Phoenix, Rising from the Ashes

Ozone and Urban Air Pollution in the Hottest City in the U.S.

Phoenix, AZ (USA): A History

Hohokam Canals

Nomadic Native American populations in the Salt River Valley (7000 to 6000 BC) followed by more agricultural-based populations in the last two millennia (Hohokam, O’odham, and Sobaipuri).

European settlers arrived and attempted settlements near present-day Tucson before 1800, followed shortly by American settlers.

Phoenix was settled in 1867 and incorporated in 1881. The population boomed in the 1950s due to the influx of WWII veterans and their families and increased adoption of air conditioning. Tech companies like Motorola and Intel established research and manufacturing centers in Maricopa county.

Phoenix Today

Today: The Phoenix metropolitan area has almost 5 million residents, making it the 11 th  largest metropolitan area in the nation (2020 census). The main industries include real estate, financial services, and manufacturing (electronics, aircraft parts, processed foods)

Emissions

2020 Breakdown of Greenhouse Gas (GHG) Emissions  MCOC041_Flyer_Template_F (maricopa.gov) 

Mobile sources contribute to the largest source category.

Mobile sources are responsible for Green House Gas Emissions (GHG) in addition to NOx and VOCs which are precursors for Ozone

EPA Total Emissions

The US Environmental Protection Agency (EPA) collects emissions from different sections like: Power Plants or Road Transpiration

In Phoenix, the 2017 annual NOx emissions see clear enhancements on the roads and major highways, indicating this is a major emission source of NOx

Importance of Ozone in Phoenix

An O 3  exceedance day occurs when the MDA8 O 3  is greater than 70 ppb on that day. The exceedance days are mostly recorded from April to September, referred to here as the “ozone season”. Pheonix has "nonattainment" status, so this standard is not met.

Guo et al., 2024

The figure shows the seasonal distribution of monthly O 3  exceedance days at the JLG Supersite in Phoenix (site no. 04-013-9997). August has the most exceedance days and will be of focus in analysis.

The Valley of the Sun as an Urban Heat Island

Importance of Temperature in Phoenix

High temperatures year round necessitate the constant use of air conditioning and other heat-mitigation methods. Location of the city on the valley floor and persistent heat also contribute to the trapping of pollutants in the urban area.

(L) Phoenix metropolitan area (R) Land surface temperature of Phoenix 07/10/2024 11:03 MST

As a desert city, Phoenix experiences an interesting twist on the urban heat island effect - the highest observed LST occur in the surrounding desert (away from the urban centers). Evaporation of surface water and the presence of "unnatural" vegetation keeps the urban center relatively cool during the day. Heavily-irrigated areas and areas surrounding canals are the coolest. However, night time cooling is more dramatic in the surrounding desert and the urban areas fail to experience the same degree of cooling, leading to relatively higher nocturnal temperatures.

Native American territory on the borders of the metropolitan area experience the highest temperatures (exceeding 150 degrees F).

Human Health Impacts

Air Pollution and Health

Air pollution can have a wide range of health effects on people, both short-term and long-term. Prolonged exposure to pollutants like ground-level ozone and particulate matter can also contribute to premature mortality. Studies have shown that there are links between outdoor air pollution and adverse health outcomes ( Lelieveld et al., 2015 )

In Phoenix, high temperatures and elevated ozone levels contribute significantly to Chronic Obstructive Pulmonary Disease (COPD). According to the American Lung Association's 2023 State of the Air report, Phoenix was ranked one of the top 5 cities with  Ozone pollution .

Using the EPA's Environmental Justice (EJ) Screening and Mapping Tool, we found that areas with high traffic proximity often overlap with low-income populations. This can result in increased exposure to pollutants and higher traffic-related risks. Studies suggest that this correlation can further increase health disparities ( Grineski et al., 2007 ;  Pratt et al., 2015 )

Traffic Proximity and Volume (Left) Low Income (Right) (EJScreen)

The EJ Index is crucial for identifying how environmental hazards like ozone pollution disproportionately affect different communities. Low-income communities are disproportionately affected by ozone pollution due to their higher exposure to traffic emissions and industrial activities.

