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 five years, 1994–98. The data processing consisted of three broad categories of filters: (1) validity of individual data points, (2) filters based on statistics for specific stations, and (3) filters based on spatial analysis. The data are presented as the aerosol extinction coefficient (Bext or haze) at the surface, seasonally aggregated over five 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 coefficient >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 qualification 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 reflectance data from the SeaWiFS satellite.