Improvement of stratospheric aerosol extinction retrieval from OMPS/LP using a new aerosol model

Abstract

The Ozone Mapping and Profiler Suite Limb Profiler (OMPS/LP) has been flying on the Suomi National Polar-orbiting Partnership (S-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 Stratospheric Aerosol and Gas Experiment (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 collocated extinction profiles retrieved at 676 nm from the SAGE III instrument on the International Space Station (ISS) show a significant improvement in agreement for the LP V1.5 retrievals. Zonal mean extinction profiles agree to within 10 % between 19 and 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.

Issues related to the retrieval of stratospheric-aerosol particle size information based on optical measurements

“…The existence of non-sulfate components of stratospheric aerosols is well established (e.g., Murphy et al, 1998;Curtius et al, 2005;Renard et al, 2008), but these particles are assumed to be of negligible impact in the current study.…”

A comparison of lognormal and gamma size distributions for characterizing the stratospheric aerosol phase function from OPC measurements

“…However, during background stratospheric aerosol conditions, OPC measurements may not provide sufficient information about smaller particles (r < 0.15 µm) to determine a robust BMLN fit as shown in a recent study to improve OMPS/LP aerosol retrievals by Chen et al (2018). In that study, Chen et al (2018) (CMF).…”

“…However, during background stratospheric aerosol conditions, OPC measurements may not provide sufficient information about smaller particles (r < 0.15 µm) to determine a robust BMLN fit as shown in a recent study to improve OMPS/LP aerosol retrievals by Chen et al (2018). In that study, Chen et al (2018) (CMF). These four BMLN distribution fits to the OPC data differed significantly from each other in the radius range between 0.01µm to 0.1µm and these differences resulted from the gaps in the OPC size bins that limited the ability of the fits to be constrained.…”

“…These four BMLN distribution fits to the OPC data differed significantly from each other in the radius range between 0.01µm to 0.1µm and these differences resulted from the gaps in the OPC size bins that limited the ability of the fits to be constrained. Consequently, the different ASDs produced P a (Θ) that differed significantly form each other as shown in Figure A2 of Chen et al (2018). Additionally, the BMLN distribution which is defined by 5 parameters (the CMF, 2 median radii, and 2 mode widths) that are independent of each other at each altitude cannot generally be determined in cases where the 150 measurements have less than 5 data points .…”

“…For both algorithms, P a (Θ) does not vary with altitude or location. (Note that the recently developed V1 (Loughman et al, 2018) and V1.5 (Chen et al, 2018) OMPS aerosol extinction retrieval use updated bi-modal lognormal and gamma phase functions respectively). The Angström exponent (AE) is computed using Equation (1), where K ext is the aerosol extinction coefficient for a particular wavelength.…”