Derek Griffith

Direct radiative forcing of urban aerosols over Pretoria (25.75°S, 28.28°E) using AERONET Sunphotometer data: first scientific results and environmental impact.

The present study uses the data collected from Cimel Sunphotometer of Aerosol Robotic Network (AERONET) for the period from January to December, 2012 over an urban site, Pretoria (PTR; 25.75°S, 28.28°E, 1449 m above sea level), South Africa. We found that monthly mean aerosol optical depth (AOD, τ(a)) exhibits two maxima that occurred in summer (February) and winter (August) having values of 0.36 ± 0.19 and 0.25 ± 0.14, respectively, high-to-moderate values in spring and thereafter, decreases from autumn with a minima in early winter (June) 0.12 ± 0.07. The Angstrom exponents (α440-870) likewise, have its peak in summer (January) 1.70 ± 0.21 and lowest in early winter (June) 1.38 ± 0.26, while the columnar water vapor (CWV) followed AOD pattern with high values (summer) at the beginning of the year (February, 2.10 ± 0.37 cm) and low values (winter) in the middle of the year (July, 0.66 ± 0.21 cm). The volume size distribution (VSD) in the fine-mode is higher in the summer and spring seasons, whereas in the coarse mode the VSD is higher in the winter and lower in the summer due to the hygroscopic growth of aerosol particles. The single scattering albedo (SSA) ranged from 0.85 to 0.96 at 440 nm over PTR for the entire study period. The averaged aerosol radiative forcing (ARF) computed using SBDART model at the top of the atmosphere (TOA) was -8.78 ± 3.1 W/m², while at the surface it was -25.69 ± 8.1 W/m² leading to an atmospheric forcing of +16.91 ± 6.8 W/m², indicating significant heating of the atmosphere with a mean of 0.47K/day.

Non-uniformity correction and bad pixel replacement on LWIR and MWIR images

To fully exploit the potential of current generation infrared focal plane arrays, it is crucial to correct for the fixed pattern noise. This paper presents two-point non-uniformity corrections (NUC) applied to infrared images acquired with long wave and medium wave infrared cameras. Pixels that are not corrected by the NUC process, defined as bad pixels, were identified and replaced using a nearest neighbor algorithm.

Laboratory panel and radiometer calibration

This paper presents the results of the laboratory and field based reflectance panel and radiometer comparisons that took place as part of the CEOS 2010 Key comparison of “techniques and instruments used for the vicarious calibration of Land surface imaging through a ground reference standard test site”. The results of the comparisons are presented which shows that the different ways in which reflectance panels are calibrated can give different results for the reflectance attributed to a test site and that changing illumination and environmental conditions can effect the measured target reflectance.

Tuz Gölü site Characteristics

The Tuz Gölü site does demonstrate some challenges for being used as an absolute calibration site. These include: spatial and temporal variability on the order of +/− 0.02, atmospheric transmittance variability on the order of between 0.52 and 0.86 at 380 nm, lower spectral reflectance levels of less than 0.10 in the longer wavelengths, and limitations on the site availability (only two month period in summer). But, with each successful campaign made at this location, these challenges are being overcome and site characteristics have become better understood and techniques for compensation are being devised. With proper techniques, instrumentation, and coincident data collection this site remains a viable candidate as a vicarious calibration site for optical imaging sensors.

Estimating uncertainty in resolution tests

Resolution testing of imaging optical equipment is still com- monly performed using the USAF 1951 target. The limiting resolution is normally calculated from the group and element that can just be resolved by an observer. Although resolution testing has limitations, its appeal lies in the fact that it is a quick test with low complexity. Resolution uncer- tainty can serve as a diagnostic tool, aid in understanding observer vari- ability, and assist in planning experiments. It may also be necessary to satisfy a customer requirement or international standard. This paper de- rives theoretical results for estimating resolution and calculating its un- certainty, based on observer measurements, while taking the target spatial-frequency quantization into account. We show that estimating the resolution by simply averaging the target spatial frequencies yields a biased estimate, and we provide an improved estimator. An application illustrates how the results derived can be incorporated into a larger un-

Comparison of slant-path scintillometry, sonic anemometry and high-speed videography for vertical profiling of turbulence in the atmospheric surface layer

