Quantification of atmospheric visibility with dual digital cameras during daytime and nighttime

and Physics cess Atmospheric Measurement Techniques Open Access Biogeosciences Open Access Atmos. Meas. Tech., 6, 2121–2130, 2013 www.atmos-meas-tech.net/6/2121/2013/ doi:10.5194/amt-6-2121-2013 © Author(s) 2013. CC Attribution 3.0 License. of the Past K. Du, K. Wang, P. Shi, and Y. Wang Open Access Quantification of atmospheric visibility with dual digital cameras during daytime and nighttime Climate Institute of Urban Environment, Chinese Academy of Sciences, Xiamen, China Earth System Dynamics Received: 21 November 2012 – Published in Atmos. Meas. Tech. Discuss.: 2 January 2013 Revised: 19 July 2013 – Accepted: 19 July 2013 – Published: 27 August 2013 Open Access Open Access Published by Copernicus Publications on behalf of the European Geosciences Union. Open Access Atmospheric visibility can be described by the maximum horizontal distance at which a target with a sky background can be visually observed by human eyes (Horvath, 1981). Usually, it is also interpreted as “visual range” (Malm, 1979), which is determined with different definitions of threshold contrast. For example, Koschmieder (1924) used a threshold Open Access Introduction Open Access 1 contrast of 0.02 to calculate Geoscientific atmospheric visibility, while the WMO (World Meteorological Organization, 1971) uses 0.05 Instrumentation as the threshold contrast. To make our results comparable Methods and to calculations of visibility reported by most research, we Data ofSystems selected the threshold contrast 0.02 in this study. Atmospheric visibility has decreased over the globe since the 1970s (Wang et al., 2009). Visibility degradation is highly associated with atmosphericGeoscientific pollution, which affects not only human health but also the safety of air and road transportaModel tion. Another issue is thatDevelopment the particles that impair visibility also contribute to a change of the global radiation balance, which, in turn, affects climate. In air quality research, visibility reflects the extent of polHydrology and lution by particulate matters in the air (Charlson, 1969), and System therefore is regulated andEarth measured regularly. Most meteorological stations in China applySciences the human visual range observation method to determine atmospheric visibility. However, human perception is influenced by a number of factors such as target illumination (brightness), background illumination, target geometry, air pollution levels along the obserOcean Science vation, and scenic characteristics (Malm, 1999). The “human eye” method requires the observer to make a visibility measurement by synthesizing the impact of these factors subjectively. Errors are introduced due to subjectivity because human eyes possess different thresholds for contrast perceptions for the same target. Middleton tested 1000 peoSolid(1952) Earth ple to find that the threshold contrast varies from 0.01 to 0.20. This difference would lead to completely different visibility estimation by these people in comparison to the meteorological range with a threshold contrast of 0.02. Therefore, optical instruments, such as transmissometer, were developed to measure the light extinction, which can be used to calculate The Cryosphere visibility. Instrumentation-based visibility measurements are Open Access Abstract. A digital optical method “DOM-Vis” was developed to measure atmospheric visibility. In this method, two digital pictures were taken of the same target at two different distances along the same straight line. The pictures were analyzed to determine the optical contrasts between the target and its sky background and, subsequently, visibility is calculated. A light transfer scheme for DOM-Vis was delineated, based upon which algorithms were developed for both daytime and nighttime scenarios. A series of field tests were carried out under different weather and meteorological conditions to study the impacts of such operational parameters as exposure, optical zoom, distance between the two camera locations, and distance of the target. This method was validated by comparing the DOM-Vis results with those measured using a co-located Vaisala® visibility meter. The visibility under which this study was carried out ranged from 1 to 20 km. This digital-photography-based method possesses a number of advantages compared with traditional methods. Pre-calibration of the detector with a visibility meter is not required. In addition, the application of DOM-Vis is independent of several factors like the exact distance of the target and several camera setting parameters. These features make DOM-Vis more adaptive under a variety of field conditions. Open Access Correspondence to: K. Du () M 2122 K. Du et al.: Quantification of atmospheric visibility with dual digital cameras more “objectively” and independent of human observations. Transmissometers quantify visibility by measuring the light extinction of the atmosphere between the transmitter and the receiver. An optical path of 300 m–2 km (Auvermann et al., 2004) is usually required. In addition, the reliability of this method relies on the stability of both the light source and the photosensitive device at the receiving end. Another type of optical instruments, called the scatterometer, is based on forward light scattering. The transmitter and receiver are placed less 1 m apart with their optical axes crossing each other at a certain angle. Light scattering is quantified based on the scattered light received by the receiver, and thereby light extinction can be calculated with assumed single scattering albedo. Visibility can then be calculated from the light extinction. This technology generates a more stable signal than transmissometry because the transmitter and receiver are fixed on one rigid frame of the scatterometer, while they are separated far apart (from 10 m to more than 1000 m for transmissometers). However, the results of scatterometer are prone to being biased by local pollution, because the small sampling volume makes the result not representative of the visibility of the ambient atmosphere over a larger spatial area. Photographic methods have been developed to estimate atmospheric visibility. In the 1980s, Richard et al. (1989) developed a method to monitor atmospheric visibility using a film camera. In this method, calibration was performed to quantify the relationship between the film density and the radiance received by the camera using a teleradiometer and panels with different grayscale values. The atmospheric visibility was calculated by analyzing the signal recorded on the film. Most recently, methods were developed to determine atmospheric visibility using digital cameras, which can be categorized into two groups according to their working principles. The first group of methods determines visibility by measuring the apparent contrast of a distant target against its background. Xie et al. (1999) developed a digital photographic visibility sys (...truncated)