Now meet the "quiet thief" that is stealing billions of dollars from American farmers every year.

It is ozone, a/k/a a major air pollutant. And a new government research report says that it is cutting the yield of corn, wheat, soybeans and peanuts by about $3.1 billion annually--a tenth of the total value of those four crops.

The new data, the most extensive yet produced, were developed by the National Crop Loss Assessment Network (NCLAN), a research program sponsored by the Environmental Protection Agency with help from Agriculture and Energy.

NCLAN's study, released yesterday by Rep. George E. Brown Jr. (D-Calif.), developed cost estimates far higher than those previously used to describe the effects of air pollution on farm production. It looked specifically at ozone, which is formed when hydrocarbons from burned fuels react with the sun's rays.

Brown, calling ozone "a quiet thief," said that crop losses from other pollutants not included in this study cost farmers billions of additional dollars yearly. In California alone, crop losses due to pollution are estimated at $1 billion a year.

Early NCLAN data reviewed during hearings last summer by Brown's House agriculture research subcommittee indicated that farm crop losses to air pollution were running between $1 billion and $2 billion.

The congressman's release of the study coincides with a symposium on the effects of air pollution on farm commodities, being held here today by the Izaak Walton League.

"These latest figures show a loss in the range of 5 percent of our total agricultural production and are based on a survey of only four major crops," Brown said. "These figures for reduced yields caused by ozone are startling. Yet they are conservative estimates of the total amount of farm and forestry production which vanishes into thin air--or bad air--every year."

In remarks prepared for delivery at the symposium, Brown also was sharply critical of the Reagan administration's plans to cut EPA spending to deal with crop-loss from air pollutants.

He said the NCLAN program, begun in fiscal 1980, was to have been funded at a level of $2 million per year. The Reagan budget would cut NCLAN funding to $300,000, and Brown said the administration has proposed rescinding some of that amount as well.

"To cut this program now, after our two-year investment is showing such results, does not make any sense," Brown said. "To terminate this program . . . before it is possible to set up a permanent monitoring network is a disservice to the farmers of America."

The NCLAN study said that of the $3.1 billion that may be lost in crop productivity for the four commodities, soybeans suffered the most--about 64 percent of the dollar losses. Others were corn, 17 percent; wheat, 12, and peanuts, 7.