Frequently asked Questions about Wintertime PM2.5 Pollution in Utah's Salt Lake Valley

During winter, the Wasatch Front suffers from multi-day wintertime air pollution episodes in which the concentrations of aerosols or fine particulate matter with diameters less than 2.5 micrometers, called PM2.5, rise above the National Ambient Air Quality Standards (NAAQS), which were established to protect human health.

Salt Lake Valley PM2.5 pollution episode. Most episodes cover the entire valley with a deep brown/gray haze. Views from within the haze are not so photogenic! Photo © Sebastian Hoch.

This webpage brings a number of disparate sources on Salt Lake City PM2.5 concentrations onto a single page so that links can be conveniently followed. If you want to access the links directly without hunting for them through the text on this page, click here.


What is the current level of PM2.5 in the Salt Lake Valley?
How big are PM2.5 particles?
Why is PM2.5 of concern?
What are the EPA standards for PM2.5?
What is the Air Quality Index (AQI) for PM2.5?
What is the composition of our PM2.5?
What are the sources of PM2.5?
How do PM2.5 concentrations build up?
What are the local factors that lead to high PM2.5 in the Salt Lake Valley?
How does weather affect PM2.5 levels?
Do we presently have high atmospheric stability in the valley?
Will the high pressure ridge persist?
How's it looking out there? Webcams
What is the pollution forecast for the next 3-days?
How does PM2.5 vary with elevation in the Salt Lake Valley?
How does PM2.5 vary spatially on the floor the Salt Lake Valley?
How deep is the pollution layer now?
What time of year is PM2.5 the worst?
What is being done about PM2.5?
What can we do to decrease our exposure to these high pollution events?
Other links

What is the current level of PM2.5 in the Salt Lake Valley?

The UDAQ Hawthorne Elementary School measurements are the most accurate measurements in the valley and the ones used to establish whether the valley meets EPA's air quality standards. Other recently established experimental sites in the valley (Neil Armstrong Academy and Mountain Meteorology Lab) use less expensive measurement equipment that is not maintained as rigorously.

UDAQ PM2.5 time series from Hawthorne Elementary School, 1675 South 600 East.

PM2.5 concentration at the Mountain Meteorology Lab on the University of Utah campus

PM2.5 concentration at the Neil Armstrong Academy in West Valley City

How big are PM2.5 particles?

PM2.5 aerosol particles are much smaller in size than a human hair. The concentration of the particulates is given in units of mass per unit volume, or micrograms of particulate matter per cubic meter of air.

Comparison of a PM2.5 particle to the diameter of a human hair, a particle of fine beach sand, or a PM10 particle.

Why is PM2.5 of concern?

Particles smaller than 2.5 micrometers are hazardous to human health because they are readily inhaled into human lungs, where they can be deposited. Some of the particles are carcinogenic (i.e., cancer-causing), and the body's normal defenses such as nose hairs and mucous in the upper respiratory tract cannot readily remove these tiny particles before they are deposited in the lungs.The health concern has been documented scientifically, with the main concern being a lodging of the small aerosol particles in the lungs. The Environmental Protection Agency's (EPA) standard has been established because of known health effects, including:

but much health effect research still remains to be done to determine the pathways of the effects seen from epidemiological and clinical studies.

PM2.5 pollution can be linked to a variety of other significant health issues. Many residents report burning eyes, sore throats, and headaches. In the Salt Lake Valley, a recent study has documented the rise in emergency room visits for pulmonary problems that is correlated with the length of pollution epsiodes. Of special concern is the health of people most at risk from PM exposure. These include people with heart or lung disease, children, older adults and people of lower socioeconomic status.

PM2.5 pollution also harms public welfare. It causes haze, reduces visibility, contributes to transportation hazards, causes soiling of materials, and its close association with temperature inversions and the buildup of shallow stratiform clouds within the valley can sometimes result in freezing drizzle or freezing rain episodes, with attendant transportation problems, including the cancellation of scheduled aircraft flights.

What are the EPA standards for PM2.5? (see footnote)

EPA has established two National Ambient Air Quality Standards for PM2.5:

Footnote: NAAQS thresholds have been lowered as additional research has documented the health effects of particulate matter. The annual average PM2.5 standard issued in 1997 was 15 micrograms per cubic meter. This was revised downward to 12 in December 2012. The daily average PM2.5 standard was 65 micrograms per cubic meter until 2006, when it was changed to 35. More information on PM2.5 standards can be found in an EPA document.The downward revisions raise the question whether there is really any level at which human health would be fully protected. Perhaps the effect is simply cumulative and is not triggered at a specific level of concentration.

What is the Air Quality Index (AQI) for PM2.5?

