Felix C. Seidel

APEX - the hyperspectral ESA Airborne Prism Experiment

The airborne ESA-APEX (Airborne Prism Experiment) hyperspectral mission simulator is described with its distinct specifications to provide high quality remote sensing data. The concept of an automatic calibration, performed in the Calibration Home Base (CHB) by using the Control Test Master (CTM), the In-Flight Calibration facility (IFC), quality flagging (QF) and specific processing in a dedicated Processing and Archiving Facility (PAF), and vicarious calibration experiments are presented. A preview on major applications and the corresponding development efforts to provide scientific data products up to level 2/3 to the user is presented for limnology, vegetation, aerosols, general classification routines and rapid mapping tasks. BRDF (Bidirectional Reflectance Distribution Function) issues are discussed and the spectral database SPECCHIO (Spectral Input/Output) introduced. The optical performance as well as the dedicated software utilities make APEX a state-of-the-art hyperspectral sensor, capable of (a) satisfying the needs of several research communities and (b) helping the understanding of the Earths complex mechanisms.

Attributing Accelerated Summertime Warming in the Southeast United States to Recent Reductions in Aerosol Burden: Indications from Vertically-Resolved Observations

During the twentieth century, the southeast United States cooled, in direct contrast with widespread global and hemispheric warming. While the existing literature is divided on the cause of this so-called “warming hole,” anthropogenic aerosols have been hypothesized as playing a primary role in its occurrence. In this study, unique satellite-based observations of aerosol vertical profiles are combined with a one-dimensional radiative transfer model and surface temperature observations to diagnose how major reductions in summertime aerosol burden since 2001 have impacted surface temperatures in the southeast US. We show that a significant improvement in air quality likely contributed to the elimination of the warming hole and acceleration of the positive temperature trend observed in recent years. These reductions coincide with a new EPA rule that was implemented between 2006 and 2010 that revised the fine particulate matter standard downward. Similar to the southeast US in the twentieth century, other regions of the globe may experience masking of long-term warming due to greenhouse gases, especially those with particularly poor air quality.

Coupled retrieval of aerosol properties and land surface reflection using the Airborne Multiangle SpectroPolarimetric Imager (AirMSPI)

The Airborne Multiangle SpectroPolarimetric Imager (AirMSPI) has been flying aboard the NASA ER-2 high altitude aircraft since October 2010. In step-and-stare operation mode, AirMSPI acquires radiance and polarization data in bands centered at 355, 380, 445, 470*, 555, 660*, 865*, and 935 nm (* denotes polarimetric bands). The imaged area covers about 10 km by 11 km and is typically observed from 9 viewing angles between ±66° off nadir. For a simultaneous retrieval of aerosol properties and surface reflection using AirMSPI, an efficient and flexible retrieval algorithm has been developed. It imposes multiple types of physical constraints on spectral and spatial variations of aerosol properties as well as spectral and temporal variations of surface reflection. Retrieval uncertainty is formulated by accounting for both instrumental errors and physical constraints. A hybrid Markov-chain/adding-doubling radiative transfer (RT) model is developed to combine the computational strengths of these two methods in modeling polarized RT in vertically inhomogeneous and homogeneous media, respectively. Our retrieval approach is tested using 27 AirMSPI datasets with low to moderately high aerosol loadings, acquired during four NASA field campaigns plus one AirMSPI pre-engineering test flight. The retrieval results including aerosol optical depth, single scattering albedo, aerosol size and refractive index are compared with AERONET aerosol reference data. We identify the best angular combinations for 2-, 3-, 5-, 7-angle observations from the retrieval quality assessment of various angular combinations. We also explore the benefits of polarimetric and multiangular measurements, and target revisits in constraining aerosol property and surface reflection retrieval.

Optimizing irradiance estimates for coastal and inland water imaging spectroscopy

Next generation orbital imaging spectrometers, with advanced global remote sensing capabilities, propose to address outstanding ocean science questions related to coastal and inland water environments. These missions require highly accurate characterization of solar irradiance in the critical 380–600 nm spectral range. However, the irradiance in this spectral region is temporally variable and difficult to measure directly, leading to considerable variance between different models. Here we optimize an irradiance estimate using data from the NASA airborne Portable Remote Imaging Spectrometer (PRISM), leveraging spectrally smooth in-scene targets. We demonstrate improved retrievals for both PRISM and the Next Generation Airborne Visible Infrared Imaging Spectrometer.

Photopolarimetric Sensitivity to Black Carbon Content of Wildfire Smoke: Results From the 2016 ImPACT‐PM Field Campaign

Detailed characterization of the aerosol content of wildfire smoke plumes is typically performed through in situ aircraft observations, which have limited temporal and spatial coverage. Extending such observations to regional or global scales requires new remote sensing approaches, such as retrievals that make use of spectropolarimetric, multiangle imaging. In this work measurements made during the Imaging Polarimetric Assessment and Characterization of Tropospheric Particulate Matter (ImPACT‐PM) field campaign in a smoke plume near the town of Lebec in Southern California by the Navy Center for Interdisciplinary Remotely Piloted Aircraft Studies Twin Otter aircraft on 8 July 2016 are used in conjunction with near‐coincident measurements from the Airborne Multiangle SpectroPolarimetric Imager (AirMSPI) on the National Aeronautics and Space Administration ER‐2 high‐altitude research aircraft to assess the sensitivity of spectropolarimetric measurements to the black carbon content of the plume. Tracking visible features in the smoke through the sequence of AirMSPI observations allowed the height of the plume to be estimated through geometric techniques. Then, by constraining the fractional amounts of the aerosol constituents with the in situ data, radiative closure was obtained through simulations performed with a polarimetric radiative transfer code, demonstrating the ability to constrain the black carbon mass fraction to approximately 5%, given the uncertainties in the AirMSPI measurements and the assumption of external mixing of aerosol components. The AirMSPI retrieval, made using a limited set of observations from the 470 nm polarimetric spectral band alone, was also generally consistent with operational retrievals of aerosol optical depth and surface reflectance made by the Multi‐Angle Implementation of Atmospheric Correction algorithm at 1 km resolution.

