The Environmental Protection Agency is proposing to revise rules governing nitrogen-dioxide air emissions, the first new NO2 standards in more than 35 years. The June 26 proposal includes a new one-hour standard of 80 to 100 parts per billion and would add monitoring for NO2 within 50 meters of major roads in cities with populations of 350,000 or more. EPA would keep the present annual average standard at 53 ppb.
Arch dams, classically curved monoliths situated in narrow valleys, are not as common as other types of dams, such as embankment dams and gravity dams. ENR takes a look at some of the biggest arch dams in the world, both completed and under construction.