Salt Lake Valley PM2.5 Pollution Study

Dec. 2015 - Jan. 2016


Figure 1: Cold air pool conditions trapping pollutants in the Salt Lake Basin. Sebastian Hoch photo.

1. Introduction

The 2015-2016 Salt Lake Valley PM2.5 Pollution Study is a multi-university study sponsored by the Utah Division of Air Quality. Researchers from the University of Utah (UU), Weber State University (WSU), Utah State University (USU), and from the Utah Division of Air Quality (DAQ) are combining their expertise and observational tools to study chemical transformation in Salt Lake Valley's persistent wintertime inversions. The goal of the study led by lead PI Munkh Baasandorj is to investigate the reactive pathways of the formation of secondary particulate pollutants and the coupling of meteorological and chemical processes.

The passing of high pressure ridges during wintertime favors the formation of persistent wintertime cold air pools in Utah's topographic basins. Under these conditions, the boundary layer is stably stratified and/or capped by a capping inversion associated with warm air advection aloft. Under these conditions, atmospheric mixing is limited and pollutants emitted near the surface are accumulating, often exceeding the National Ambient Air Quality Standard (NAAQS) for PM2.5. A typical winter in the Salt Lake City Basin sees about 6 multi-day events comprising 18 days (Whiteman et al. 2014) when the NAAQS is exceeded. 

My role is to capture the meteorological conditions during the air pollution episodes. Instrumentation ranging from small inexpensive temperature dataloggers deployed along an elevation-transect from the valley floor up the basin sidewall, to sophisticated remote-sensing equipment such as ceilometers and a Doppler Wind LiDAR, are used to monitor the spatial and temporal variation of the atmospheric conditions of the valley cold pools.

Key components of the meteorological measurements include the timing and strength of mixing in the early morning, when two reservoirs of air, the surface layer and the upper part of the cold pool, are combined and allow chemical reactions to take place.

The influence of other meteorological phenomena, such as the lake breeze, clean air inflows from tributary valleys (Parleys Canyon, Mill Creek, etc.), and top-down erosion of the basin cold air pools are also investigated.

Quicklooks of the collected meteorological data sets can be found at

2. Background

An excellent background overview (click here) on the PM2.5 pollution problem in the Salt Lake City Basin has been compiled by Prof. C. David Whiteman. His webpage aims at answering the "frequently asked questions" on the topic and provides links to observations in the Salt Lake City Basin.

PM2.5 particulate pollution consists of suspended particles with a diameter of or below 2.5 micrometers (for comparison, human hair has a diameter of 50-70 micrometers). A large fraction of the PM2.5 mass observed during persistent wintertime inversions in the Salt Lake Basin is secondary in origin. This means that the particulates are formed in the atmosphere through chemical reactions of precursor gases. The main chemical component of the wintertime PM2.5 in the Salt Lake Valley is ammonium nitrate, which forms via reversible reactions of gas phase ammonia and nitric acid. Hence a proper understanding of the particulate pollution requires detailed information on the particulate matter and its gas phase precursors. 

3. Experimental design

On order to understand the spatial and vertical variation of pollutants and precursors and the influence of meteorological processes, in-situ and mobile observations on the valley floor, along the valley sidewall, and along vertical profiles are combined.

Observation Sites

A map of the main observation sites can be found here. We thank all the volunteers and institutions that are hosting some of our equipment in their back yards. The two main reference sites are the DAQ Hawthorne Elementary site and the roof-top of a high building on the University of Utah Campus. The elevation difference between these two locations amounts to 155 m (~500 ft). Under the assumption of horizontal homogeneity, two different levels within the cold air pool are sampled.

Mobile Observations

Transects with the University of Utah "Nerd Mobile" ( through the Salt Lake City Basin reveal the spatial variation of pollutants. The Nerd Mobile is operated by Ryan Bares of Prof. John Lin's LAIR group during strong inversion events, and air quality monitors on a UTA TRAX train are available via Prof. John Horel's mesowest data interphase. See for mobile TRAX measurements and for a compilation by Alex Jacques of air quality observations throughout the Salt Lake basin.

Tethered Balloon Measurements

Weber State University's aerostat, a tethered balloon system, is being deployed by Prof. John Sohl's HARBOR group to observe the vertical variation in pollutant concentrations in the lowest 500 ft above ground. Preliminary data and images of the aerostat operation can be found here.

The pseudo-vertical approach

The observation of the vertical variation of temperature, humidity of pollutant concentrations in the atmosphere is difficult and expensive. Measurements from a tethered balloon system are subject to limitations of the FAA in the busy air space of the Salt Lake Valley and thus limited to the lowest 500 ft.

