Comparison of OMI NO<sub>2</sub> observations and their seasonal and weekly cycles with ground-based measurements in Helsinki

Abstract

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.

The version 3 OMI NO<sub>2</sub> standard product

“…3). For most other polluted locations the new SPv3 data are lower than the previous version, as confirmed with direct-sun Pandora comparisons in Helsinki (Ialongo et al, 2016). Some reasons for the discrepancies between satellite-and ground-based NO 2 retrievals include the spatial averaging inherent in the large OMI field of view; the still quite coarse sampling of the a priori profiles Figure 11.…”

Lightning NOx Emissions: Reconciling Measured and Modeled Estimates With Updated NOx Chemistry

“…Our model-satellite comparisons make use of averaging kernels Eskes & Boersma, 2003), so the comparisons are insensitive to the choice of a priori profiles in the retrieval. While a number of studies validating OMI tVCD NO2 products using aircraft-or ground-based measurements or models have been done (Bucsela et al, 2008;Hains et al, 2010;Ialongo et al, 2016;Lamsal et al, 2010Lamsal et al, , 2014Oetjen et al, 2013;Russell et al, 2011), they have mixed results on which product is more accurate, and no Geophysical Research Letters 10.1002/2017GL074436 single study evaluates the versions of both the SP and DOMINO product used in this work; therefore, we use both products. The NASA SPv3 based on the revised slant columns described in Marchenko et al (2015) was released after this analysis was completed.…”

Monthly top‐down NOx emissions for China (2005–2012): A hybrid inversion method and trend analysis

“…Trends in OMI NO 2 retrievals at regional scales may be susceptible to biases in the retrieval process itself [e.g., Herron-Thorpe et al, 2010;Zheng et al, 2014;Ialongo et al, 2016]. To test the robustness of the trends driving our inversion, we next compare the two retrievals of OMI NO 2 product here.…”