of emissions are similar regardless of altitude. Airport ground vehicles, those used by passengers and staff to access airports, emissions generated by airport construction and aircraft manufacturing also contribute to the greenhouse gas emissions from the aviation industry. Nitrogen oxides (, nitric oxide and nitrogen dioxide) In the tropopause, emissions of favor ozone () formation in the upper troposphere. At altitudes from , emissions result in greater concentrations of than surface emissions and these in turn have a greater global warming effect. The effect of surface concentrations are regional and local, but it becomes well mixed globally at mid and upper tropospheric levels. emissions also reduce ambient levels of methane, another greenhouse gas, resulting in a climate cooling effect, though not offsetting the forming effect. Aircraft sulfur and water emissions in the stratosphere tend to deplete , partially offsetting the -induced increases, although these effects have not been quantified. Light aircraft and small commuter aircraft fly lower in the troposphere, not in the tropopause. Contrails and cirrus clouds Fuel burning produces water vapor, which condenses at high altitude, under cold and humid conditions, into visible line clouds: condensation trails (contrails). They are thought to have a global warming effect, though less significant than emissions. Contrails are uncommon from lower-altitude aircraft. Cirrus clouds can develop after the formation of persistent contrails and can have an additional global warming effect. Their global warming contribution is uncertain and estimating aviation's overall contribution often excludes cirrus cloud enhancement. Particulates Compared with other emissions, sulfate and soot particles have a smaller direct effect: sulfate particles have a cooling effect and reflect radiation, while soot has a warming effect and absorbs heat, while the clouds' properties and formation are influenced by particles. Contrails and cirrus clouds evolving from particles may have a greater radiative forcing effect than emissions. As soot particles are large enough to serve as condensation nuclei, they are thought to cause the most contrail formation. Soot production may be decreased by reducing the aromatic compound of jet fuel. In 1999, the IPCC estimated aviation's radiative forcing in 1992 to be 2.7 (2 to 4) times that of alone − excluding the potential effect of cirrus cloud enhancement. This was updated for 2000, with aviation's radiative forcing estimated at 47.8 mW/m2, times the effect of emissions alone, 25.3 mW/m2. In 2005, research by David S. Lee, et al, published in the scientific journal Atmospheric