Table 1.
Terms and definitions related to the visibility tracking metric used in regional haze planning.
| Term | Definition |
|---|---|
| Impairment or anthropogenic impairment | As defined in U.S. EPA 2017: “any humanly perceptible difference due to air pollution from anthropogenic sources between actual visibility and natural visibility on one or more days. Because natural visibility can only be estimated or inferred, visibility impairment also is estimated or inferred rather than directly measured.” |
| Most impaired days | The 20% of IMPROVE monitored days in a calendar year with the highest fraction of total haze attributed to anthropogenic sources. |
| U.S. anthropogenic | U.S. emissions defined in national, state, and local inventories as being emitted by anthropogenic activities |
| Natural haze | Fraction of total haze attributed to naturally occurring phenomena |
| Episodic natural | Aerosol light extinction representing episodic extreme natural events of carbon (organic matter carbon plus elemental carbon) or dust (coarse mass plus fine soil) and calculated as the minimum annual 95th percentile value in the period 2000 to 2014 |
| Routine natural | Aerosol light extinction representing for each aerosol species the fraction of daily non-episodic aerosol light extinction assigned to natural emissions. The fraction is calculated as the average Natural Conditions II aerosol extinction divided by the total annual average non-episodic aerosol extinction. |
| U.S. background (USB) | Combined U.S. natural emissions and international natural plus anthropogenic emissions |
| Natural conditions | As defined in U.S. EPA 2017: “Natural conditions reflect naturally occurring phenomena that reduce visibility as measured in terms of light extinction, visual range, contrast, or coloration, and may refer to the conditions on a single day or a set of days. These phenomena include, but are not limited to, humidity, fire events, dust storms, volcanic activity, and biogenic emissions from soils and trees. These phenomena may be near or far from a Class 1 area and may be outside the United States.” |