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Atmos. Meas. Tech., 9, 5203-5212, 2016
https://doi.org/10.5194/amt-9-5203-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
24 Oct 2016
Comparison of OMI NO2 observations and their seasonal and weekly cycles with ground-based measurements in Helsinki
Iolanda Ialongo1, Jay Herman2, Nick Krotkov2, Lok Lamsal2,3, K. Folkert Boersma4,5, Jari Hovila1, and Johanna Tamminen1 1Earth Observation Unit, Finnish Meteorological Institute, Helsinki, Finland
2Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
3GESTAR, Universities Space Research Association, Columbia, Maryland, USA
4Royal Netherlands Meteorological Institute, Climate Observations Department, De Bilt, the Netherlands
5Wageningen University, Meteorology and Air Quality Group, Wageningen, the Netherlands
Abstract. We present the comparison of satellite-based OMI (Ozone Monitoring Instrument) NO2 products with ground-based observations in Helsinki. OMI NO2 total columns, available from NASA's standard product (SP) and KNMI DOMINO product, are compared with the measurements performed by the Pandora spectrometer in Helsinki in 2012. The relative difference between Pandora no. 21 and OMI SP total columns is 4 and −6 % for clear-sky and all-sky conditions, respectively. DOMINO NO2 retrievals showed slightly lower total columns with median differences about −5 and −14 % for clear-sky and all-sky conditions, respectively. Large differences often correspond to cloudy fall–winter days with solar zenith angles above 65°. Nevertheless, the differences remain within the retrieval uncertainties. The average difference values are likely the result of different factors partly canceling each other: the overestimation of the stratospheric columns causes a positive bias partly compensated by the limited spatial representativeness of the relatively coarse OMI pixel for sharp NO2 gradients. The comparison between Pandora and the new version (V3) of OMI NO2 retrievals shows a larger negative difference (about −30 %) than the current version (V2.1) because the revised spectral fitting procedure reduces the overestimation of the stratospheric column.

The weekly and seasonal cycles from OMI, Pandora and NO2 surface concentrations are also compared. Both satellite- and ground-based data show a similar weekly cycle, with lower NO2 levels during the weekend compared to the weekdays as a result of reduced emissions from traffic and industrial activities. The seasonal cycle also shows a similar behavior, even though the results are affected by the fact that most of the data are available during spring–summer because of cloud cover in other seasons.

This is one of few works in which OMI NO2 retrievals are evaluated in a urban site at high latitudes (60° N). Despite the city of Helsinki having relatively small pollution sources, OMI retrievals have proved to be able to describe air quality features and variability similar to surface observations. This adds confidence in using satellite observations for air quality monitoring also at high latitudes.


Citation: Ialongo, I., Herman, J., Krotkov, N., Lamsal, L., Boersma, K. F., Hovila, J., and Tamminen, J.: Comparison of OMI NO2 observations and their seasonal and weekly cycles with ground-based measurements in Helsinki, Atmos. Meas. Tech., 9, 5203-5212, https://doi.org/10.5194/amt-9-5203-2016, 2016.
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Short summary
We present the comparison between satellite- and ground-based atmospheric NO2 observations in Helsinki (Finland). The results show that, despite some limitations due to cloud contamination and low solar angles, satellite data are able to describe urban air quality features such as the weekly and seasonal cycles. The results support air quality satellite data exploitation at high latitudes and prepare for similar applications for future missions.
We present the comparison between satellite- and ground-based atmospheric NO2 observations in...
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