Nitrogen oxide (NOx) and carbon dioxide (CO2) greenhouse gas emissions in Paris dropped over the period 2002-2007 according to a recent report on the air quality in the capital. NOx emissions (a major factor in smog and acid rain) fell by 32 per cent, or the equivalent of 5.2 tonnes per day, according to the study carried out by AIRPARIF. Improvements in automobile technology and better traffic management are thought to be behind the reduction. Greenhouse gas emissions dropped by nine percent, or 496 tonnes per day, thanks partly to local government policy which has seen conspicuous investments in public transport.