Megacity emissions sensed from space

The emissions of nitrogen oxides can be determined from satellite measurements of the pollution levels by using a mathematical trick
 
The amount of nitrogen oxides released into the air in densely populated areas can now be determined directly, independently from extrapolations and model simulations.

From pollution to emissions: The left graph shows, how nitrogen dioxide is distributed on average over the Middle East for calm. Red indicates high pollution levels, white low. The right graph shows the distribution of nitrogen dioxide around Riyadh for different wind directions and calm (centre). The distinct downwind plumes allow scientists to determine how quickly the pollutant disappears from the atmosphere and thus how large the emitted amounts in Riyadh are.

Researchers at the Max Planck Institute for Chemistry established how long the nitrogen oxides stay in the atmosphere, using satellite observations of the current pollution and meteorological data. From the lifetime and the current pollution levels, actual emissions can then be calculated. The survey of the emissions allows targeted measures to control air pollution. So far, emissions are estimated based on projections, which often leads to inaccurate results.
 
In megacities vast quantities of pollutants are released into the atmosphere. In particular road traffic produces thousands of tons of harmful nitrogen oxides daily, which still pass unfiltered into the air in many countries. Researchers at the Max Planck Institute for Chemistry in Mainz, together with colleagues at the University of Heidelberg and the Royal Netherlands Meteorological Institute De Bilt, determined how high these emissions are on average by using extensive measurements of the satellite instrument OMI, which measures the nitrogen oxides pollution globally.

"We found an independent way of investigating the average lifetime of nitrogen oxides in the air using satellite data," says Steffen Beirle, lead author of the study, which is now published in the research journal Science. Actually, the lifetime is a prerequisite to infer the emissions from the current pollution level.

The amounts of exhaust gases emitted into the atmosphere have been listed globally in emission inventories for several years. However, these inventories are mostly based on projections of traffic volume and energy consumption of the respective states, which are often inaccurate or may not even exist, particularly in emerging and developing countries.

Amongst other locations, the researchers applied their independent method to Riyadh, the capital city of Saudi Arabia, which has grown enormously in recent years and suffers from continuously increasing nitrogen oxide pollution. The city is particularly suited as a test case for the Mainz´ method because of its isolated position on the Arabian peninsula. Hence the nitrogen oxide emissions do not mix with those from other major sources, which would further complicate the analysis.

To determine the average lifetime of nitrogen oxides, the researcher team examined the rate of degradation of the pollutants in the megacities' exhaust plumes. To gather clear downwind plumes from the satellite observations, they used a trick: they sorted the satellite observations by wind direction and created separate mean maps of the nitrogen oxides pollution for Riyadh. The lifetime of nitrogen oxide in the atmosphere and thus the actual emissions in Riyadh can then be determined with a mathematical operation.

"Our method of determining emissions of nitrogen oxides is independent of previous methods and can be applied to megacities globally," says Steffen Beirle. The data collected can help to optimize global models of atmospheric chemistry. Ultimately, the data will also help to better adapt measures for combatting air pollution to local conditions.

The work of the researchers in Mainz was part of the European Research Project MEGAPOLI whose objective is to determine the effects of air pollution from megacities on climate. Twenty-two renowned research institutions take part in MEGAPOLI.