The impact of atmospheric particulate matter pollution on visibility

This paper describes overall visibility impairment situation of Milan city by atmospheric particulate matter pollution and its chemical composition during the time duration from August’02 to December’03. Visibility impairment is a basic form of air pollution that people can see and recognize without special instruments. Visibility impairment is a major atmospheric pollution problem in many mega cities around the world. The impact of air pollution by anthropogenic sources on human health and visibility has been studied for decades. Many analyses have been conducted worldwide not only to estimate the health benefits from air pollution abatement but also to identify scientific and technical understanding of how the air pollutants impair visibility. It is known that visibility impairment can result directly or indirectly from particle emission and is dominated by fine particulate matter (PM). Air pollutants can be roughly divided into two classes: gases and particles. Particles are composed of liquids or solids and are collectively referred to as particulate matter (PM). Most gaseous pollutants are invisible to the human eye including ozone, sulfur dioxide, and carbon dioxide. The individual particles in PM are so small that they are invisible (or nearly invisible) to the human eye but collectively they create haze. The visibility effects of haze are similar to the effects of fog; the main distinctions being that smog particles are smaller and are composed primarily of air pollutants not water. Examples of PM include diesel exhaust from motor vehicles, smoke from chimneys, and sulfuric acid droplets formed in the atmosphere. Although the PM from individual sources seems to disappear as it disperses in the atmosphere, it does not – it is merely diluted. In the process, the distinct plumes from individual sources merge into a featureless, uniform haze. PM can persist in the atmosphere for several days or weeks and be transported thousands of miles, affecting visibility locally, regionally, and globally. Particle pollution (also called particulate matter or PM) is the term for a mixture of solid particles and liquid droplets found in the air. Some particles, such as dust, dirt, soot, or smoke, are large or dark enough to be seen with the naked eye. Others are so small they can only be detected using an electron microscope. Particle pollution includes "inhalable coarse particles," with diameters larger than 2.5 micrometers and smaller than 10 micrometers and "fine particles," with diameters that are 2.5 micrometers and smaller. The chemical compositions stated are based on results collected in studies in most parts of the United States. The results indicate that sulphate, ammonium, hydrogen ions, elemental carbon, primary and secondary organic compounds from cooking and combustion, as well as certain transition metals are predominantly found in the fine particle mode, while crustal materials such as calcium, aluminium, silicon, magnesium, and iron are mostly discovered in coarse particles. Also found in the coarse mode are primary organic materials such as pollen, spores, and plant and animal debris. Furthermore, nitrate and potassium are found in both the fine and the coarse modes as their sources and mechanisms of formation may vary. Traditionally, visibility has been defined by meteorologists as the visual range defined as the furthest distance at which a black object can be distinguished against the horizon sky (EPA, 1979). As a consequence, visibility can only be quantified for a sight path and depends on the illumination of the atmosphere and the direction of view. When determining it as a quantity, the concentration of particles in the atmosphere plays a key role. Particles and gases in the atmosphere attenuate light on its way from an object to the observer. Visual range is the distance at which a given object can be seen with the unaided eye. The deciview scale is zero for pristine conditions and increases as visibility degrades. Each deciview change represents a perceptible change in visual air quality to the average person. Generally, a one deciview change in the haze index is likely perceptible by a person regardless of background visibility conditions.