WHAT IS TROPOSPHERIC OZONE AND WHERE DOES IT COME FROM?

Ozone is a molecule consisting of three oxygen atoms (O3). It is a very reactive oxidant (an oxidant oxidizes other compounds; removes electrons, often through combining with oxygen).

Ozone is called a "secondary pollutant". This isn't because it is secondary in importance, but because it is not emitted directly, but is formed from reactions of other pollutants in the atmosphere. (Ozone itself doesn't come out of an automobile's tailpipe, for example.)

Ozone is one of the main constituents in Los Angeles type smog, also referred to as "photochemical smog." (It is one of the constituents most injurious to human health.)

Ozone is also called a "photochemical oxidant." The 'photo' part of the name refers to the fact that the energy of sunlight is required for ozone production, and the 'chemical' part just refers to the fact that chemical reactions are involved. 'Oxidant' is included because ozone is an oxidant, chemically.

Ozone is produced in a cycle of reactions involving two basic classes of compounds:

NITROGEN OXIDES:

NOx's are involved not only with ozone production, but they are also heavily involved in producing acid deposition. They are included with the "reactive nitrogen compounds" that we talked about earlier -- those compounds whose production has been greatly accelerated by humans, with unknown, but likely significant, consequences for ecosystems, both terrestrial and oceanic.

NOx are produced whenever fossil fuels are burned at high temperatures and pressures. Their production results partly from oxidation of organic forms of nitrogen in the fuel and partly from oxidation of N2 gas in the atmosphere.

What is a situation in which fossil fuels are burned at high temperatures and pressures? Yes, in the internal combustion engine. Thus, the transportation sector accounts for about 40% of the atmospheric input of NOX in the US.

(Actually NO is 90% of direct NOx emissions, but it is readily oxidized to NO2 in the atmosphere.)

There are also other industrial and utility sources of NOx, and it is readily volatilized from fertilizers in agriculture. In addition to being volatilized (essentially, evaporated), bacteria in soils convert some forms of nitrogen fertilizer (particularly ammonia-containing fertilizers [NH3) into NOx's, via a process called "nitrification," which are then released into the atmosphere.

Globally, natural emissions of NOx are probably much larger than anthropogenic (human-caused) emissions, but it is difficult to estimate natural emissions, leading to much disagreement between various scientists about the relative proportions of natural versus anthropogenic sources. (See articles on human influences on the global nitrogen cycle listed with supplementary readings in the Study Guide for agricultural issues if you want more information, and recall that Vitousek et al. in the article in your readings packet on "Human domination..." point out that the global rate of nitrogen fixation has more than doubled under human influence. Not all of this fixation of N2 results in production of NOx, but a considerable fraction of it does!)

There is consensus, however, that for populated areas on Earth, anthropogenic emissions are much greater than natural emissions. The National Acidic Precipitation Assessment Program estimates that, for the US, natural sources produce about 11% of the total (natural + anthropogenic) NOx.

NATURAL SOURCES OF NOx

Primary natural sources of NOx include the following:

ANTHROPOGENIC EMISSIONS TRENDS FOR NOx IN THE US

NOx emissions in the US increased very rapidly from 1940-1970, approximately tripling over this time. The increases were attributable to our increasing population and the increasing intensity of our fossil fuel energy use, particularly related to cars.

Emissions declined slightly between 1978-88 (1988 emissions were about 9% lower than in 1978), and the decline is continuing.

Decreases in NOx emissions have been forced by various pieces of legislation. For example, the first NOx emissions standards for cars and trucks were imposed in 1973, as part of the US Clean air Act. Emissions standards have been tightened since that time. (Catalytic converters reduce NOx to N2 gas.)

Between 1978 and 1983, NOx emissions from cars + trucks decreased by 9% despite a 5% increase in miles driven! This is a nice demonstration that cleaning up doesn't necessarily mean stopping growth. However, everything isn't rosy: between 1970 and 1995 there was an average decrease of 1/2 - 1/3 in NOx emissions per car, BUT over that same time interval, the number of vehicles increased by 85% and vehicle miles increased by 105%. Thus, the decrease in emissions per car wasn't able to do much more than offset the increase in mileage.

A bright spot! As of the year 2000, use of trains and buses for transit increased in the US more than did automobile use for the fourth year in a row. The number of trips by public transit increased ~20% over the preceding 5 years, while the growth in driving rates was only 11%. Also in the year 2000, the number of miles driven by car in the US was flat for the first time in 20 years, according to the Federal Highway Administration. Increases in transit use are credited to several factors, including increasingly congested roads, improved transit systems, and increased public support for mass transit.

Under the Clean Air Act Amendments of 1990, another large decrease in tailpipe NOx emissions was mandated by the year 2000 and NOx emissions from existing coal-fired steam electricity generating facilities were also required to decrease 2 million tons by 2000. Emissions from such "stationary sources" [as opposed to transportation-related sources], including industrial and utility sources,were tightened again in 2011.