A new color-coded EPA Air Quality Index advises citizens of air quality levels. The recently revised color scheme and breakpoints are shown below. This figure will help you determine the actual air pollution concentrations when you hear that the day is green, yellow, red, etc.


What is the composition of our PM2.5?

UDAQ has a long-term program in which daily samples of PM2.5 pollution are collected on filters. Analyses of the filters collected at 3 or 6 day intervals, depending on the site, provide information on the composition of PM 2.5 particles.

The individual particles are not overwhelmingly primary aerosols - that is, particles that are emitted directly into the atmosphere by pollution sources. Rather, most of the particles form in the atmosphere over time due to chemical and photochemical reactions. All of the chemical mechanisms leading to these so-called secondary aerosols are not well known, but major reactants include oxides of nitrogen (abbreviated NOx) and volatile oganic compounds (abbreviated VOCs). The primary source of NOx is high temperature combustion from vehicles, space heating, industrial operations, etc. VOCs have many sources, including gasoline vapors, solvents and emissions from vegetation.

The bulk of the airborne particles are ammonium nitrate and ammonium sulfate, along with elemental and organic carbon. Recent research by other UU investigators that will soon be published (and will be linked here upon publication) indicates that on days when PM2.5 exceeds 20 micrograms per cubic meter, ammonium chloride can contribute 10-15% of the total PM2.5 mass. A small fraction of the particulates are small dust particles and other, unspecified components. Some particles are hygroscopic, meaning that they will swell by absorbing water vaper when relative humidity exceeds about 80%. Some small particles will swell sufficiently to outgrow the 2.5 micrometer size fraction.

Composition of PM2.5 on winter days when PM2.5 concentrations rise above the NAAQS, from 2006-2010. A chemical reaction mechanism is shown that leads from NOx and VOCs to ammonium nitrate particulates. From UDAQ.

What are the sources of PM2.5?

Emission inventories produced by UDAQ are generally broken down into point sources, mobile sources, and area sources. Point sources are large stationary industrial or commercial facilities that emit more than 100 tons per year of a regulated pollutant. Mobile sources are generally non-stationary sources such as vehicles, trains and aircraft, and area sources are smaller stationary sources that, due to their greater number, are accounted for as a group. These include, for example, emissions from space heating of structures, smoke from wood burning, dust from roadways, and emissions from restaurants, dry cleaners, printing/graphics, and auto body refinishing shops.


This figure shows primary emissions for the Salt Lake, Davis, Utah, and Weber counties (the Wasatch Front) for a typical winter day as estimated for 2014 from UDAQ's 2010 emission inventory. Included are direct emissions of PM2.5 and emissions of chemical precursors (VOCs, NOx, and SOx) that lead to secondary PM2.5. Detailed emission inventories are available for VOC, NOx, and SOx for other counties and even for individual industrial sources. Recent information suggests that, while the attribution of primary sources of pollution is in general agreement with the inventory, the role of wood burning in airborne PM2.5 concentrations may be somewhat underestimated. A research paper on this topic has passed peer review and will be published soon. Further information on pollution sources can be found in annual reports and annual monitoring plans linked from the DAQ webpage.

An increasing population generally increases the rate of pollution emissions as infrastructure expands to accommodate the rising population. Vehicle emissions, for example, will increase with the number of vehicle miles traveled. This increase may overcome the pollution reductions mandated by federal vehicle emissions standards.

Population of Salt Lake County, 1850-2013. From U.S. government census records.

How do PM2.5 concentrations build up?

Concentrations build up over a period of days when high stability in the valley atmosphere (an inversion is one type of high stability event) decouples the valley from the generally stronger winds aloft. Pollution and reactants released in the valley then fail to mix vertically out of the valley and are trapped in the stable air between the Wasatch and Oquirrh Mountains. Chemical and photochemical processes build up secondary PM2.5 particulates during this time. There appears to be a certain stability that has to be attained for the valley to become decoupled. It is typical in these events for PM2.5 concentrations to attain higher and higher values from day to day, unless the high stability period is interrupted by passing low-pressure weather systems. A recently completed National Science Foundation-funded research program in the Salt Lake Valley has provided new information on the influences of different weather events on our cold-air pool. A summary article giving an overview of the project is available here, the project website can be found here and a resaerch article investigating the history of particulate measurements in the Salt Lake Valley and the relationship between particulate concentrations and meteorology can be found here.

What are the local factors that lead to high PM2.5 in the Salt Lake Valley?


