EPA Administrator Lisa Jackson speaks about new mercury emission regulations during a news conference at the Children's National Medical Center on December 21, 2011 in Washington, D.C. - 

The Obama Administration announced this morning the head of the Environmental Protection Agency -- Lisa Jackson -- is stepping down.

This is the time, at the start of a second term, when many Cabinet-level jobs turn over. Jackson says she's ready for new opportunities and more time with her family. The president praised her for making our air and water safer.

Jackson is a Princeton-educated chemical engineer, who took over after an administration headed by an oilman president.

She walked in with great expectations by the environmental community -- which today is hailing great accomplishments by the EPA under her watch – from climate to air and water to cars.

And, not surprisingly, perhaps the most vilified regulator in the eyes of congressional republicans and fossil fuel interests, notably coal which is the dirtiest fuel.

What kind of impact has she had on your life? Now, your next car will have stricter mileage and carbon pollution standards; the next new power plant in your neighborhood and mine will likely not run on coal; and smokestack industries will belch less soot and mercury in the air, because of EPA rules.

Her friends say one of the biggest moments was when the agency declared carbon dioxide emissions contribute to global warming and are a threat to public health.

And a lot of rules pushed through despite the White House, which put the climate change issue in its pocket after it failed in Congress in 2010.

The unfinished business she leaves behind? What to do about hydraulic fracturing.  The EPA’s currently doing a big study of risks to drinking water that's due in 2014.

We’re waiting for whether they come out with new rules on existing power plants; regulation of toxic chemicals and water quality may also be on the way.

What she also leaves behind is a toxic political environment. Several people inside agency say it’s worse than they can remember.

Follow Scott Tong at @tongscott