NOx emissions have followed similar temporal trends in Europe, but are increasing on a global basis as industrialization (and cars) increase in the lesser developed nations, particularly of Asia, and in nations of the former USSR.

Ninety five percent of global NOx emissions from fossil fuel combustion are estimated to come from countries North of the equator.

HYDROCARBONS:

The other big contributors to O3 production are hydrocarbons. These are sometimes referred to as "VOC's" (volatile organic compounds). Basically, they include hydrocarbons and oxygen-containing organic compounds, which have high vapor pressures at ordinary room temperatures. They range from simple compounds such as methane (CH4) through compounds such as benzene to complex, high molecular weight polycyclic aromatic hydrocarbons.

Most important from the perspective of ozone-forming potential are the highly reactive and volatile compounds such as aldehydes (for example, formaldehyde). Methane is less reactive than most VOC's and not usually important in ozone formation except in remote atmospheres (which means that globally it may be very important). Thus, you will sometimes read about "NMOC" (nonmethane organic compounds) instead of VOC's in connection with ozone formation. (Methane is an important greenhouse gas, however!)

Natural sources of hydrocarbons are larger than anthropogenic sources, globally. (We'll explore this further when we discuss greenhouse gases and methane.) Important natural sources include:

In most urban areas, however, anthropogenic sources are more important, with incomplete combustion as from automobiles being a major component. The "gassy" smell at gas stations is partly hyrdocarbons. Additional sources include bakeries and dry cleaners, with their "fugitive emissions" of VOC's, as well as some paints, solvents, and charcoal lighter fluids.

There are some highly vegetated cities -- such as Atlanta, GA -- in which anthropogenic sources represent only about 24% of hydrocarbon emissions. The bulk of emissions in Atlanta (59%) is isoprene from vegetation. (Emissions of many hydrocarbons from plants are also higher in summer when it is hotter; the peak O3 forming time....)

The 1990 Amendments to the US Clean Air Act called for a 30% decrease in VOC emissions from tailpipes by 2000. (Three way catalytic converters, which have been required on all automobiles for many years now, remove NOx, carbon monoxide [CO], and VOC's.) Today's cars emit 80-90% less VOC's than did cars of the early 1970's (but remember, there are many more cars now than there were then!).

FORMATION OF OZONE

Now let's look at how these compounds interact in the production of ozone. Please note: I do not expect you to memorize these reactions, but simply to understand the process conceptually. I'll give a stripped-to-the-essentials version in lecture!

NO is emitted from autos, but in bright sunlight it is photochemically oxidized to NO2. It can also be oxidized to NO2 by reactions involving reactive oxygen-containing organic molecules, such as VOC's.

Ways of getting NO2:

(1) NO + O3 ----hv----> NO2 + O2 [note that this reaction requires energy from the sun ["hv"] and that it actually consumes O3]

OR

(2) NO + RO2 -----------> NO2 + RO [where RO2 is a reactive oxygen-containing molecule; note that this does not consume O3 but still produces NO2; the RO2 could be a VOC, as described above]

First, we are going to look at O3 production in an atmosphere that does not contain abundant hydrocarbons. (This would be characteristic of many remote areas, but not of urban areas.)

NO is emitted, and, in sunlight, is photochemically oxidized to NO2 (using O3 or another oxidant, as in eqns. 1 & 2, above)

NO2 is then split into nitric oxide (NO) and an oxygen atom (O) in the presence of sunlight. This is called "photodissociation"):

(3) NO2----------hv-------> NO + O (where "hv" indicates high energy solar radiation)

The atomic O then reacts with the normal O2 of the air to form O3:

(4) O + O2 + m -------> O3 + m (where "m" is an energy-accepting third body).

The O3 may then react with NO to reform NO2 and O2:

(5) O3 + NO <--------> NO2 + 02

These (reactions 3 - 5) then comprise a balanced cyclic reaction and O3 doesn't build up; for every consumption of O3 by oxidation of NO to NO2, there is the production of one O3 by photodissociation of NO2. In this situation, characteristics of unpolluted air, NO consumption = NO2 production.

Summary reaction: O3 + NO <------hv-----> NO2 + O2

O3 can also be destroyed by ultraviolet photodissociation:

(6) O3 <-----hv------> O2 + O [note that this reaction runs both ways]

Now, in atmospheres containing compounds that the NO can react with (such as VOCs):

So, we take the same cycle as described above, but put hydrocarbons in it, and they interfere with that balanced, cyclic reaction. With hydrocarbons:

(1) NO2 is produced without using O3 and

(2) that NO2 can then go on to produce O3.

Think of it this way:

Some of the products of the reaction of hydrocarbons with NO also produce other nasty photochemical pollutants such as PAN (peroxyacetyl nitrate).

Click on "where is it a problem?" to jump to information on situations in which ozone levels become elevated. To return to the index of subjects dealing with tropospheric ozone, click on tropospheric ozone. Click "contents" to return to the master Table of Contents for this BI 301 home page. For reminders on how to navigate within and among these pages, click "Navigate."

Page maintained by Patricia Muir at Oregon State University. Last updated Dec. 3, 2012.