Author: Antonio Da Roza
Nitrogen dioxide (NO2) is mainly formed from oxidation of nitrogen oxides (NOx) emitted from fuel combustion. Emissions from power stations and motor vehicles are the two major sources of NOx in Hong Kong. The latter has greater impact on roadside air quality. 
Nitrogen oxides and electricity generation
Public electricity generation was also the major source of nitrogen oxides emissions which contributed to 46% of the total emission in 2007. In 1993, because of the implementation of low nitrogen oxide burner programme at coal-fired power generating units, the emission had reduced drastically. In 2003 and 2004, the emissions increased due to the increased power generation from coal-fired units. 
Since 2005, emission caps had been set, leading to the emissions decreasing. However, emissions in 2007 increased by 7% from 2006 due to the increased use of coal-fired generating units. 
Nitrogen oxides and transport
Emissions from vehicles are one of the major sources of nitrogen oxides emission which contributed to 22% of total emissions in 2007. From 1995 to 2006, the vehicle emission standards were tightened progressively from Euro I to Euro IV, so emissions of nitrogen oxides reduced. In 2007, various incentive schemes were launch to encourage owners to phase-out their pre-Euro and Euro I commercial vehicles, and this helped reduce the emissions. 
Emissions from navigation and civil aviation have been increasing in recent years due to rapid economic developments in the PRD region which increased the traffic of passenger vessels and aircraft.