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Aerosol: a dispersion of microscopic solid or liquid particles in a gaseous medium, such as smoke and fog


Aerosol Light Extinction (Bep):   The attenuation of light due to scattering and absorption of aerosols. According to the old IMPROVE algorithm, extinction is calculated according to the following equation:

           Bep = 3F(RH)[Sulfate] + 3F(RH)[Nitrate] + 4[OMC] + 10[LAC] + 1[Soil] + 0.6[CM]

In the following table the brackets [ ] denote concentrations of that chemical species.







All elemental S is from sulfate.  All sulfate is from ammonium sulfate.

Fossil fuel combustion



Denuder efficiency is close to 100%.  All nitrate is from ammonium nitrate.

Industrial and automobile emissions, organic decomposition
Organic Mass by Carbon (OMC) 1.4 * OC Average organic molecule is 70% carbon. Biomass burning, automobile emissions, fossil fuel combustion, gas-to-particle conversion of hydrocarbons
Light absorbing Carbon (LAC) EC1+EC2+EC3-OP
(see definitions below)
Incomplete combustion of fossil and biomass fuels

SOIL (fine soil)



[Soil K]=0.6[Fe].  FeO and Fe2O3 are equally abundant. A factor of 1.16 is used for MgO, Na2O, H2O, CO2.

Desert dust, construction, road Dust

CM (coarse mass)

[PM10] - [PM2.5]

Consists only of insoluble soil particles

Crushing or grinding operations, dust from paved or unpaved roads












AIRS: The EPA Aerometric Information Retrieval System


Alberta Clipper:


Area source:


ASOS: Automated Surface Observation System




BIA: Bureau of Indian Affairs


Bermuda-Azores High:


Biogenic source:


BLM: Bureau of Land Management


CASTNET: Clean Air Status and Trends network


CENRAP: Central Regional Air Planning organization




Chinook winds:


Cluster Analysis:


CM: Abbreviation for coarse mass.  This is defined as the difference in mass of the PM10 fraction minus the fine fraction or PM2.5.


CMB: Chemical Mass Balance receptor model


Conditional Probability: The conditional probability typically is the fraction of backtrajectories for which when air passed over a given area it met some condition (i.e. within 20% worst haze. What we have plotted here is the percent residence time over an area for worst sulfate, etc. days divided by the average residence time over an area. Values greater than 1 indicate more transport than average for the given condition (e.g. worst sulfate); values less than 1 indicate less frequent than average transport over an area during those conditions.


COOP: National Weather Service Cooperative Observation station. This is a network of surface meteorological observations stations in the US for the collection of daily maximum and minimum temperatures, precipitation and sometimes snow fall.


Deciview (DV): A metric of haze proportional to the logarithm of the atmospheric extinction (Bext).

Units: Unitless

Benefit: Under many circumstances a change in one deciview will be perceived to be the same on clear and hazy days.

Drawback:  Deciview is not easily related to gaseous and aerosol concentrations


DV = 10*ln(Bext/10)

DV = 10*ln(K/V.R./10) 


DRI: Desert Research Institute


EC: or sometimes indicated by LAC. This represents elemental carbon aerosols.  This type of aerosol is from incomplete combustion of fossil and biomass fuels.


EDAS: Eta Data Assimilation model system.  This is the primary meteorological data input to the HYSPLIT trajectory model used in the Causes of Haze Assessment.  For the years 1997 to April 2004 we used the 80-km resolution dataset.   From May 2004 to the present we use the 40-km version that is available from the NOAA Air Resource Laboratory's ftp site.


Emission Inventory: An emission inventory is a database of air emissions information with input from numerous State and local air agencies, tribes, and from industry.  This database contains information on stationary and mobile sources that emit criteria air pollutants and their precursors, as well as hazardous air pollutants (HAPs). The database includes estimates of annual emissions, by source, of air pollutants on an annual basis. Emissions Inventories are the basis for numerous efforts including trends analysis, regional, and local scale air quality modeling, regulatory impact assessments, and human exposure modeling.


FCIA: Federal class I Area


Fugitive Dust:


FWS: US Fish & Wildlife Service


GIS: Geographic Information System


Great Basin High:




IMPROVE: Interagency Monitoring of PROtected Visual Environments


IMPROVE protocol: Interagency Monitoring of PROtected Visual Environments


Incremental Probability: Also called residence time difference.  Residence time difference maps give the percent residence time for some condition (e.g. high sulfate) minus the percent residence time for all conditions ( i.e. average residence time).


