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We present the comparison of satellite-based OMI (Ozone MonitoringInstrument) 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 thePandora spectrometer in Helsinki in 2012. The relative difference betweenPandora no. 21 and OMI SP total columns is 4 and −6 % for clear-skyand all-sky conditions, respectively. DOMINO NO2 retrievals showedslightly lower total columns with median differences about −5 and−14 % for clear-sky and all-sky conditions, respectively. Largedifferences often correspond to cloudy fall–winter days with solar zenithangles above 65°. Nevertheless, the differences remain within theretrieval uncertainties. The average difference values are likely the resultof different factors partly canceling each other: the overestimation of thestratospheric columns causes a positive bias partly compensated by thelimited spatial representativeness of the relatively coarse OMI pixel forsharp NO2 gradients. The comparison between Pandora and the newversion (V3) of OMI NO2 retrievals shows a larger negative difference(about −30 %) than the current version (V2.1) because the revisedspectral fitting procedure reduces the overestimation of the stratosphericcolumn. The weekly and seasonal cycles from OMI, Pandora and NO2 surfaceconcentrations are also compared. Both satellite- and ground-based data showa similar weekly cycle, with lower NO2 levels during the weekendcompared to the weekdays as a result of reduced emissions from traffic andindustrial activities. The seasonal cycle also shows a similar behavior,even though the results are affected by the fact that most of the data areavailable during spring–summer because of cloud cover in other seasons. This is one of few works in which OMI NO2 retrievals are evaluated ina urban site at high latitudes (60° N). Despite the city of Helsinkihaving relatively small pollution sources, OMI retrievals have proved to beable to describe air quality features and variability similar to surfaceobservations. This adds confidence in using satellite observations for airquality monitoring also at high latitudes

atmospheric measurement techniques

Comparison of OMI NO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; observations and their seasonal and weekly cycles with ground-based measurements in Helsinki