Evaluation of Near-UV/blue Aerosol Optical Thickness Retrieval from Airborne Hyperspectral Imagery

An aerosol retrieval algorithm is currently under development in scope of the upcoming APEX hyperspectral imager. It will be able to close the gap between global remote sensing and one-dimensional in situ measurements of atmospheric particles. This paper presents a feasibility study of the proposed APEX aerosol retrieval approach for hyperspectral imagery with high spatial resolution. The extraction of a sample aerosol optical thickness is done by fitting radiation transfer model results to measured at-sensor radiances at two near-UV/blue bands (394 and 404 nm) from the PHILLS imager. The PHILLS hyperspectral data are used to simulate the APEX Visible Near- Infrared detector and comprehend a dark surface reference target to avoid most uncertainties from the surface reflection contribution.

Improving MISR AOD Retrievals With Low-Light-Level Corrections for Veiling Light

Operational retrievals of aerosol optical depth (AOD) from Multi-angle Imaging SpectroRadiometer (MISR) data have been shown to have a high bias in pristine oceanic areas. One line of evidence involves comparison with Maritime Aerosol Network (MAN) observations, including the areas of low aerosol loading close to Antarctica. In this paper, a principal reason for the AOD overestimation is identified, which is stray light measured by the MISR cameras in dark regions of high-contrast scenes. A small fraction of the light from surrounding bright areas, such as clouds or sea ice, is redistributed to dark areas, artificially increasing their brightness. Internal reflections and light scattering from optical elements in MISR’s pushbroom cameras contribute to this veiling light effect. A simple correction model is developed that relies on the average scene brightness and an empirically determined set of veiling light coefficients for each MISR camera and wavelength. Several independent methods are employed to determine these coefficients. Three sets of coefficients are further implemented and tested in prototype MISR 4.4-km AOD retrievals. The results show dramatic improvements in retrieved AODs compared against MAN observations and the currently operational V22 MISR retrievals. For the best performing set of coefficients, the bias is reduced by 51%, from 0.039 to 0.019, the RMSE is lowered by 19%, from 0.062 to 0.050, and 84% of retrievals fall within the uncertainty envelope compared with 66% of retrievals in V22. The best performing set will be implemented operationally in the next V23 MISR AOD product release.

Imaging spectroscopy to understand the controls on cryospheric melting in a changing world

The global retreat of Earths cryosphere is the iconic symbol of climate change. Decades of satellite, airborne, and ground observations show clear evidence of increased melting of glaciers and ice sheets, declines in sea ice, and decreasing spring snow cover. This increased melting of cryosphere cover makes Earth more absorptive of sunlight and moves enormous volumes of stored water from frozen state to liquid, raising sea level and changing water availability to large populations. However, the distribution of forcings controlling this accelerated melting is poorly known. Atmospheric warming from greenhouse gases is contributing to this acceleration but its magnitude is uncertain due to our uncertainties in the controls on the dominant contributor to annual melt, absorbed sunlight, itself controlled by albedo. Despite this crucial role of albedo and solar radiation in snow and ice melt, sparse measurements have kept us from understanding the global distribution of controls on albedo, grain size and impurities, and from accurately modeling melt processes worldwide. Such an understanding is crucial to determining cryosphere melt and projecting its future behavior. Global spectroscopic measurements are required to improve our understanding of controls on cryosphere melt rates, enabling better predictions of future changes that will affect humankind.

Spectrophotometry Applications: Remote Sensing

Abstract Remote sensing measurements, and the spectral analysis of these data, provide scientists with a rich information source to better understand Earth processes. This chapter provides examples of how these measurements can assist in climate and other Earth-process studies. Remote sensing is the study of imagery and measurements made from balloon, aircraft, or in-orbit sensors. Global measurements can be made using remote sensing sensors that are space based. As an example, Greenhouse gases Observing SATellite has been measuring atmospheric carbon dioxide, CO2, and other greenhouse gases since 2009. Such measurements are important for training atmospheric transport models, which will allow us to better understand the sources and sinks of CO2 and the processes that control greenhouse gas exchanges between the Earths surface and its atmosphere. Cloud, snow, and ice studies are important to climate scientists, who wish to better understand the role these elements play in the hydrological cycle and in the Earths radiant energy fluxes. Remote-sensing data can also assist in volcano eruption predictions, and well as posteruption ash dispersion studies. Studies, such as these, reflect our desire to expand our understanding of Earth processes, as we learn about the impact that we have on our environment and seek to control and monitor that impact. Remote sensing spectrophotometry is an important and emerging field assisting us in this endeavor.