Tropospheric Ozone

What is ozone?

• O₃
• Properties: blue color, characteristic smell (a little like chlorine) , absorbs UV light
• highly reactive compound
• discovered by Schoenbein, 1940
• There are two problems associated with ozone, 'good' and 'bad' ozone (Fig)

Stratospheric ozone

• 85 to 90% of ozone found in the stratosphere
• UV produces Ozone in stratosphere, maximum at about 30 km height
• 3 O₂ + UV ->  2 O₃
• destruction:
• O₃ + UV, visible light -> O + O₂
• O₃ + X -> XO + O₂ (where X may be O, NO, OH, Br or Cl)
• vertical concentration distribution (Fig)
• typical maximum concentration: 12 ppm
• Dobson Unit: the unit of measure for total ozone. If you were to take all the ozone in a column of air stretching from the surface of the earth to space, and bring all that ozone to standard temperature (0 Celsius) and pressure (1013.25 millibars, or one atmosphere, or "atm"), the column would be about 0.3 centimeters thick. Thus, the total ozone would be 0.3 atm-cm. To make the units easier to work with, the "Dobson Unit" is defined to be 0.001 atm-cm. Our 0.3 atm-cm would be 300 Dobson Units (DU).
• ozone hole, caused by CFC's -> increase in UV -> skin cancer
• ozone hole, southern hemisphere (Fig), still growing (Fig)
• ozone hole, northern hemisphere (Fig)
• today's ozone concentration
• 1987 Montreal protocol: phase out use of CFC's by 1996/2010

Tropospheric ozone

• Troposheric, or ground-level ozone, is the major ingredient in smog and continues to pose a health risk to many Americans
• health effects:
• ozone attacks cells and breaks down tissue
• decreased ability to breathe, coughing, increased susceptibility to respiratory diseases such as pneumonia and bronchitis
• increased sensitivity to allergens
• long-term exposure may result in permanent lung damage
• Risk groups: sensitive healthy adults (5-25%), children, individuals with respiratory disease, exercising individuals
• according to EPA, about 15,000 Americans die every year from exposure to air-borne pollutants, and exposure to ozone causes hundreds of thousands of acute asthma attacts
• ozone is a plant toxin, enforced by presence of SO₂ and NO_x
• ozone also damages materials such as nylons, rubber, and certain fabrics
• economic impacts
• damage of agricultural crops, forests, and wilderness areas
• lowering of ozone levels by 25% may increase US crop yield by $0.5 to 1.0 billion per year (mainly soybeans, corn, and tobacco;Office of Technology Assessment)
• natural tropospheric ozone concentration: 10 ppb
• in 1997, EPA indroduced a new ozone standard:
• In establishing the 8-hour standard, EPA is setting the standard at 0.08 parts per million (ppm) and defines the new standard as a "concentration-based" form, specifically the 3-year average of the annual 4th-highest daily maximum 8-hour average ozone concentrations.
• The 0.12 ppm 1-hour standard will not be revoked in a given area until that area has achieved 3 consecutive years of air quality data meeting the 1-hour  standard. The purpose of retaining the current 1-hour standard is to ensure a smooth, legal, and practical transition to the new standard.
• Nearly 100 major cities in the United States are periodically exposed to concentrations of ozone that exceed health-based air-quality standards.
• According to the U.S. EPA, 80 million people lived in areas designated "non-attainment" for ozone in 1991 (according to the old standards)

Formation of tropospheric ozone

• chemistry of ozone formation (Ozone cycle)
• complex
• involved are: VOC (RH), OH (hydroxyl radical), NOx, M (inert body, N₂, or O₂); O₃ can be dissociated by UV and will form two OH -> chain process
• VOC and NO_x concentrations control ozone concentrations in a complicated way (Fig)
• O3 formation is probably NOx limited, rather than VOC
• (NOx is the sum of NO and NO₂)
• removal of ozone
• oxidation of all sorts of things
• interaction with surfaces
• reaction with NOx, that's why in polluted areas ozone drops to low values at night
• what is needed for formation of ozone:
1. VOCs: volatile organic compounds: mostly emitted by motor vehicles, vegetation, industrial, and commercial, dry cleaners, paints
2. NOx: nitrogen oxides,motor vehicles, power plants, industrial facilities, biomass burning, lightning
3. sunlight
4. high temperature (Fig)
5. (low windspeed)
• meteorology is also influencing ozone concentrations (Fig)
• highest O₃ concentrations can be found in the summer during dry high pressure conditions
• during inversions (warm air above cooler air) pollutants often get trapped resulting in high ozone concentrations
• Sources of pre-cursors: National emissions inventory (1985)
• NOx, 5.67 Teragrams nitrogen/y (Fig)
• VOC's, 20.02 Teragrams/year (Fig)

Ozone in NYC this summer

• measurement of ozone by UV absorption
• today's ozone concentrations
• July 1999 was the warmest July on record in NY
• maximum and average daily temperature at LGA airport in July (Fig), climatology of July 1999 at LGA (PDF file)
• ozone concentrations in 1999 in the Northeast
• Air Quality Index (AQI), indroduced by EPA for reporting the levels of ozone and other common air pollutants:

 

Ozone Concentration (ppm) (8-hour average, unless noted)	Air Quality Index Values	Air Quality Descriptor
0.0 to 0.064	0 to 50	Good
0.065 to 0.084	51 to 100	Moderate
0.085 to 0.104	101 to 150	Unhealthy for  Sensitive Groups
0.105 to 0.124	151 to 200	Unhealthy
0.125 (8-hr.) to 0.404 (1-hr.)	201 to 300	Very Unhealthy

Trends in tropospheric ozone

• Trends in US
• percentiles, box plots
• VOC's definitely decreasing
• NOx is fairly constant, maybe slightly decreasing
• Ozone in US slightly decreasing

Resources:

Stratospheric Ozone

The TOMS home page, NASA

The Ozone hole tour at the University of Cambridge

Goddard DAAC: Atmospheric Chemistry and Resources, NASA

Tropospheric Ozone

WHO/UNEP (1994) Air pollution in the World's Megacities. Environment, 36, 2.

National Research Council (1992) Rethinking the ozone problem in urban and regional air pollution. National Academy Press, Washington, D.C., 500pp.

EPA: The National Emission Trends Update 1970 -1999

NorthEast Airshed Regional Data Analysis Toolshed - NEARDAT

Nat'l Academy Press, Rethinking the Ozone Problem in Urban and Regional Air Pollution (1992), Table of Contents

US EPA Region 2: New York State

Air Pollution - US EPA Office of Air and Radiation

EPA: Smog--Who Does it Hurt?

Air Pollution - Aerometric Information Retrieval System (AIRS)

EPA's Decision on New Air Quality Standards

AIRNOW - Real Time Air Pollution Data

Commonly asked questions about Ozone, NOAA

OAR Publication: 1997 National Air Quality Trends Brochure - Ground Level Ozone