Question: Why did environmental groups support cap-and-trade programs for SO2 emissions but not for NOx and heavy metals? View Available Hint(s) for Part A The cap-and-trade programs for SO2 emissions have not resulted in any decreases in SO2 emissions. The effects of emissions of SO2 and heavy metals are felt over a broad region, but the effects of NOx are mostly in the immediate vicinity of the source. The effects of emissions of SO2 are felt over a broad region, but the effects of NOx and heavy metals are mostly in the immediate vicinity of the source. It is too difficult to accurately measure NOx and heavy metal emissions.