Atmospheric haze is a phenomenon where the clarity of the sky is diminished, creating an opaque, milky-white, or brownish appearance. This reduction in visibility is a direct result of sunlight being scattered and absorbed by a concentration of extremely small airborne particles known as aerosols. These aerosols are a suspension of solid or liquid matter in the atmosphere, ranging in size from a few nanometers up to several micrometers. Haze is distinct from clouds or fog, which are formed primarily by condensed water droplets, because haze particles exist as dry or semi-liquid suspensions and do not require near-saturation humidity to form.
Haze from Industrial and Vehicle Emissions
Combustion sources associated with industry and transportation are significant contributors to atmospheric opacity in populated areas. These processes release gaseous pollutants, such as sulfur dioxide (\(text{SO}_2\)) and nitrogen oxides (\(text{NO}_x\)), which are colorless upon emission. These gases undergo complex chemical reactions in the atmosphere, converting into microscopic solid or liquid particles known as secondary inorganic aerosols.
The primary components of this industrial haze are fine particulate matter (\(text{PM}_{2.5}\)), specifically ammonium sulfates and ammonium nitrates. These particles are typically less than \(2.5\) micrometers in diameter, making them highly efficient at scattering visible light. This scattering, especially of blue light, is what creates the characteristic gray or brownish smog that blankets urban and industrial regions. Vehicle exhaust also contributes black carbon, a form of elemental carbon (soot), which absorbs light and further darkens the haze layer.
The Impact of Wildfire Smoke and Biomass Burning
Haze from wildfires, agricultural burns, and prescribed forest management fires introduces a distinct type of aerosol into the atmosphere on a massive, regional scale. This biomass burning generates a complex mixture of primary pollutants, with a large fraction consisting of black carbon and organic aerosols. The organic fraction, sometimes referred to as brown carbon, is what gives smoke plumes their noticeably different coloration.
Unlike the gray-brown tint of industrial smog, fresh smoke often imparts a more yellowish or reddish hue to the sky because brown carbon strongly absorbs light in the blue and ultraviolet wavelengths. The composition of this smoke plume evolves over time, with volatile organic compounds (VOCs) reacting to form secondary organic aerosols (SOA) as the plume travels. This aging process alters the particle’s physical properties and its light-scattering efficiency.
Windblown Dust and Natural Geological Sources
Natural geological processes contribute to haze through the mobilization of mineral dust and other crustal particles. Large-scale events, such as desert dust storms originating from the Sahara or the Gobi desert, inject massive amounts of material into the atmosphere, forming vast, long-lasting layers. This aeolian dust is composed of heavy mineral particles, including silicates, metal oxides, and calcium compounds.
These geological aerosols are generally larger and denser than combustion-generated particles, falling mostly into the coarse particulate matter fraction (\(text{PM}_{10}\)). Their transport is heavily reliant on strong wind systems, which can lift them to high altitudes and carry them across continents and oceans, such as the Saharan Air Layer. Smaller, localized haze can also be created by agricultural tilling or volcanic eruptions, which release ash plumes containing pulverized rock and glass fragments.
How Humidity Magnifies Haze
The presence of atmospheric water vapor significantly enhances the haziness caused by dry aerosol particles. This magnification occurs because many common aerosols, particularly the sulfates and nitrates from industrial sources, are hygroscopic, meaning they readily attract and absorb water vapor. As the relative humidity (\(text{RH}\)) increases, these particles act as condensation nuclei, leading to a process called hygroscopic growth.
During this growth, the particle’s diameter can swell dramatically, transforming into a much larger, water-rich droplet. This increase in size shifts the physics of light interaction from Rayleigh scattering to Mie scattering, which is far more efficient at scattering visible light in all directions. Consequently, the same amount of pollution on a dry day will cause less visibility impairment than on a humid day, when the water uptake has substantially increased the aerosol’s optical efficiency.