Clean Air Initiative: GlobalClean Air Initiative: AsiaIniciativa del Aire Limpio: América LatinaClean Air Initiative: Sub-Saharan Africa
Advanced Search
Dialogue room
Mailing List


Regional and Urban Haze
Regional haze is visibility impairment that occurs over a wide geographical area. This is different than urban haze which obscures visibility over cities.

Satellite Imagery of Regional Haze over China on Jan. 12 2004

The principle cause of regional and urban haze, or visibility impairment, is light extinction caused by fine particles, primarily sulfates. It can also be caused by organic and elemental carbon, nitrates and crustal matter. Particles between 0.1 and one micrometers in size are most effective at scattering light, in addition to being of greatest concern for human health.

Haze in the US

In the eastern U.S., reduced visibility is mainly attributable to secondary particles. Based on data collected by the Interagency Monitoring of Protected Visual Environments (IMPROVE) network for visibility monitoring, sulfate particles account for about 50-70 percent of annual average light extinction in eastern locations. Sulfate plays a particularly significant role in the humid summer months, most notably in the Appalachian, northeast, and mid-south regions. Nitrates, organic carbon, and elemental carbon each account for between 10-15 percent of total light extinction in most eastern locations. Rural areas in the eastern U.S. generally have higher levels of impairment than most remote sites in the western U.S., generally due to the eastern U.S.'s higher levels of man-made pollution, higher estimated background levels of fine particles, and higher average relative humidity levels. The relative contribution of individual pollutants to visibility impairment varies geographically. While secondary particles still dominate in the West, direct particulate emissions from sources such as wood smoke contribute a larger percentage of the total particulate load than in the East. In the rural western U.S., sulfates also play a significant role, accounting for about 25-40 percent of estimated total light extinction in most regions. In some areas, such as the Cascades region of Oregon, sulfates are estimated to account for over 50 percent of annual average light extinction. Organic carbon typically is estimated to be responsible for 15-35 percent of total light extinction in the rural western U.S. and elemental carbon (absorption) accounts for about 15-25 percent, so the total carbonaceous contribution is between 30 and 60 percent. Soil dust (coarse PM) accounts for about 10-20 percent. Nitrates typically account for less than 10 percent of visibility impairment. Visibility is greatly affected by ambient PM2.5 concentration, with PM2.5 concentrations below the NAAQS being sufficient to impair visibility. Black elemental carbon particles are a dominant light adsorbing species in the atmosphere, and a major component of diesel exhaust.


Go to the BAQ 2004 website
Regional and global effects
Regional and global effects > Regional haze

Secretariat: The World Bank & Asian Development Bank