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For decades, NASA has flown a fleet of single-view radiometers that have been used for characterizing the global aerosol system. These have included the Total Ozone Mapping Spectrometer (TOMS), the MODerate resolution Imaging Spectroradiometer (MODIS) on both Terra and Aqua satellites, the Ozone Monitoring Instrument (OMI) on Aura, the Ozone Mapping Profiler Suite (OMPS) and the VIsible InfraRed Sensor (VIIRS) on Suomi-National Polar orbiting Partnership (S-NPP) satellite.

During this era, multiple algorithms have been developed to extract the specific information offered by each sensor and retrieve properties of global aerosol. Algorithms derived for TOMS, OMI and OMPS make use the ultraviolet (UV) portion of the reflected solar spectrum. Algorithms developed for MODIS and VIIRS rely on the visible (VIS) through the shortwave infrared (SWIR). Each spectral range provides information that has been exploited for aerosol characterization, some of which is unique to the specific spectral range and some of which overlaps between ranges. For example, the UV is particularly sensitive to aerosol absorption characteristics, the SWIR sensitive to size distribution and all wavelengths useful to derive aerosol loading in the form of aerosol optical depth (AOD).

With the Ocean Color Instrument (OCI) on PACE, there is opportunity to exploit the full reflected shortwave spectrum for retrieving global aerosol properties. Although we expect there will be at-launch algorithms applied to OCI, these will be based on the heritage set, which was tuned to either UV or VIS/NIR/SWIR. Although these products would include AOD in the UV and VIS over land and ocean, and the fine mode fraction of the AOD over oceans, they would NOT include any information on aerosol absorption, aerosol layer height, nor aerosol above clouds. To retrieve these important characteristics, and narrow uncertainties in estimating aerosol effects on climate change and air quality, we must mine PACE-OCI's full potential. Using OCI's broad spectrum from the UV to the SWIR we can bring home spectral AOD, size parameter, absorption, layer height over ocean and land, and aerosol above clouds. Furthermore, we can use OCI's hyperspectral capability through the oxygen absorption bands to provide an independent measure of aerosol layer height.

We propose a comprehensive aerosol characterization algorithm for OCI-alone, rooted in heritage, but yet completely innovative. By joining forces, we are cutting out the redundancies (and inconsistencies) of multiple aerosol algorithm groups working on separate algorithms on separate UV and VIS/NIR/SWIR sensors. We are not ignoring the added benefit that polarimeters can bring to aerosol characterization, but will not rely on this information for real-time aerosol retrievals.

We intend to provide a unified aerosol algorithm for OCI-unified in its spectral approach from UV to SWIR and unified in bringing multiple heritage groups together into one team. Together spectrally and together in experience, we can produce the best aerosol characterization possible in an operational environment.

Aerosol absorption is a key aerosol parameter required for quantification of direct aerosol radiative forcing and has been linked to changes in atmospheric circulations and large scale precipitation patterns and to cloud macrophysical properties over large regions. Thus, a satellite aerosol product that quantifies absorbing aerosols will make a significant contribution to climate science. Absorbing aerosols also confuse atmospheric correction over oceans and interfere with determination of ocean water-leaving radiances, even at low aerosol loading. Thus, both the atmospheric and oceanic communities are united by their need to identify and quantify absorbing aerosols over the global oceans. The Pre-Aerosols-Clouds-Ecosystem (PACE) mission will attempt to push oceanographic science a full step forward by defining the Ocean Color Instrument (OCI) with hyperspectral capability from the ultraviolet (UV) to the near-infrared (NIR), and with additional wavelengths in the shortwave infrared (SWIR). We propose here that the basic PACE OCI instrument, with no enhancement, will also make a significant improved contribution to aerosol science. OCI is an exciting instrument for aerosol scientists because it will be the first truly broad spectrum U.S. instrument, effectively combining the aerosol-retrieval capabilities of MODIS and OMI, but on the same instrument, at the same spatial resolution. Sensitivity tests tell us that we should be able to retrieve 3 pieces of aerosol information from this configuration: nominally loading, absorption and either particle size or layer height. These published sensitivity tests are far from complete and they do not cover the wide range of circumstances that are required to identify optimal wavelength configurations and retrieval assumptions necessary to prepare for producing an aerosol product from OCI. Here we propose a series of theoretical studies to determine the uncertainties involved in 1) identifying absorbing aerosol at low aerosol optical depth (AOD) for the purposes of atmospheric correction, and 2) retrieving aerosol information including AOD and absorption at moderate to high AOD. Our focus is on the over ocean retrievals, where the new broad spectrum OCI offers enhanced possibilities, but this will be overlaid on a global (ocean and land) product that would represent adapting existing OMI and MODIS algorithms to OCI radiances. We are offering a perspective towards atmospheric correction that is aerosol-centered, proposed by investigators who are undisputed experts in aerosol retrieval from an OCI type of sensor. This work represents the major theoretical exploration and defining of uncertainties necessary for producing an operational product from satellite data. In addition, the PI (Remer) is volunteering to serve in a leadership capacity on the PACE Science Team.