The U.S. Court of Appeals for the District of Columbia Circuit quested May 3 whether the Environmental Protection Agencyʼs recent announcement on how it will implement the sulfur dioxide air quality standard suspends a key argument made by petitioners who are challenging the standard. North Dakota and other state petitioners told the court during oral arguments that the agencyʼs final rule revising the SO2 standard required implementation through air quality modeling rather than monitoring data. However, in April 12 letters, the agency indicated that it would not require state implementation plans, due next June, to rely on modeling.