Megacity Emissions and Lifetimes of Nitrogen Oxides Probed from Space
Abstract
Megacities are immense sources of air pollutants, with large impacts on air quality and climate. However, emission inventories in many of them still are highly uncertain, particularly in developing countries. Satellite observations allow top-down estimates of emissions to be made for nitrogen oxides (NOx = NO + NO2), but require poorly quantified a priori information on the NOx lifetime. We present a method for the simultaneous determination of megacity NOx emissions and lifetimes from satellite measurements by analyzing the downwind patterns of NO2 separately for different wind conditions. Daytime lifetimes are ~4 hours at low and mid-latitudes, but ~8 hours in wintertime for Moscow. The derived NOx emissions are generally in good agreement with existing emission inventories, but are higher by a factor of 3 for the Saudi Arabian capital Riyadh.
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References and Notes
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In this study, we define “megacity” as a city with more than 5 million inhabitants, following the definition used within the MEGAPOLI project (http://megapoli.dmi.dk/nlet/MEGAPOLI_NewsLet01.pdf)
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Because the respective satellites have repeat cycles of only 1 to 6 days, it is generally not possible to track a particular air mass, as done with in situ aircraft transects. Instead, spatial patterns of temporally averaged (seasonal/multi-annual) NO2 tropospheric columns have been analyzed under special conditions, resulting in lifetimes of 4 hours (12) up to a day (7, 17). The latter estimates are biased toward high values because they ignore effects of the satellite’s coarse spatial resolution.
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Detailed methods are available as supporting material on Science Online.
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Though specific emission inventories exist for various megacities (with highly variable quality), we present a comparison to the global EDGAR inventory, which is widely used in global atmospheric chemistry models.
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Published In
Science
Volume 333 | Issue 6050
23 September 2011
23 September 2011
Copyright
Copyright © 2011, American Association for the Advancement of Science.
Submission history
Received: 2 May 2011
Accepted: 3 August 2011
Published in print: 23 September 2011
Acknowledgments
Acknowledgments: The research leading to these results received funding from the European Union’s Seventh Framework Programme FP/2007-2011 within the project MEGAPOLI (grant 212520). We acknowledge the free use of tropospheric NO2 column data from the OMI sensor from www.temis.nl. We thank the ECMWF for providing wind fields. EDGAR NOx emissions for 2005 were provided by the European Commission, Joint Research Centre (JRC)–Netherlands Environmental Assessment Agency (PBL) (EDGAR version 4.1, http://edgar.jrc.ec.europa.eu/, 2010). We thank S. Dörner for valuable assistance in processing ECMWF data.
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