President Reagan is set to name James R. Mahoney, a conservative massachusetts meteorologist and business consultant, to head the Environmental Protection Agency. The appointment could be announced this week.

Considered an expert on the Clean Air Act, Mohoney is cofounder and senior vice president of Environmental Research and Technology Inc. (ERT), a giant industry-oriented consulting and environmental monitoring firm based in Concord, Mass. His patrons, both in business and in Reagan's councils, are the utility executives, chemical company officials and industry leaders whose companies he has helped steer through the labyrinth of EPA regulations since the agency was born.

Mahoney, 42, has no illusions that his appointment would cause any joy among environmentalists. Cornered at an ERT seminar for businessmen on the Clean Air Act recently, the hefty, energetic scientist, while declining to confirm his impending nomination, said it would be "entirely legitimate" for senators to question him closely at any confirmation hearings on his willingness to listen to nonindustry viewpoints.

"We went after industry business, we consult to them and I'm proud of that," Mahoney said, "but I understand that environmentalists really are the constituency of that [Epa] job and businesses are the reactors to what it does." He refused to discuss that issues that will face EPA, saying that his current job demands understanding of the laws rather than opinions about them, and that he was had no indication he will head the EPA. o

ERT, which was purchased by the Communications Satellite Corp. (Comsat) in 1979, is the nation's largest firm of its type, employing 775 persons at 15 locations in 10 states and Puerto Rico. About 400 employes are scientists, Mahoney said.

He and a friend, Dr. Norman E. Gaut, set the company up in 1968 "with more than a little chutzpah," Mahoney recalled, well before environmental issues became a national concern. Gaut, also a meteorologist, was teaching at the Massachusetts Institute of Technology, where both had completed doctoral degrees, and Mahoney was an assistant professor at the Harvard University School of Public Health, where he specialized in the behavior of air Pollution in the atmosphere.

The company began as a consulting firm but expanded to offer technical services covering air, water, weather and radiation monitoring. It has prepared air pollution control programs for governments and private planning agencies, written envoronmental impact statements and developed and sold instruments and scientific techniques to help with those jobs. Mahoney for a time ran the firm's international division, which is now an independent entity.

"There is no matter having to do with the environment that ERT lacks the ability to measure and study," said a Comsat publication, Satellite Pathways. "Corporations, agencies and institutions required by law to write Environmental Impact Statements may turn to ERT for help with every aspect of the task, including testimony in public hearings and litigations."

The firm took in $28 million in 1978, the publication said, with a gross profit of about $9 million. Its clients include 40 utilities, the California Air Resources Board and Department of Transportation, Continental Oil, Dow Chemical, the National Science Foundation, Procter & Gamble, the New Jersey Turnpike Authority, Edison Electric Institute, and International Paper Co., among many others. Saudi Arabia is among the foreign nations that have employed ERT. o

"We are not advocates," said Mahoney in the informal conversation. "Basically we consult to businesses on how to do what they want to do within the law."

Environmentalists say the firm is more than that. "ERT has been very much involved in forming the positions of the industries he'd have to regulate at EPA," said Richard Ayres, air pollution specialist at the Natural Resources Defense Council. Analyses ERT prepared to help its utility clients oppose the 1977 Clean Air Act amendments were based on "truly outrageous assumptions and were very misleading," Ayres said. "The firm had to play a big part in forming those arguments because only they had the expertise to do it."