EJ Index for Ozone (left), Low Income (right) ( EJScreen )

Heat Related Health Impacts

Extreme summer heat poses significant risks to human health. Prolonged exposure to high temperatures increases mortality rates, with vulnerable populations particularly at risk.

As temperatures continue to rise we find that that the human health impacts will rise during the high summer heat. Maricopa county's  heat-related illness and death data  are updated weekly during the heat season.

In 2023 majority of heat related deaths occurred between July and August. Of the total 645 heat-related deaths, 378 were directly caused by heat while 267 had heat as a contributing factor.

2023 Heat Related Deaths in Phoenix ( MCPH )

The combination of high temperatures contributing to the elevated ozone levels can have compounded health effects. In 2023, 68% of heat related deaths were premature deaths.

Heat Related Deaths by Age Group ( MCPH )

Chemistry of Ozone

Phoenix has been classified by the EPA as moderate non-attainment under the 2015 ozone guidelines, meaning the city has exceeded the 8-hour exposure standard of 70 ppb of ozone for multiple years in a row.

Formation of ozone and PM 2.5  above the Phoenix skyline from volatile organic compound and NO 2  emissions  ( image adapted ) 

Emissions in urban areas contribute to the formation of ozone that people breathe in daily. Emissions of volatile organic compounds (or VOCs) contribute to both the formation of ozone, as well as particulate matter. VOCs are emitted from sources such as industrial solvents, cleaning products, personal care products, and combustion engine traffic. Importantly, car and truck traffic also emit NO 2 , another urban pollutant that contributes to ozone formation. In the presence of sunlight (readily abundant in the Valley of the Sun), NO 2  can photolyze and lose one of its oxygen atoms. This oxygen can then react with molecular oxygen (O 2 ) to form ozone.

As seen in the scheme above, NO 2  is directly emitted from Phoenix, but it also can be formed through reactions with VOCs. After VOCS are emitted, they can be oxidized with OH to form HO 2 , which in turn can reform photolyzed NO 2  from NO. These cycles will generally continue to produce ozone as long as there is sunlight.

Phoenix has another factor contributing to its elevated ozone concentrations: the record-breaking temperatures Phoenix experiences drive the reactions faster, meaning Phoenix produces high levels of ozone quickly.

As seen to the left, the modeled ozone in Phoenix increases linearly with the temperature, making ozone production a growing concern as temperatures rise in the city.

RSIG visualization

To the right is a timelapse showing variation between day and night ozone in Phoenix. The concentration increases after the sun rises and peaks in the middle of the day. On this day, the peak concentration hits 101 ppb. As night falls, ozone will stop forming and its concentration will decrease as it is lost to other sources, until it gets back down to 0 ppb.

Timelapse of AirNow ozone monitors on July 28th, 2023

Integrated Air Quality System

In Situ Observations

In situ measurements require that the instrumentation be located directly at the point of interest and in contact with the subject of interest. 

AirNow reports air quality using the official U.S.  Air Quality Index  (AQI), a color-coded index calculated from in situ observations. The AQI is designed to communicate whether air quality is healthy or unhealthy for you. When you know the AQI in your area, you can take steps to protect your health.

AirNow is a partnership of the U.S. Environmental Protection Agency, National Oceanic and Atmospheric Administration (NOAA), National Park Service, NASA, Centers for Disease Control, and tribal, state, and local air quality agencies. 

Below are the Air Now in situ monitoring locations for the greater Phoenix area.

AirNow Interactive Map

Model Simulation

WRF-Chem

WRF-Chem is the Weather Research and Forecasting (WRF) model coupled with Chemistry. The model simulates the emission, transport, mixing, and chemical transformation of trace gases and aerosols simultaneously with the meteorology. The model is used for investigation of regional-scale air quality, field program analysis, and cloud-scale interactions between clouds and chemistry.

Atmospheric chemistry models help us understand what in situ observations see. They are used to help us understand drivers of ozone production.

The model simulated Phoenix NO 2  and Ozone for August, 2023.

Ozone experiences a diurnal variation as shown here with WRF-Chem (Model – red) and Observations (Obs – black). O 3  level rises in the morning, peaks in the afternoon and then gradually declines at night. O 3  production could be driven by the increase in NOx emissions from morning transpiration.

