It is great news for people in Maine and along the East Coast that ozone pollution has been reduced. But the improved air quality is the result of stringent federal rules, not simply market-based trading, as administration officials claim. The lesson from ozone control programs is that mandatory reductions work and can be applied to other pollutants, such as carbon dioxide and mercury.

When the Environmental Protection Agency last week released a report showing huge decreases in the emission of pollutants that form ozone, the head of the agency pointed to the success of a trading program. EPA Administrator Stephen Johnson called the reductions “yet another example of how market-based trading programs are significantly reducing emissions of air pollutants.” He’s telling only half the story.

In the early 1990s, the EPA set out to reduce the emissions of nitrogen oxides and volatile organic compounds, which undergo a chemical reaction in heat and sunlight to form smog. The agency set strict limits on the emissions of nitrogen oxides and set a deadline for states to meet the emissions limits.

To meet the federal requirements, states were allowed to come up with their own implementation plans, although the EPA said they would rewrite them if they were inadequate. The agency suggested that requiring power plants to reduce emissions was the most cost-effective way to meet the federal rules and it came up with a trading program that allowed companies to buy and sell pollution credits to meet the federal requirements. The system is working well, according to the EPA.

Without required emissions reductions and a firm deadline, however, the trading plan would not have worked. Companies would not have bought credits if they didn’t have to meet emissions requirements.

The emission reduction requirement coupled with a trading system has reduced nitrogen oxides pollution from power plants by 50 percent since 2000. This has reduced summertime ozone by 10 percent.

Contrast this with carbon dioxide, a major contributor to climate change. The United States signed onto an international accord with a goal of lowering carbon dioxide emissions to 1990 levels by 2000. The reductions were voluntary and U.S. carbon dioxide emissions have increased 16 percent since 1992.

For mercury, the EPA earlier this year finalized a rule requiring the nation’s 600 coal-fired power plants to cut their total mercury emissions, through pollution control technologies and trading, to 15 tons by 2018, a reduction of nearly 70 percent. The Clean Air Act required power plants to install pollution control technology that would reduce mercury emissions by 90 percent by 2009. Another rule from the EPA removes coal- and oil-fired utility steam- generating units from the Clean Air Act requirements on mercury emissions.

Worse, the Clean Air Act requirements would have forced power plants to install technology that would have also reduced emissions of nitrogen oxides and sulfur dioxide, which is a component of acid rain. Three problems could have been solved by one good rule.

The ozone rules are a success story. While trading is part of it, there would be no story without strict federal requirements for pollution reductions.