If you come away from tonight's "Nova" program on acid rain feeling a bit frustrated and confused over the planet's latest environmental megaproblem, you've probably got the right reaction.

"Acid Rain: New Bad News," at 8 tonight on Channel 26 and Maryland Public Television stations, illustrates the damage with a helicopter excursion over the skeletal remains of West Germany's noble Black Forest, a scuba tour of the algae-cloaked bottom of one of Sweden's dead lakes and a close-up of the eroded faces on the Cologne cathedral's stone saints.

But who's to blame? And what to do about it?

That depends on which scientist you find most believable. This quick Cook's tour of acid rain researchers offers many voices, from the scientist who demonstrates the basics of soil chemistry with a lemon and a roll of Tums to the one who runs an elaborate mountaintop cloud-sampling operation.

Much of the documentary, however, is devoted to exploring the proposition that nitrogen (becoming nitric acid in the atmosphere) is the primary culprit, a theory that is gaining some ground in scientific circles even as lawmakers and environmentalists are crafting strategies that involve sharp cutbacks in sulfur emissions.

But then again, the scientists muse, maybe something else is killing the trees and lakes. Overfertilization from nutritionally enriched air, perhaps, or a magnesium deficiency caused by . . . by something.

"Nova" tosses in the standard but irresistible interview with the frustrated Adirondacks angler, gazing at the slightly turgid waters of an Upstate New York lake that hasn't produced a trout in 15 years. It also couldn't pass up the gaunt trunk of a Black Forest tree, painted with a skull and crossbones and the plaintive message "Hilf Uns (Help Us)."

But mostly this is a cautious, dispassionate treatment of a difficult scientific problem. And if one preeminent acid rain researcher wants to make dire predictions about "global habitability" and "Spaceship Earth," why, that's the voice of science.