Air Quality in U. S.

Air Quality Index

The latest image available is for July 29, 2014 as of 3:10 pm MST. If the image below is older than this, you will need to force your browser to reload this page by clicking on the browser "Reload" or "Refresh" button.

Map showing current Ozone levels in U. S..

PLEASE NOTE: Data in this image is collected from TREX air monitoring sites, local agencies, and private monitoring networks. This data has not been verified by the TREX or the responsible entity and may change. While this is the most current data, it is not official until it has been certified by the appropriate technical staff. This image is updated hourly.

The image above shows the Air Quality Index (AQI) ratings for each of the NAAQS pollutants that are measured real-time and the critical pollutant that is driving the AQI rating in each , other area where pollutant levels are monitored by the TREX. The critical pollutant is the pollutant with the highest AQI rating measured in the area. The image is updated each hour and covers the period from midnight through the indicated ending time.

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Which Pollutants Can I Monitor Using the AQI?

There are four pollutants that go into the Air Quality Index: ozone, carbon monoxide, sulfur dioxide, and particulate matter. In the image above, each monitoring area is represented by a small box which is color coded to match the AQI rating for the day (see Interpreting the AQI). Inside the box, the pollutant that is driving the AQI rating is identified by its abbreviation (see the table below). At the bottom of each box is a small legend that indicates which pollutants are actively measured in that area. Please note that not all pollutants are measured in all areas or at all sites. The table below briefly describes each pollutant that goes into the AQI.

OzoneO3Ozone is a form of oxygen with three atoms instead of the usual two atoms. It is a photochemical oxidant and, at ground level, is the main component of smog. Unlike other gaseous pollutants, ozone is not emitted directly into the atmosphere. Instead, it is created in the atmosphere by the action of sunlight on volatile organic compounds and nitrogen oxides.

In general, higher levels of ozone usually occur on sunny days with light winds, primarily from March through October. An ozone exceedance day is counted if the measured eight-hour average ozone concentration exceeds the standards.

Carbon MonoxideCOCarbon monoxide is a colorless, odorless, very toxic gas produced by the incomplete combustion of carbon-containing fuels, most notably by gasoline powered engines, power plants, and wood fires.

The eight-hour standard can be exceeded during winter months when very stable atmospheric conditions exist.

Sulfur DioxideSO2Sulfur dioxide is produced by burning sulfur-containing fuels (such as coal), smelting metallic ores containing sulfur, and removing sulfur from fuels. There are three sulfur dioxide standards which include a 24-hour average, an annual average, and a three-hour average.
Particulate MatterPM2.5

Particulate matter is anything that is suspended in the air. It can be caused by natural phenomena or come from man-made sources. In high enough concentrations, particulates can aggravate existing respiratory problems or even trigger new ones.

Particulate matter is broken down by the size of the suspended particles. The TREX measures or plans to measure particulate matter in the range of 10 microns and smaller (known as PM-10) and 2.5 microns and smaller (known as PM-2.5). A micron is one-millionth of a meter or one 25-thousandth of an inch - by comparison, a human hair is about 50 microns in diameter. There are several methods that can be used to measure particulate matter which involve measuring mass accumulation directly or through radiation attenuation. The only difference between a PM-10 instrument and a PM-2.5 instrument is the inlet filter.

Coarse particulates (PM-10) come from sources such as windblown dust from the desert or agricultural fields (sand storms) and dust kicked up on unpaved roads by vehicle traffic. PM-10 data is the near real-time measurement of particulate matter 10 microns or less in size from the surrounding air. This measurement is made at standard conditions, meaning it is corrected for local temperature and pressure.

Fine particulates (PM-2.5) are generally emitted from activities such as industrial and residential combustion and from vehicle exhaust. Fine particles are also formed in the atmosphere when gases such as sulfur dioxide, nitrogen oxides, and volatile organic compounds, emitted by combustion activities, are transformed by chemical reactions in the air. Large-scale agricultural burning or sand storms can produce huge volumes of fine particulates. PM-2.5 data is the near real-time measurement of particulate matter 2.5 microns or less in size from the surrounding air. This measurement is made at local conditions, and is not corrected for temperature or pressure.

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Interpreting the AQI

Air Quality Index

Each NAAQS pollutant has a separate AQI scale, with an AQI rating of 100 corresponding to the concentration of the Federal Standard for that pollutant. Additional information about the AQI and how it can be used is available from the EPAExit.

Place your mouse pointer over the scale displayed above to view information about the Air Quality Index, and each of the rating levels.

The actual index calculation is different for each parameter measured and is specified by the EPA. The following table shows the various breakpoints used in calculating the AQI.

AQI Breakpoint Definitions
AQI Range 1hr Ozone
in ppm
8hr Ozone
in ppm
8hr Carbon Monoxide
in ppm
24hr Sulfur Dioxide
in ppm
24hr PM-10
in ug/m3
24hr PM-2.5
in ug/m3
0 - 50 Not Defined 0 - 0.059 0 - 4.4 0 - 0.034 0 - 54 0 - 15.4
51 - 100 Not Defined 0.06 - 0.075 4.5 - 9.4 0.035 - 0.144 55 - 154 15.5 - 40.4
101 - 150 0.125 - 0.164 0.076 - 0.095 9.5 - 12.4 0.145 - 0.224 155 - 254 40.5 - 65.4
151 - 200 0.165 - 0.204 0.096 - 0.115 12.5 - 15.4 0.225 - 0.304 255 - 354 65.5 - 150.4
201 - 300 0.205 - 0.404 0.116 - 0.374 15.5 - 30.4 0.305 - 0.604 355 - 424 150.5 - 250.4
301 - 400 0.405 - 0.504 Not Defined 30.5 - 40.4 0.605 - 0.804 425 - 504 250.5 - 350.4
401 - 500 0.505 - 0.604 Not Defined 40.5 - 50.4 0.805 - 1.004 505 - 604 350.5 - 500.4
500 Not Defined Not Defined Not Defined Not Defined 605 - 4999 500.5 - 999.9

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