EPA is tightening the limit for emissions of nitrogen oxides (NOx) from nitric acid plants. A Clean Air Act rule issued on May 17 establishes a limit of 0.5 lb of NOx released per ton of nitric acid produced. The new standard is significantly tighter than the former limit of 3 lb of NOx per ton of acid produced, which has been in effect since 1971. The regulation also requires operators of nitric acid plants to install continuous emissions monitoring systems for NOx. The rule, which will take effect once it is published in the Federal Register, applies to all nitric acid plants built, modified, or reconstructed after Oct. 14, 2011. EPA decided against including standards for nitrous oxide and other greenhouse gas emissions in the rule, saying it “does not yet have adequate information” to take action. But the agency added that it is gathering data and plans to propose greenhouse gas emission standards for nitric acid plants.