AS August fades into September, Americans are ready, once again, to celebrate their Labor Day holiday weekend. Light up the outdoor grills. Throw on the hamburgers.And, this year, do it with at least a thought for the subtle way such delightful cookery dirties the air. Smoking charcoal and evaporating lighter fluids are obvious polluters. What have not been so obvious are the particles and chemicals coming from the cooking meat itself. These now turn out to be the most significant pollutants of all. Environmental chemist Glen R. Cass and his colleagues at the California Institute of Technology in Pasadena, working with Bernd R.T. Simonelt of Oregon State University at Corvallis report that "meat cooking operations are a major source of organic aerosol emissions in the urban atmosphere." In their research paper published earlier this summer in Environmental Science & Technology, they explain that such emissions "comprise up to 21 percent of the primary fine organic carbon particle emissions in the Lo s Angeles area." As a numerical example, they note an estimate that, every day in 1982, meat cookery contributed 4,400 to 4,900 kilograms for charbroiling plus 1,400 kg for pan frying to the fine organic particle emissions in the Greater Los Angeles area. That beats the contributions of more obvious sources such as forest fires, fireplaces, or motor-vehicle exhausts. Meat cookery has been a suspected polluter for some years. Laboratory tests have revealed substantial emissions of particles and various chemical species. But there was little proof that such emissions were actually an important part of the real air-pollution mix. Air monitors needed distinctive tracer chemicals to pin down the meat pollution. This is what Cass and his colleagues now have provided. They identified some 75 such marker chemicals. One of the best of these tracers turns out to be that old nutritional bugaboo cholesterol. The scientists report that "fine aerosol cholesterol can be regarded as a potentially important marker for food cooking operations." The pollution depends partly on the type and quality of the meat and on how it is cooked. In estimating emissions, the researchers concentrated on beef hamburger - the most widely used type of dry cooked meat in the United States. They used these data to approximate emissions for meat cooking of all types. In general, they found: "Fine aerosol emissions from charbroiling of regular hamburger meat [21 percent fat] are higher than from charbroiling extra lean meat [10 percent fat], which in turn are much higher than the emissions from [pan] frying the same amount of hamburger meat." The research team's findings need not put a damper on Labor Day cookouts. Their main significance lies in identifying meat cookery as a major polluter and in providing chemical fingerprints to trace that pollution. Commercial operations are the biggest source of meat-cooking pollution, by far. And there is little regulation of their meat cooking emissions. Establishing regulations to curb these emissions, including requirements for filtering exhausted air, would cut the pollution substantially. So throw another burger on the grill. But remember, one day this source of pollution may also have to yield to the overriding need for clean air.