As we've already
seen, ozone becomes a problem only in areas where air is polluted
both with hydrocarbons and with NOx. Since the two are typically
emitted together, however, they are ususally elevated in the same
areas (see previous section for sources
of both).
There is some O3 naturally present in the troposphere, resulting
from incursions from stratosphere (during thunderstorms or other
mixing times) and from photochemical reactions involving natually-produced
precursors. Background levels are, however, much lower than in
areas strongly affected by pollutants; preindustrial background
levels are estimated to have been on the order of 0.01 - 0.015
ppmv (parts per million by volume), while modern background levels
range from 0.02 - 0.04 ppmv (39 - 78 ug/m3)
Ozone concentrations haven't been being monitored for very long using current methods. They were monitored in the mid-1800's at a number of sites in Europe, but the comparability of those methods to modern monitoring methods is uncertain, making it difficult to describe accurately trends in concentration from pre-industrial times to the present. However, in areas of West Germany and Great Britain, monitoring has been going on using modern methods since the 1950's, and ambient concentrations in those areas have increased as would be expected given increased population and increased intensity of fossil fuel use over that time. Data from a variety of midlatitude sites in the Northern Hemisphere indicate that ambient concentrations increased by about 12% between 1970 and 1981, however trends depend very much on the monitoring sites chosen and the period over which trends are examined. While much of the increase undoubtedly relates to increases in precursor emissions, patterns in weather (hot & sunny vs cloudy) and patterns of stratospheric intrusion (as in thunderstorms) are also influential.
However, ozone concentrations in the US decreased markedly -- on average, between 1976 and 1997, ambient ozone concentrations decreased by 30.9 % (despite a 58 % increase in the GDP, a 45 % increase in energy consumption, and a 143 % increase in vehicle miles driven!). It isn't clear whether this decline continued into the 2000's.
Highest levels occur on warm sunny days with relatively still air (in which precursors build up and there is plenty of solar energy available). This is why air alerts for cities with ozone problems (such as Los Angeles) often occur on the nicest sunny days.
Such conditions are often associated with thermal inversions, and that is partly why inversion-prone cities such as LA and Denver have such ozone troubles (in addition to their abundances of precursors, of course!)
Here is how an inversion works and how it allows buildup of pollutants such as ozone (briefly):
As you know, air normally is cooler as you go up in elevation
During an inversion, a layer of warmer air sits on top of a layer of cooler air near the earth; that is, the temperature profile with elevation gets turned upside down, or inverted.
Valley or bowl-shaped topographies (such as surround Denver and LA) favor development of inversions, because cool air, being denser, drains into the valley (like overnight) and can then get trapped there when conditions are still. Still conditions associated with these topographies also result in pronounced radiative cooling at night which leads to near surface cool temperatures. The less dense warm air then floats like a cap over the cooler air in the valley and "traps" it there until winds or other disturbances disrupt these conditions.
In inversions then, pollutants can really build up since air movement is restricted by still conditions and the cap of warm air.
Seasonally, ozone levels are usually highest in mid to late summer when there is an abundance of solar energy to drive the reactions and temperatures are warm.
Diurnally, the typical pattern in urban areas is for there to be a sharp peak in O3 concentrations between late morning and early afternoon, with levels often falling to near zero overnight. This pattern results from:
What is the fate of O3? As a global average, about 1/3 of O3 is deposited on surfaces and then consumed by reacting with surfaces such as leaves and materials (oxidative reactions), while the other 2/3 is consumed by chemical reactions in the atmosphere.
When scientists first discovered the existence
of elevated levels of tropospheric O3, it was believed that it
was strictly an urban problem. This made sense, because urban
areas are where the precursors are produced in abundance and also
tend to be where air quality monitoring is done.
Subsequent air quality monitoring and analysis of patterns in
biological effects caused by ozone revealed, however, that its
levels are elevated in broad regional areas, distant by even hundreds
of miles from major sources.
That is, ozone is a "transported pollutant." It has a 60 hr atmospheric residence time, on average. Not only is ozone itself transported, but as air masses containing precursor pollutants travel and "cook," reactions producing ozone continue during daylight hours. NOx can persist in the atmosphere for several hours on average before being further oxidized or consumed, during which time reactions generating ozone can continue. Hydrocarbons also are transported over long distances. In addition, some of the products of reactions of hydrocarbons with NO (such as PAN) can serve as reservoirs for NOx on later days of transport -- reactions are reversible so that NO can be regenerated, enabling production of additional ozone.
In fact, sometimes a phenomenon called "urban quenching" is observed, in which ozone levels are higher just downwind of a city than right over it. This seems to occur because there is so much NO in the atmosphere over the city that a lot of the ozone produced there back reacts with it immediately. As the air moves downwind from the city, less of the NOx is present as NO and more is present as oxidized NO2 (or other reaction products), so there is less NO for ozone to back react with, and its concentrations increase.
OZONE CONCENTRATIONS CAN BE ELEVATED IN
RURAL AREAS FROM:
1. Its production in air masses that contain precursor pollutants
as they travel
2. Transport of ozone itself that was produced in urban areas
The diurnal ozone pattern in rural areas is typically quite different from that in urban areas. While the peak hour (maximum concentration) is often lower than in the urban and near-urban areas, the peak area is broader. That is, rather than a relatively brief, high spike in concentrations, concentrations in rural areas are elevated over muchof the day (and even night), but elevated to lower maxima than in urban areas.
This diurnal pattern in rural areas is caused by the facts that:
In fact, in some remote areas, O3 levels often don't even drop over night because it continues to arrive in air masses from elsewhere.
The result is that, in rural areas, daily mean (average) ozone concentrations can actually be higher than in urban or near-urban areas! (This doesn't mean that the maximum hourly concentration is higher in rural areas, but averaged over the entire day, concentrations in rural areas can be higher.)
The importance of ozone for ecosystems depends in part on which pattern of exposure -- rural or urban -- causes the most injury. For example, the longer exposure in rural areas may be more damaging than the shorter, acute blasts in cities. Even though peaks are lower in rural areas, they are sustained so much longer that resultant injury may actually be greater.
Thus, ozone is truly a regional pollutant -- it is of concern in much more than in urban areas.
In addition to ozone levels in rural areas being elevated becasue of transport of urban pollutants, there are increasing reports of ozone "episodes" (periods of high concentration) in tropical and subtropical regions far from urban areas. In these cases, ozone production is related to emissions of hydrocarbons and NOx produced from biomass burning (as in cutting and burning of tropical forests). The intense sunlight, coupled with high emissions of precursors, favors ozone production. Concentrations are highest in the dry season (September - November) and can be as high as in polluted areas of North America! Large tropical oceanic regions are affected each year by these episodes.
In contrast, tropical forests are also a "sink" for O3 because plants take it up very efficiently (where it causes problems, as we'll see).
To move to the next section of notes, on effects of ozone, or on what is meant by a "criteria pollutant," click the highlighted phrases. Click "tropospheric ozone" to return to the index of subjects discussed relative to this topic, or on "contents" to return to the master Table of Contents for these pages. Click "navigate" for reminders on how to move about within and among these pages.
Page maintained by Patricia Muir at Oregon State University. Last updated November 19, 2007.