The Wasatch Mountains east of the valley, the Oquirrh Mountains to the west, and the Traverse Mountain to the south form a basin-like topography that traps cold air and shields the valley from the generally stronger winds aloft. The valley is open to the Great Salt Lake to the northwest. Weak nighttime down-valley drainage flows often carry polluted air over the lake. Here, it is sometimes stored overnight and then carried back into the valley as a lake breeze the following day.

How does weather affect PM2.5 levels?

Episodes generally occur when a persistent high-pressure ridge forms over or is advected into the Intermountain Basin (IB) and atmospheric stability in the Salt Lake Valley reaches, exceeds and maintains a threshold value over multiple days. Topography plays an important role, too, as air pollutants are trapped in the valley by the surrounding topography. But, similar high atmospheric stability occurs in other locations in the IB during these events, and there are other IB locations surrounded by mountainous topography. Thus, while weather and topography play strong roles, the CAUSE of the PM2.5 air pollution episodes is NOT the weather, it is the emissions of PM2.5 and their chemical precursors into the stable valley atmosphere.

Do we presently have high atmospheric stability in the valley?

Latest Salt Lake City atmospheric sounding

Note, a description of the figure is found just below the plot. Check the sounding time to be sure that you are looking at the latest sounding. The 12Z or 1200 UTC sounding is 0500 MST on the date indicated, 00Z is at 1700 MST on the day pevious to the date indicated on the chart. When the sounding temperature line somewhere in the lowest 1200 m of the atmosphere (the valley depth) slopes off to the right of the 45 degree right-sloping blue lines a temperature inversion layer is present in the valley (bad news for pollution, especially if seen in a succession of 12-hourly soundings).

Don't like the fact that the plot has sloping temperature lines? Then, the same sounding, but in a more user-friendly format in which vertical lines are temperatures, and horizontal lines are atmospheric pressures, can be found on the University of Wyoming's weather website by selecting GIF:Stuve as the type of plot and clicking on SLC on the map.

Do we presently have a high pressure ridge over the Intermountain Basin?

Existing or approaching large-scale, high-pressure ridges can be seen on mid-tropospheric weather charts. Here a chart is drawn for the 500 mb pressure level - a pressure level found at about 18000 feet or 5500 m above sea level. The contour lines on this chart give the heights above sea level of the 500 mb pressure level. A ridge on this chart is indicated by arched contour lines that are concave downwards, exactly like a ridge on a topographic map. Sometimes the ridge will have a closed contour, like a peak on a topographic map.

Current 500 mb weather chart

Raw data are plotted on this map at each radiosonde location, including Salt Lake City. Radiosondes are balloon-borne weather sounding devices launched twice per day. Data plotted at each sounding location include temperature (degrees C, upper left of the plus sign), dew point depression (an indicator of humidity, the difference between temperature and dew point temperature, deg C, lower left of plus sign), height of the 500 mb surface (10's of m above sea level, upper right), and wind direction and speed (funny looking arrow with fletches). Each full fletch is 10 nautical miles per hour (kts), half-fletches are 5 kts and blackened triangle fletches are 50 kts. Add up the fletch values to get the total speed, within 5 kts. The arrow shaft, directed toward the plus sign, shows the wind direction. Black lines are 500 mb height contours, dashed lines are temperature contours. Green colors are contours of dew point depressions (indicators of humidity and clouds).

Will the high pressure ridge persist?

Generally, the longer the ridge of high-pressure persists over the Intermountain Basin, the worse the air pollution. Here is a link to 500 mb forecasts.

Days 3-7 500 mb forecasts

How's it looking out there? webcams webcam from UU William Browning Building, Atmos. Sci. Dept., looking E

Real-time photos, Suncrest development on Traverse ridge at S end of valley, looking NNW

Last two hour time lapse, Suncrest development on Traverse ridge at S end of valley, looking NNW

Wilkerson webcam/weather on east sidewall, looking NW

Neil Armstrong Academy (West Valley City) webcam, looking E

UU Mountain Meteorology Lab webcam, looking SSW

What is the current weather in the Salt Lake Valley?

MesoWest Network (click the button "View Profile Without Logging In")

What is the pollution forecast for the next 3-days?

Three-day pollution forecast. From UDAQ.

How does PM2.5 vary with elevation in the Salt Lake Valley?

A recent study measured PM2.5 concentrations on a line running up the Avenues residential district in north Salt Lake City. Although the data set covered only part of one winter, PM2.5 concentrations were found to generally decrease with elevation above the lowest few hundred feet of the valley, suggesting that residents living at higher elevations might suffer less exposure to pollutants. But, the aerosol layer typically extends to elevations well above the valley floor, often enveloping all residential areas. The pollution events sometimes break up in such a way that the highest elevations are the first to experience clean air.

How does PM2.5 vary spatially on the floor of the Salt Lake Valley?

