Aerosol and Radiation Model for Atmospheres (CARMA) calculated results. A cumulative distribution fit derived from the gamma function ASD gives better agreement with CARMA results at small particle radii than bimodal or unimodal functions. The new ASD also explains the spectral dependence of LP measured radiances better than the V1.0 ASD. We find that the impact of our choice of ASD on the retrieved extinctions varies strongly with the underlying reflectivity of the scene. Initial comparisons with co-located extinction profiles retrieved at 676 nm from the SAGE III/ISS instrument show a significant improvement in agreement for the LP V1.5 retrievals. Zonal mean extinction profiles agree to within 10% between 19-29 km, and regression fits of collocated samples show improved correlation and reduced scatter compared to the V1.0 product. This improved agreement will motivate development of more sophisticated ASDs from CARMA results that incorporate latitude, altitude, and seasonal variations in aerosol properties.
Highlights
Accurate estimation of stratospheric aerosol is important because aerosols in the stratosphere have an important influence on climate variability through their contribution to direct radiative forcing, the magnitude of this term is still uncertain (Ridley et al, 2014)
This paper describes the derivation of a revised aerosol size distribution function to retrieve aerosol extinction profiles from Ozone Mapping and Profiler Suite Limb Profiler (OMPS/LP) limb scattering radiance measurements
We find that using an aerosol size distribution (ASD) based on a gamma function fit requires fewer free parameters than our previous choice of a bimodal lognormal ASD and is more consistent with the Community Aerosol and Radiation Model for Atmospheres (CARMA) particle size results
Summary
Accurate estimation of stratospheric aerosol is important because aerosols in the stratosphere have an important influence on climate variability through their contribution to direct radiative forcing, the magnitude of this term is still uncertain (Ridley et al, 2014). The LP instrument collects limb-scattered radiance data and solar irradiance data on a 2-D charge-coupled device (CCD) array over a wide spectral range (290–1000 nm) and a wide vertical range (0–80 km) through three parallel vertical slits These spectra are primarily used to retrieve vertical profiles of ozone (Rault and Loughman, 2013; Kramarova et al, 2018), Published by Copernicus Publications on behalf of the European Geosciences Union. Instruments that measure scattered radiation need to assume some form of aerosol size distribution (ASD) to convert their measured information into aerosol extinction These instruments include limb-scattered instruments such as the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) (von Savigny et al, 2015; Malinina et al, 2018), Optical Spectrograph and Infrared Imaging System (OSIRIS) (Bourassa et al, 2008, 2012; Rieger et al, 2014, 2018), OMPS/LP (Loughman at al., 2018; Chen et al, 2018), and space- and ground-based lidars such as Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) (Winker et al, 2009). This work extends the previous results shown in Chen et al (2018) to provide improved validation of the LP V1.5 aerosol extinction product
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