"PREPOSTEROUS" seemed about right for the statement that Sen. Kennedy attributed to Ronald Reagan in his speech Tuesday: "80 percent of air pollution comes from plants and trees." But immediately from Reagan headquarters in California came word that the senator had misquoted the former governor.What Mr. Reagan had in fact said, his deputy staff chief informed the press, that "80 percent of nitrogen oxide pollutants in the air come from plants and trees, not 80 percent of all air pollution." Oh.

If something tells you that still sounds fishy, you're right. Like so many other campaign clarifications, this one only makes matters worse. So here, for the record, is more than you will ever again want to know about nitrogen oxides. If it begins to sound too much like high school chemistry class for comfort, bear with us: there will be no quiz at the end.

There are three types of nitrogen oxides in the atmosphere. There names sound alike but they aren't -- two of them are pollutants and one of them isn't. The one that isn't, nitrous oxide, N2O, amounts to more than 90 percent of the total. If he was talking about anything, this is the one Mr. Reagan was probably talking about. It is given off by a relatively small class of bacteria known as denitrifying bacteria. Plants and trees do not emit any gaseous nitrogen oxides -- in fact, they spend most of their time and energy taking in nitrogen from the soil. Nitrous oxide (which you might also recognize as laughing gas, although this is a serious matter) is a biologically natural and benign gas. Its chief role is to control the ebbs and flows of the ozone layer in the stratosphere.

The other two nitrogen oxides, nitric oxide, NO, and nitrogen dioxide, NO2, are the pollutants. Both are principally the products of combustion -- the burning of coal in a power plant or gasoline in an automobile engine. Nitric oxide is a colorless gas that in itself is not particulary toxic. However, it reacts with hydrocarbons and sunlight to produce smog that can harm or kill plants and animals. It is also oxidized in the air to form the orangish-brown nitrogen dioxide, which is quite toxic. It interferes with the blood's ability to carry oxygen and can produce various types of lung damage. Nitrogen dioxide also reacts with water vapor to form nitric acid, which gives us acid rain.

We like to be helpful, so here is what Mr. Reagan can say if -- though it seems unlikely -- he should ever again wish to comment on the nitrogen cycle: "90 percent of all nitrogen oxides come from denitrifying bacteria." It hardly seems a slogan to campaign on, but it has the advantage of being right. For those of you who were wondering if you should pave over your back yards in the interest of cleaner air, forget it -- plants and trees are innocent of polluting the air. Man-made air pollution, Mr. Reagan notwithstanding, is still the problem.