The optical effect of atmospheric turbulence greatly inhibits the achievable range of Detection, Recognition and Identification (DRI) of targets when using imaging sensors within the surface layer. Since turbulence tends to be worst near the ground and decays with height, the question often arises as to how much DRI range could be gained by elevating the sensor. Because this potential DRI gain depends on the rate of decay of turbulence strength with height in any particular environment, there is a need to measure the strength profile of turbulence with respect to height in various environments under different atmospheric and meteorological conditions. Various techniques exist to measure turbulence strength, including scintillometry, sonic anemometry, Sound Detection and Ranging (SODAR) and the analysis of point source imagery. These techniques vary in absolute sensitivity, sensitivity to range profile, temporal and spatial response, making comparison and interpretation challenging. We describe a field experiment using multiple scintillometers, sonic anemometers and point source videography to collect statistics on atmospheric turbulence strength at different heights above ground. The environment is a relatively flat, temperate to sub-tropical grassland area on the interior plateau of Southern Africa near Pretoria. The site in question, Rietvlei Nature Reserve, offers good spatial homogeneity over a substantial area and low average wind speed. Rietvlei was therefore chosen to simplify comparison of techniques as well as to obtain representative turbulence profile data for temperate grassland. A key element of the experimental layout is to place a sonic anemometer 15 m above ground at the centre of a 1 km slant-path extending from ground level to a height of 30 m. An optical scintillometer is operated along the slant-path. The experiment layout and practical implementation are described in detail and initial results are presented.

Spectral reflectance measurement methodologies for Tuz Golu field campaign

A field campaign had been organized in August 2010 on Tüz Gölü salt lake, Turkey, with the aim of characterizing the site for satellite optical sensor vicarious calibration, and of comparing different methodologies of surface reflectance factor characterization. Several teams have made ground-based reflectance measurements with a field spectrometer on different areas of the salt lake of 100 m × 300 m and 1km × 1 km size. Different types of sampling strategies and measurements methods have been used by the participants, and are described in this paper. Preliminary results on one area are presented, that show a good agreement between the different measurements.

Global Sources of Fine Particulate Matter: Interpretation of PM2.5 Chemical Composition Observed by SPARTAN using a Global Chemical Transport Model

Exposure to ambient fine particulate matter (PM2.5) is a leading risk factor for the global burden of disease. However, uncertainty remains about PM2.5 sources. We use a global chemical transport model (GEOS-Chem) simulation for 2014, constrained by satellite-based estimates of PM2.5 to interpret globally dispersed PM2.5 mass and composition measurements from the ground-based surface particulate matter network (SPARTAN). Measured site mean PM2.5 composition varies substantially for secondary inorganic aerosols (2.4-19.7 μg/m3), mineral dust (1.9-14.7 μg/m3), residual/organic matter (2.1-40.2 μg/m3), and black carbon (1.0-7.3 μg/m3). Interpretation of these measurements with the GEOS-Chem model yields insight into sources affecting each site. Globally, combustion sectors such as residential energy use (7.9 μg/m3), industry (6.5 μg/m3), and power generation (5.6 μg/m3) are leading sources of outdoor global population-weighted PM2.5 concentrations. Global population-weighted organic mass is driven by the residential energy sector (64%) whereas population-weighted secondary inorganic concentrations arise primarily from industry (33%) and power generation (32%). Simulation-measurement biases for ammonium nitrate and dust identify uncertainty in agricultural and crustal sources. Interpretation of initial PM2.5 mass and composition measurements from SPARTAN with the GEOS-Chem model constrained by satellite-based PM2.5 provides insight into sources and processes that influence the global spatial variation in PM2.5 composition.

Comparison between different measurement techniques investigating optical turbulence along a slant path in the atmospheric surface layer

The vertical distribution of optical turbulence was investigated on a measurement campaign over an Highveld grassland in South Africa. Results of measurements along a slant path in the atmospheric surface layer with different techniques are presented and discussed.

Exploration of satellite-derived data products for atmospheric turbulence studies

The quality, availability and diversity of satellite-derived earth observation data products are continuously improving. Such satellite products can provide an extensive and complementary view on many matters with respect to intensive but localised in-situ or ground measurements. A search has been undertaken on the available types and sources of satellite data products that could be applicable in the study of the spatio-temporal distribution of aero-optical turbulence in the atmospheric boundary layer. This has included all satellite data products that are relevant to the surface energy balance such as surface reflectance, temperature and emissivity. It was also important to identify active archive data services that can provide preprocessed and quality-filtered time-series products. Products derived from the Moderate Resolution Imaging Spectrometer (MODIS) and other sensors on the NASA Terra and Aqua platforms were of special interest. The use of climatological shortwave and longwave radiative transfer models, combined with satellite-derived data was explored as a method of elucidating the surface heat balance. An in-situ dataset from the Rietvlei vertical turbulence profiling campaign of 2013 was used to validate a number of aspects of the satellite-derived heat balance approach.