Phoenix, AZ Air Quality Index
Friday Forecast:
An Ozone High Pollution Advisory has been issued for tomorrow in Phoenix. This HPA is not related to the Mulch Fire in Mesa.
We are in a short transitional period between two disturbances making their way through the southwestern U.S. Expect lighter winds and slightly higher temperatures over the next three days. Forecasted cloud cover will help to limit ozone growth today and on Saturday. Tomorrow however, timing of incoming cloud cover is just outside of the afternoon period where we see the most ozone growth throughout the day.
By Sunday, another system will move through the area, bringing with it stronger southwesterly winds, slightly lower temperatures, and occasional cloud cover. These indices will help to keep ozone out of the USG AQI category, and is expected to remain within the Moderate AQI.
PM10 (dust) will be mainly locally driven over the next few days, with the normal morning signatures we see at our most active sites. Levels will remain in the Moderate AQI until the weekend, dropping into the Good AQI as local activity diminishes...
- B. Droppleman
ADEQ Meteorologist
[Note: Data unavailable from source ~0000-0600 MST.]
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| Air Quality Forecast | |
| Forecast Location: Phoenix, AZ Forecast Agency: Arizona Department of Environmental Quality | |
| Two Day Forecast | |
| Day One Forecast | Day Two Forecast |
| Date: 06/09/2023 Type: Ozone Index: Moderate AQI Value: 87  | Date: 06/10/2023 Type: Ozone Index: Unhealthy for Sensitive Groups AQI Value: 101  |
| Last Update: Thu, 08 Jun 2023 11:45:10 MST | |
Computation of the AQI requires an air pollutant concentration over a specified averaging period, obtained from an air monitor or model. Taken together, concentration and time represent the dose of the air pollutant. Health effects corresponding to a given dose are established by epidemiological research.[4] Air pollutants vary in potency, and the function used to convert from air pollutant concentration to AQI varies by pollutant. Its air quality index values are typically grouped into ranges. Each range is assigned a descriptor, a color code, and a standardized public health advisory.
The AQI can increase due to an increase of air emissions. For example, during rush hour traffic or when there is an upwind forest fire or from a lack of dilution of air pollutants. Stagnant air, often caused by an anticyclone, temperature inversion, or low wind speeds lets air pollution remain in a local area, leading to high concentrations of pollutants, chemical reactions between air contaminants and hazy conditions.[5]
United States[edit]
| AQI | Level of health concern | Color |
|---|---|---|
| 0 to 50 | Good | Green |
| 51 to 100 | Moderate | Yellow |
| 101 to 150 | Unhealthy for sensitive groups | Orange |
| 151–200 | Unhealthy | Red |
| 201–300 | Very unhealthy | Purple |
| 301–500 | Hazardous | Maroon |
The United States Environmental Protection Agency (EPA) has developed an Air Quality Index that is used to report air quality. This AQI is divided into six categories indicating increasing levels of health concern. An AQI value over 300 represents hazardous air quality and below 50 the air quality is good.[10]
The AQI is based on the five "criteria" pollutants regulated under the Clean Air Act: ground-level ozone, particulate matter, carbon monoxide, sulfur dioxide, and nitrogen dioxide. The EPA has established National Ambient Air Quality Standards (NAAQS) for each of these pollutants in order to protect public health. An AQI value of 100 generally corresponds to the level of the NAAQS for the pollutant.[10] The Clean Air Act (USA) (1990) requires the EPA to review its National Ambient Air Quality Standards every five years to reflect evolving health effects information. The Air Quality Index is adjusted periodically to reflect these changes.
Computing the AQI[edit]
The air quality index is a piecewise linear function of the pollutant concentration. At the boundary between AQI categories, there is a discontinuous jump of one AQI unit. To convert from concentration to AQI this equation is used:[41]
(If multiple pollutants are measured, the calculated AQI is the highest value calculated from the above equation applied for each pollutant.)
where:
- = the (Air Quality) index,
- = the pollutant concentration,
- = the concentration breakpoint that is ≤ ,
- = the concentration breakpoint that is ≥ ,
- = the index breakpoint corresponding to ,
- = the index breakpoint corresponding to .
