Distribution of continental surface aerosol extinction based on visual range data

The global continental haze pattern was evaluated based on daily average visibility data at 7000 surface weather stations over "ve years, 1994}98. The data processing consisted of three broad categories of "lters: (1) validity of individual data points, (2) "lters based on statistics for speci"c stations, and (3) "lters based on spatial analysis. The data are presented as the aerosol extinction coe$cient (Bext or haze) at the surface, seasonally aggregated over "ve years. The data reveal that the continental haze is concentrated over distinct aerosol regions of the world. The haziest regions of Asia are the Indian subcontinent, eastern China, and Indochina where the 75 percentile seasonal Bext exceeds 0.4 km~1. In Africa, the highest year around extinction coe$cient’0.4 km~1 is found over Mauritania, Mali and Niger. During December, January, February, the savanna region of sub-Saharan Africa shows similar values. The haziest region of South America is over Bolivia, adjacent to the Andes mountain range, with a peak during August}November (0.4}0.6 km~1). In North America and Europe, there are isolated haze pockets, such as the San Joaquin Valley in California and the Po River Valley in the northern Italy. In many regions of the world the size, shape, and intensity of hazy pockets is determined by the topographic barriers. A major quali"cation of this work is that the haze maps are based on daily average visibility which emphasizes humid regions with hygroscopic aerosols (nighttime peak Bext) and de-emphasizes arid, dusty regions with daytime maximum extinction. Regional haze episodes over several continental aerosol regions are illustrated by truecolor rendering of the re#ectance data from the SeaWiFS satellite. ( 2000 Elsevier Science Ltd. All rights reserved.

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