Air quality emission levels by mode are only compiled at National level by the National Atmospheric Emissions Inventory published at http://naei.defra.gov.uk/data/data-selector, funded by the Department for Environment, Food and Rural Affairs and the Department of Energy and Climate Change. This estimates national emissions of Greenhouse Gases and air quality pollutants across all sources, including road traffic, aviation and shipping.
Local authorities are responsible for reviewing and assessing air quality under the Local Air Quality Management system. As part of this assessment London councils undertake additional monitoring. Information can be found on their individual websites. Greater London Authority hold the London Atmospheric Emissions Inventory, which can be found here: http://data.london.gov.uk/dataset/london-atmospheric-emissions-inventory-2010 .