The EPA's table of breakpoints is:[42][43][44]
| O3 (ppb) | O3 (ppb) | PM2.5 (μg/m3) | PM10 (μg/m3) | CO (ppm) | SO2 (ppb) | NO2 (ppb) | AQI | AQI |
|---|---|---|---|---|---|---|---|---|
| Clow – Chigh (avg) | Ilow – Ihigh | Category | ||||||
| 0–54 (8-hr) | — | 0.0–12.0 (24-hr) | 0–54 (24-hr) | 0.0–4.4 (8-hr) | 0–35 (1-hr) | 0–53 (1-hr) | 0–50 | Good |
| 55–70 (8-hr) | — | 12.1–35.4 (24-hr) | 55–154 (24-hr) | 4.5–9.4 (8-hr) | 36–75 (1-hr) | 54–100 (1-hr) | 51–100 | Moderate |
| 71–85 (8-hr) | 125–164 (1-hr) | 35.5–55.4 (24-hr) | 155–254 (24-hr) | 9.5–12.4 (8-hr) | 76–185 (1-hr) | 101–360 (1-hr) | 101–150 | Unhealthy for sensitive groups |
| 86–105 (8-hr) | 165–204 (1-hr) | 55.5–150.4 (24-hr) | 255–354 (24-hr) | 12.5–15.4 (8-hr) | 186–304 (1-hr) | 361–649 (1-hr) | 151–200 | Unhealthy |
| 106–200 (8-hr) | 205–404 (1-hr) | 150.5–250.4 (24-hr) | 355–424 (24-hr) | 15.5–30.4 (8-hr) | 305–604 (24-hr) | 650–1249 (1-hr) | 201–300 | Very unhealthy |
| — | 405–504 (1-hr) | 250.5–350.4 (24-hr) | 425–504 (24-hr) | 30.5–40.4 (8-hr) | 605–804 (24-hr) | 1250–1649 (1-hr) | 301–400 | Hazardous |
| — | 505–604 (1-hr) | 350.5–500.4 (24-hr) | 505–604 (24-hr) | 40.5–50.4 (8-hr) | 805–1004 (24-hr) | 1650–2049 (1-hr) | 401–500 | |
Suppose a monitor records a 24-hour average fine particle (PM2.5) concentration of 26.4 micrograms per cubic meter. The equation above results in an AQI of:
which rounds to index value of 81, corresponding to air quality in the "Moderate" range.[45] To convert an air pollutant concentration to an AQI, EPA has developed a calculator.[46]
If multiple pollutants are measured at a monitoring site, then the largest or "dominant" AQI value is reported for the location. The ozone AQI between 100 and 300 is computed by selecting the larger of the AQI calculated with a 1-hour ozone value and the AQI computed with the 8-hour ozone value.
Eight-hour ozone averages do not define AQI values greater than 300; AQI values of 301 or greater are calculated with 1-hour ozone concentrations. 1-hour SO2 values do not define higher AQI values greater than 200. AQI values of 201 or greater are calculated with 24-hour SO2 concentrations.
Real-time monitoring data from continuous monitors are typically available as 1-hour averages. However, computation of the AQI for some pollutants requires averaging over multiple hours of data. (For example, calculation of the ozone AQI requires computation of an 8-hour average and computation of the PM2.5 or PM10 AQI requires a 24-hour average.) To accurately reflect the current air quality, the multi-hour average used for the AQI computation should be centered on the current time, but as concentrations of future hours are unknown and are difficult to estimate accurately, EPA uses surrogate concentrations to estimate these multi-hour averages. For reporting the PM2.5, PM10 and ozone air quality indices, this surrogate concentration is called the NowCast. The Nowcast is a particular type of weighted average that provides more weight to the most recent air quality data when air pollution levels are changing.[47][48] There is a free email subscription service for New York inhabitants – AirNYC.[49] Subscribers get notifications about the changes in the AQI values for the selected location (e.g. home address), based on air quality conditions.
Public availability of the AQI
Real time monitoring data and forecasts of air quality that are color-coded in terms of the air quality index are available from EPA's AirNow web site.[50]
Other organizations provide monitoring for members of sensitive groups such as asthmatics, children and adults over the age of 65.[51]
- Historical air monitoring data including AQI charts and maps are available at EPA's AirData website.[52]
- Detailed map about current AQI level and its two-day forecast is available from Aerostate web site.[53]
History of the AQI[edit]
The AQI made its debut in 1968, when the National Air Pollution Control Administration undertook an initiative to develop an air quality index and to apply the methodology to Metropolitan Statistical Areas. The impetus was to draw public attention to the issue of air pollution and indirectly push responsible local public officials to take action to control sources of pollution and enhance air quality within their jurisdictions.
Jack Fensterstock, the head of the National Inventory of Air Pollution Emissions and Control Branch, was tasked to lead the development of the methodology and to compile the air quality and emissions data necessary to test and calibrate resultant indices.[54]
The initial iteration of the air quality index used standardized ambient pollutant concentrations to yield individual pollutant indices. These indices were then weighted and summed to form a single total air quality index. The overall methodology could use concentrations that are taken from ambient monitoring data or are predicted by means of a diffusion model. The concentrations were then converted into a standard statistical distribution with a preset mean and standard deviation. The resultant individual pollutant indices are assumed to be equally weighted, although values other than unity can be used. Likewise, the index can incorporate any number of pollutants although it was only used to combine SOx, CO, and TSP because of a lack of available data for other pollutants.
While the methodology was designed to be robust, the practical application for all metropolitan areas proved to be inconsistent due to the paucity of ambient air quality monitoring data, lack of agreement on weighting factors, and non-uniformity of air quality standards across geographical and political boundaries. Despite these issues, the publication of lists ranking metropolitan areas achieved the public policy objectives and led to the future development of improved indices and their routine application.
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> "High Pollution Advisory" or "HPA" means the highest concentration of pollution may exceed the federal health standard.
Active children, adults and people with lung disease such as asthma should reduce prolonged or heavy outdoor exertion.
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