Light Absorbing Carbon or LAC:


Light Extinction (Bext): The attenuation of light due to scattering and absorption as it passes through a medium.

Units: inverse distance, e.g. inverse mega meters (Mm-1)

Benefit:  Light extinction can be directly related to gaseous and aerosol concentrations.

Drawback:  Light extinction is non-linearly related to a person perception of changes in haze.  For example, a 10 Mm-1 increase in bext will have a larger perceived impact on a scene at Bext = 20 Mm-1 than at Bext = 100 Mm-1


Bext = K/V.R. 
Where K is the Koschmieder Coefficient – the natural log of the contrast threshold of the human eye,  K ranges from 3 to 3.9

Bext = 10*exp(DV/10) 


Marine Boundary Layer:


Microgram: One millionth of a gram (10-6 g) and also symbolized by g


Micron: One millionth of a meter (10-6 m) and also symbolized by m


Mixed layer:


Mm-1: A unit of light extinction called "Inverse megameters"


Mobile source:


Mountain/valley circulations:






Nighttime drainage flow:


NP: National Park


NPS: National Park Service


NO3: This refers to nitrate aerosol.  This is a secondary pollutant primarily from industrial and automobile emissions, and organic decomposition.




NOx: Gaseous nitrogen oxides


NWS: National Weather Service


OC: Organic carbon aerosol


Perceived Visual Air Quality (PVAQ): An index that relates directly to how human observers perceive changes in visual air quality.

Units:  Unitless

Benefit: Quantitatively, measures haze or visibility impairment to how human observers perceive changes in visual air quality.


Physiographic region:


PM10: Particulate matter with aerodynamic diameter less than 10 microns


PM2.5: Particulate matter with aerodynamic diameter less than 2.5 microns


PMF: Positive Matrix Factorization receptor model


Point source:


RAOB: Radiosonde observation.  This is way to measure meteorological parameters as a function of height at one site.  Meteorological parameters collected by a radiosonde include pressure, temperature, humidity, wind speed and wind direction.  Routine radiosonde observations are taken world-wide at 00 and 12 UTC daily.  See this National Weather Service Radiosonde page that describes the instrument used.  There are 92 radiosonde sites in North America operated or overseen by the National Weather Service as shown in this network map.


Rayleigh Gas Scattering or Rayleigh Scattering: Light scattering due to gas phase.

            A standard value of 10 Mm-1 is used in visibility calculations regardless of site elevation


RAWS: Remote Automated Weather Station


Receptor Model:


Regional Haze:


Residence Time: Residence time is the fraction of the total time of backtrajectories that the backtrajectory was over a given area of 1 degree latitude by one degree longitude. What is important is not the absolute residence time number, but the shape of the residence time distribution (e.g. mostly from the east during hazy days, etc. The absolute number is arbitrary because it depends on the size of the area considered and the length of the backtrajectories in days.

Residence time difference: Residence time difference maps give the percent residence time for some condition (e.g. high sulfate) minus the percent residence time for all conditions (i.e. average residence time)


RPO: Regional Planning Organization


SO2: Sulfur Dioxide


Soil: This refers to the soil derived fraction in the aerosol.


Source Attribution:




SO4: This refers to sulfate aerosol. Sometimes we represent SO4 as S (elemental sulfur) in this website.  In this case we assume that all elemental sulfur is from sulfate that is from ammonium sulfate.  This is a secondary pollutant primarily from fossil fuel combustion.


Temperature Inversion:


Trade-wind flow:


Trajectory Model:


Upslope/downslope flow:


USDA FS: United States Department of Agriculture Forest Service


UTC: Coordinated Universal Time




Visual Range (V.R.):The greatest distance at which an observer can just see a black object viewed against the horizon sky.   

Units:  Distance, e.g. kilometers (km)

Benefit:  This metric is most useful in situation where only the distance that one can see is important, such as for aviation.

Drawback:  Does not account for changes in scene, such as loss of textures and discoloration as haze increases.


V.R. = K/Bext
Where K is the Koschmieder Coefficient – the log of the contrast threshold of the human eye,  K = 3 – 3.9


VOC: Volatile Organic Compounds


Vog: Conditions occurring in the Hawaiian islands during volcanic eruptions where fog and volcanic ash to mix to create visibility impairment.


WA: Wilderness Area


Wind rose:


WRAP: Western Regional Air Partnership