Visibility: Science and Regulation

Abstract

The 1999 Regional Haze Rule provides a context for this review of visibility, the science that describes it, and the use of that science in regulatory guidance. The scientific basis for the 1999 regulation is adequate. The deciview metric that tracks progress is an imperfect but objective measure of what people see near the prevailing visual range. The definition of natural visibility conditions is adequate for current planning, but it will need to be refined as visibility improves. Emissions from other countries will set achievable levels above those produced by natural sources. Some natural events, notably dust storms and wildfires, are episodic and cannot be represented by annual average background values or emission estimates. Sulfur dioxide (SO2) emission reductions correspond with lower sulfate (SO4(2-)) concentrations and visibility improvements in the regions where these have occurred. Non-road emissions have been growing more rapidly than emissions from other sources, which have remained stable or decreased since 1970. Simpler models representing transport, limiting precursor pollutants, and gas-to-particle equilibrium should be used to understand where and when emission reductions will be effective, rather than large complex models that have insufficient input and validation measurements. Examples of model-based source attribution show large differences among estimates from various modeling systems and with ambient measurements.

Airborne measurements of aerosol extinction in the lower and middle troposphere over Wyoming, USA

“…They result primarily from the overall lowmass loading of the sampled air, and, possibly, from the adsorption of trace gases on the filter media, and/or the loss of volatile species. The sum of the mass loadings of individual species, measured on the channel II quartz filter, is invariably greater than the gravimetrically determined total mass from the deposit on the channel I Teflon filter, suggesting possible loss of volatile material Watson et al, 2001;Watson, 2002). The most abundant ionic species were sulfate and ammonium, with lesser amounts of nitrate, and even smaller amounts of sodium and chloride.…”

“…While some hydration of the aerosol might be anticipated at higher RH values (Tang, 1996;Day et al, 2000;Gasso et al, 2000;Day and Malm, 2001), the low RH (o15%) within the nephelometer inside the aircraft would ensure that particles would be effectively dried. Warming of the sampled airstream could also result in a shift in size distribution to lower sizes as a result of partial or total volatilization of some particles (Bergin et al, 1997;Watson, 2002), resulting in a reduced scattering signal. Were observed scattering extinctions to have been as high as those calculated, the average observed SSA value would become 0.8570.05, and the ratio of calculated to observed SSA would fall to 0.9770.07.…”

Carbonaceous and Secondary Inorganic Aerosols during Wintertime Fog and Haze over Urban Sites in the Indo-Gangetic Plain

“…Among these, fog-haze formation and reduction in visibility is of major concern (Watson, 2002;. The formation of fog and haze is associated with high aerosol loading from anthropogenic sources as well as formation of secondary aerosols via gas-to-particle conversion under favorable meteorological conditions (Reilly et al, 2001;Husain et al, 2004;Sun et al, 2006;Fang et al, 2011;).…”

Changes in visibility with PM2.5 composition and relative humidity at a background site in the Pearl River Delta region

“…As the deteriorated visibility is mainly due to visible light extinction by fine particles, the light extinction coefficient (b ext ) is a vital parameter to reveal ambient air pollution by PM 2.5 (Malm et al, 1994;Watson, 2002). Theoretically, b ext is affected by the nature of airborne particles, such as size distribution, chemical composition, mixing state and hygroscopic behavior (Seinfeld and Pandis, 2006).…”