In December 2014, particulate, greenhouse gas, and meteorological sensors were mounted on the top of a Utah Transit Authority TRAX light rail car. That car is at times stationary in the UTA yard but deployed frequently onto either the Red (Daybreak to University Hospital) or Green (West Valley to Airport) lines. PM2.5 and other data from these sensors are updated continuously and the web pages provide information on current conditions as well as data collected since the sensors were deployed. This effort is made possible by UTA and involves a team of faculty, postdoctoral researchers, and graduate and undergraduate students in the Department of Atmospheric Sciences. For further information, contact Logan Mitchell ( or Erik Crosman (

How deep is the pollution layer now?

The depth of the pollution layer, the presence of elevated or mutiple layers of aerosols, a qualitative idea of the vertical mixing and relative concentration of airborne particulates, the presence of fog, and the heights of stratus or other cloud layers can all be determined from laser ceilometers that have been installed in the valley for the winter of 2012-2013. It takes some experience, however, to interpret the ceilometer data. Click on the first link to learn how to interpret the data, and then click on the second and third links to see ceilometer data from the north and south ends of the Salt Lake Valley.

Guide to interpreting laser ceilometer data.

Daily laser ceilometer data from site W of SLC airport

What time of year is PM2.5 the worst?

Pollution buildup is maximized in winter when high pressure weather systems sit over the valley for multi-day periods. Local residents refer to the high air pollution events as "inversions". But the atmosphere can be stable, with little vertical mixing, even when an inversion (an atmospheric layer in which temperature increases with height) is not present. Inversions frequently form during the nights in all seasons. However, in summer incoming solar radiation heats the ground following sunrise, and rising convection currents heat the near-ground atmosphere and destroy the inversion from below on a daily basis. In winter the incoming solar radiation is gteatly reduced and daytime convection is just too weak to destroy many inversions, especially when the radiation is reflected back to space by an intervening cloud deck or by the high reflectivity of wintertime snow cover. Thus, the worst PM2.5 episodes are most likely to occur in mid-winter and to last for days under high-pressure weather systems. These inversions are typically destroyed by the passage of strong frontal systems, which bring in strong winds, cold air aloft and rising motions. A recent 38-year climatology of particulate episodes and high atmospheric stability episodes for the Salt Lake Valley (not yet published) has provided new information on the number and strength of high stability and particulate pollution events. The number of events is quite variable from winter to winter depending on the frequency of high pressure stagnation events over the Intermountain Basin.


The frequency of high PM2.5 concentrations in any week of the year can be estimated from PM2.5 observations from 1999 through 2011. Shown are the weeks of the year (Week 1 is 1-7 January; week -1 is 25-31 December; etc.) and the percentage of days in these weeks when PM2.5 concentrations rose above the NAAQS of 35 micrograms per cubic meter (red) or 17.5 micrograms per cubic meter (blue). More than half of the days in some mid-winter weeks have concentrations above 17.5, while over 25% of the days have concentrations above the NAAQS. The probability of high concentrations is greatest in the first few weeks of January, but similar concentrations can also occur in December.

What is being done about PM2.5?

UDAQ prepared a State Implementation Plan (SIP) that has been submitted to the U.S. EPA after public comments. The document details the state's plans to implement additional air pollution controls to bring PM2.5 concentrations below the federal NAAQS standards. Guidance for the new air pollution controls comes from air pollution models that are tested against historical wintertime PM2.5 episodes. At present, the models are showing that the proposed new air pollution controls will not be successful in meeting the NAAQS standards until 2019.

What can we do to decrease our exposure to these high pollution events?

The normal response to air pollution problems is to blame others and apply political pressure to force emissions reductions. But, you know, the problem is really, demonstrably, us and any solution will require a community response. Voluntary restrictions are not working and we'll have to implement controls that to some extent limit individual freedom (to burn wood, to drive cars, etc.). There are lots of things we could do; some have already been implemented in California, for example. But they'll be painful and affect us all.

There is a somewhat counterproductive regulatory situation that is affecting the state's response to these elevated PM2.5 concentrations. The state could be penalized if it does not meet air quality standards within a certain time frame. The state, however, gets 'credit' with the EPA only for permanent pollution controls. Thus the focus seems to be almost entirely on permanent controls, with little incentive to initiate temporary controls during bad air pollution episodes. If the health of the citizens were uppermost in the equation rather than EPA credits, temporary emissions restrictions might be imposed to improve air quality during the worst events.

MesoWest weather
Wasatch Weather Weenies blog

Webpage prepared by:

University of Utah
Atmospheric Sciences Department
Mountain Meteorology Group

Last update: 5 January 2015