The pseudo-vertical approach assumes horizontal homogeneity of the quantity being measured, and a pseudo-vertical profile of temperature or of a pollutant concentration is obtained by measurements along a topographic gradient. During our PM2.5 pollution study, detailed observations of pollutant concentrations and precursor gases are made at the valley floor (DAQ Hawthorne site) and on top of a tall building on the University of Utah campus, 155 m above the valley floor site. Concentration gradients can then be related to atmospheric stability (resistance to vertical mixing related to the temperature gradient), and the vertical variation of wind speed and wind direction in the basin.

Thermally driven up-slope and down-slope flows may develop under clear-sky cold-pool conditions as illustrated in Fig 3 and may undermine the assumptions the pseudo-vertical approach is based on. Measurements of the wind and temperature structure along the basin sidewall are made to evaluate their potential impact.
Figure 2: Illustrations of meteorological processes that could undermine the assumption of horizontal homogeneity. During the night (top panel), drainage flows from surrounding slopes above the inversion top or from tributary valleys may impinge on the cold air pool and advect cleaner air to the sidewall observation sites.  During daytime (bottom panel), up-slope flows may transport pollutants up the valley sidewalls and lead to observations of pollutant levels higher than those that would be observed the same elevation within the center of the basin.


The aerostat measurements, although sporadic, will be highly useful to further evaluate the pseudo-vertical assumptions.

4. Air Chemistry

Trace gases and precursors

A large fraction of the PM2.5 pollutants observed during persistent wintertime inversions in the Salt Lake Basin are secondary particulates, which means that they have formed within the atmosphere through chemical reactions from precursor gases. Our trace gas measurements are focused on two main precursors for the PM formation, gas-phase ammonia (NH3) and nitric acid (HNO3). Key questions we seek to address are (1) what the relative levels of the total nitrate and ammonium are, (2) what mechanism dominates the nitric acid formation, (3) what contributes to ammonia, and (4) which precursor plays the most important role in particulate formation.  This information is essential for formulating effective control strategies.

Nitric Acid Formation         We are measuring a wide suite of trace gases that play important role in atmospheric chemistry and in nitric acid formation including CO, NOx (NO and NO2), O3, NO3, and N2O5. Latter two species are the main atmospheric nighttime radicals, whose hydrolysis lead to the formation of nitric acid.  We are conducting in-situ measurements of NO3 and N2O5 in the Salt Lake Basin using cavity ring-down spectroscopy technique in collaboration with Dr. Steven Brown, scientist from NOAA (

Gas-phase ammonia (NH3)     One of these key components in the formation of the secondary particulate, ammonia nitrate, is gas-phase ammonia (NH3); however little is currently known about the availability of ammonia in the Salt Lake Basin airshed. Prof. Randy Martin from Utah State University is leading the effort to measure the concentration and spatial distribution of NH3 along the Wasatch Front, as well as in Cache Valley for comparison.

Particulate measurements, size distribution and chemical composition

We are not only monitoring the particulate mass concentration in real time using a tapered element oscillating microbalance (TEOM; at DAQ Hawthorne and UU rooftop site) but also investigating the chemical composition and size distribution of particulates. Prof. Kerry Kelly (UU) and Prof. Randy Martin (USU) are combining their observational resources to investigate the size distribution and chemical composition of the particulate pollution. Their instrumentation can differentiate ultra fine particulates with diameters between 0.015 to 0.7 micrometers and fine to coarse particles from 0.5 to 20 micrometers. The evolution of the particle size distribution throughout an inversion event can give insight into possible sources and formation mechanisms. More traditional PM2.5 filter-based sample are also being collected for speciated chemical analysis to further delineate likely particle sources. Additionally, ultra fine particles are particularly interesting because they have a high surface area, and a number of studies point to the importance of surface area on health outcomes.

Greenhouse gases as passive tracer

Supported by other sources of funding, Prof. John Lin is working with Prof. Jim Ehleringer and Prof. Dave Bowling to carry out measurements of greenhouse gases at a network of sites around Utah.  Greenhouse gases such as carbon dioxide (CO2) are often co-emitted with precursor gases that form PM2.5 but are chemically inert in the atmosphere, thereby serving as an useful passive tracer.  The greenhouse gas observations are displayed online in real-time at:

5. Meteorological Observations

A wide variety of meteorological observations are available in the Salt Lake City Basin. These are, however, mostly limited to the basin floor. To investigate the formation, evolution and destruction of the persistent cold air pools, information about the vertical structure of the atmosphere (temperature stratification, wind profile) are needed. Besides the twice-daily radiosonde ascents conducted by the National Weather Service at the Salt Lake City Airport, pseudo-vertical temperature soundings are compiled from temperature observations with inexpensive automatic temperature data-loggers deployed in a transect up the valley sidewall. The wind profile within the basin is measured using a Doppler wind LiDAR (Fig. 6). 

Valley Cold Pool Strength

The upper panel of Fig. 3 shows the time series of potential temperatures at the valley floor (~1300 m MSL) and at the mean top height of the Salt Lake City Basin, ~2200 m MSL, derived from the radiosonde observations by the National Weather Service from the Salt Lake City Airport. The difference between the two curves indicates how stable the valley atmosphere is stratified. When the two curves touch, the atmosphere is referred to as neutral for dry adiabatic processes and there is no resistance to vertical mixing. The further the two curves deviate, the higher is the stability and the harder it is to mix pollutants emitted at the surface. 

