Visibility And Air Pollution
Author(s)
J. G. Watson & J. C. Chow
Abstract
Haze is caused by the scattering and absorption of visibility light by particles and
gases. Light is an electromagnetic wave, and just as a plane water wave is
deflected by a barrier from its original direction, light waves are scattered when
they encounter particles and gas molecules that are approximately the same size
as the light’s wavelength. The sky is blue because particle-free air also scatters
light, but the gas molecules are so small that they scatter the shorter wavelength
blue light more than they scatter the longer wavelength red light. Total
extinction (bext), expressed in inverse megameters (Mm-1), is multiplied by the
distance between an observer and a target to indicate the amount of light
removed from the site path by scattering and absorption by particles and gases,
and is the most common measurement used to quantify visibility. Total
extinction does not take into consideration light scattered into the sight path that
further reduces the target’s contrast (ratio of light transmitted from the target to
the light transmitted from the horizon). Defining natural conditions is a scientific
challenge. Annual average estimates are currently in use, but these will
eventually need to be made more event-specific. Wildfires, dust storms, and
other natural events will affect visibility on a case-by-case basis. Transport from
outside of provincial and national boundaries needs to be considered because this
is largely beyond the control of authorities.
Keywords: haze, visibility, PM.
1 Introduction
This paper provides an overview of urban and regional haze and its relationships
to other pollutants. It describes the nature and causes of poor visibility, identifies
relationships to fine particles and other pollutants, explains regulatory
approaches to local and regional haze, and discuses relevance to other regional
Keywords
haze, visibility, PM.
Related Book
Other papers in this volume
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