The major source of nitrogen dioxide in Australia is burning of the fossil fuels coal, oil and gas. In cities, the predominant source is on-road vehicle emissions. The standards were met at all compliance sites in 2010–14, and have not been exceeded since the early 1990s. Peak 1-hour and annual average concentrations are equal to about one-third to one-half of the standards.
The United States National Aeronautics and Space Administration has used nitrogen dioxide levels measured by satellites to monitor air quality across the globe (Duncan 2014, NASA 2014). Its maps show the hotspots in Australia and in other countries. It reports average decreases of 20–30 per cent between 2005 and 2014 in Australian cites. Although these measurements are an average across 30 kilometre × 30 kilometre grid cells, the reported decreases are consistent with changes in annual average nitrogen dioxide concentrations reported from NEPM compliance sites, and indicate the success of tighter regulatory controls.
Although ambient concentrations are well below the standards, nitrogen dioxide concentrations up to 0.7 ppm have been measured in Sydney road tunnels (Advisory Committee on Tunnel Air Quality 2016). In the past decade, nitrogen dioxide has replaced carbon monoxide as one of the key criteria in the design of road tunnel ventilation systems to protect the health of tunnel users (the other is PM). New South Wales has recently set a limit for tunnel average concentrations of 0.5 ppm as a rolling 15-minute average (Advisory Committee on Tunnel Air Quality 2016).