WRF-Chem does well to simulate the variation of ozone observed from in situ monitors, but it fails to capture the exact values, and is biased low.

Satellite Observations

TEMPO

NASA's first Earth Venture Instrument mission will measure pollution of North America, from Mexico City to the Canadian oil sands, and from the Atlantic to the Pacific hourly and at high spatial resolution. TEMPO observations are from the geostationary vantage point, flying on a telecommunications host spacecraft launched in 2023.

TEMPO NO2

The TEMPO instrument is a UV-visible spectrometer, and will be the first ever space-based instrument to monitor air pollutants hourly across the North American continent during daytime. It will collect high-resolution measurements of ozone, nitrogen dioxide and other pollutants, data which will revolutionize air quality forecasts.

Mean TEMPO NO2 Tropospheric columns at 3 hourly intervals.

NO 2  columns observed from TEMPO show the highest columns in the morning time, which could correspond to increased road emissions when citizens are commuting to work and dirve Phoenix ozone production and lead to exceedances.

Comparison of TEMPO NO 2  column and WRF-Chem surface NO 2 

WRF-Chem and TEMPO produce similar diurnal variations, a double peak, where there is a peak of NO 2  in the morning hours and decreases throughout the day and begins to peak again toward the evening hours

This variation is mainly due to photochemical formation and meteorological conditions. In daytime, an increase in global solar radiation and the height of the mixing layer results in a decrease in NOx concentration or reacts to from O 3 .

The double peaks could be caused by increased emission by road traffic as well as other chemical and meteorological variables.

Phoenix in the Future 

Increased Wildfire Risk and Exposure

Rising global temperatures and associated aridity will drive an increase in wildfire occurrence and emissions worldwide. Such emissions can travel over large distances and contribute to Phoenix's existing air pollution problem.

HRRR-Smoke (NOAA GSL) - York Fire (July 29 2023)

Increasing Temperature and Demand on the Electrical Grid

The rise of Phoenix was made possible by air conditioning, which still drives much of the power consumption in the city to this day.

We must consider the resiliency (or vulnerability) of urban areas to extended electrical grid failures happening concurrently with extreme heat waves as climate change drives more frequent extreme heat waves worldwide.

Stone et al. (2023) estimated current and future heat-related mortality and morbidity risks for three cities (Phoenix, Detroit, and Atlanta) during heat events.

(R) We can already see an increasing trend in the total number of major electrical grid failures in the U.S. since 2015, motivating the need for such a study.

Considered six cases:

  1. Power on: present day, operational electrical grid
  2. Power off: present day, nonoperational grid. No cooling for 48 hours, gradual reintegration over the next 72 hours
  3. Street Trees: present day, nonoperational grid. Suppose residential roadways covered by 50% tree canopy
  4. Cool Roofs: present day, nonoperational grid. Suppose roof material albedo for all buildings set to 0.88
  5. Mid century: 2055, nonoperational grid. No heat management strategies in effect
  6. Late century: 2085, nonoperational grid. No heat management strategies in effect

(R) Emergency department visits per 100,000 people over concurrent five day blackout and heat wave

Climate Action Plan

This isn’t to say that Phoenix is doomed! The people of this city are committed to following a  climate action plan  to reduce emissions contributing to ozone pollution. This plan includes continuing work done to shift city services to carbon-neutral electrical use, increase energy efficiency, and expand public transit.

Meet the Team:

From left to right: Audrey, Sean, Tabitha, Halima

Acknowledgments & Citations

Many Thanks to NCAR!

AirNow

WRF-Chem

TEMPO

RSIG

EJScreen

2020 Breakdown of Greenhouse Gas (GHG) Emissions  MCOC041_Flyer_Template_F (maricopa.gov) 

Guo et al., 2024

EJ Index for Ozone (left), Low Income (right) ( EJScreen )

2023 Heat Related Deaths in Phoenix ( MCPH )

Heat Related Deaths by Age Group ( MCPH )

Formation of ozone and PM 2.5  above the Phoenix skyline from volatile organic compound and NO 2  emissions  ( image adapted ) 

RSIG visualization

Timelapse of AirNow ozone monitors on July 28th, 2023

From left to right: Audrey, Sean, Tabitha, Halima