The bottom panel of Fig. 3 shows the Valley Heat Deficit a measurement of the energy that would be needed to bring the atmosphere to a neutral stratification. A correlation between the Valley Heat Deficit and PM2.5 pollution (red curve) becomes evident, as previously reported by Whiteman et al. (2014). 

Fig. 3 further illustrated that the strong wintertime inversions are often caused by warm air advection aloft rather than through enhance cooling near the surface.

Figure 3: Potential temperature at the base and top of the Salt Lake City Basin from radiosonde observations by the National Weather Service (top), and the Valley Heat Deficit and the smoothed observations of PM2.5 pollutant concentrations from the DAQ Hawthorne site.

6. The 26-30 January 2016 pollution event

Figure 4 shows selected time series of meteorological variables and PM2.5 concentrations observed during the 26-30 January pollution episode. The atmosphere in the Salt Lake Basin has been stably stratified, limiting vertical mixing. Typically, and under dry conditions, the temperature decreases with height by 9.8°C/km. Under "Inversion" conditions this temperature structure is inverted, which means that the temperature increases with height. Indications for this inversion can be seen in the 2nd panel of Fig. 4.; the temperature measured at NAA, for example, is lower then that measured at the University of Utah campus (blue).

The influence of nocturnal down-valley flows on the local air quality is nicely illustrated in Fig. 4. A down-valley flow established during the nights at the mouth of Red Butte Canyon (MTN site, green curves), indicated by a steady easterly flow direction. This flow tends to bring cleaner air into the Salt Lake City Basin, and a reduction in PM2.5 pollutant concentrations are observed (green curve, bottom panel).   

Figure 4: Overview over selected data collected during the 26-30 Jan 2016 pollution episode. Top panel shown shortwave or solar radiation received at the University of Utah campus (solid black line), the shortwave reflected radiation (dotted line) and the incoming longwave or thermal radiation received at the surface. Panels 2 through 5 shows time series of temperatures (2nd), relative humidity (3rd), wind speed (4th) and wind direction (5th), from Neil Armstrong Academy (NAA, red), the Hawthorne Elementary DAQ site (orange), the base (dark blue) and the top (light blue) of the WBB building on the University of Utah campus, and from the Mountain Meteorology Laboratory (MTN, green) at the mouth of Red Butte Canyon. The 6th panel shows the magnitude of turbulent kinetic energy (TKE) which serves as a proxy of the intensity of horizontal and vertical mixing in the atmosphere, measured at WBB rooftop (light blue) and the DAQ Hawthorne (orange) site. The bottom panel shows preliminary concentrations pf PM2.5 pollution at Hawthorne (orange), NAA (red), WBB rooftop (light blue) and MTN (green).

Atmospheric Backscatter from ceilometer observations

Laser ceilometers, deployed at Hawthorne Elementary, the University of Utah campus, and at the mouth of Red Butte Canyon, emit laser signals that are scattered back to the instruments by natural aerosols, hydrometeors (cloud droplets, rain, etc.), and anthropogenic aerosols such as particulate pollutants (PM10, PM2.5). By combining the backscatter signals, in-situ observations of PM2.5 concentrations, and LiDAR retrievals of the vertical wind profile, we are investigating the atmospheric mixing processes that influence the vertical and horizontal distribution of particulate pollution. Fig. 3 shows a time-height cross section of atmospheric backscatter coefficient measured at Hawthorne Elementary during the 26-30 January 2016 pollution episode. The layered structure and a large day-to-day variation in the height of the aerosol layer becomes apparent. Precipitation events are indicated by elevated backscatter values (29 Jan. 0100 MST, 1600-1900 MST).


Figure 5: Time-height cross section of atmospheric backscatter coefficient measured at Hawthorne Elementary during the 26-30 Jan 2016 pollution episode.

7. Images


Figure 6: Weber State HARBOR Aerostat deplyment in the Salt Lake City Basin (left panel), measurements of incoming solar and infrared radiation (upper middle panel) and wind speed, direction and turbulence (lower middle panel) on a roof top at the University of Utah. Lead PI Munkh Baasandorj (Utah Division of Air Quality) with news reporter in front of the "nerd mobile" (upper right panel), and Doppler Wind Lidar deployment on the valley floor(bottom right panel). Sebastian Hoch photos.

8. In the Press

We received some press coverage:

9. The 6-16 February Pollution Episode

Another pollution episode followed the episode illustrated above. A persistent cold air pool developed in early-mid February 2016 and led to an exceedance of the NAAQS on 8 consecutive days. This episode continues to be studied and several publications covering this episode are expected to appear in the scientific literature.

10. Selected References

last edited - 09/26/2016    sebastian.hoch *