Background concentrations in Norway - Temporal averaging and uncertainty assessment.
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A system for providing approximate spatial and temporal estimates of the surface background concentrations of PM10 , PM2.5, NO2 , and O3 for a typical year in Norway has been established at NILU in recent years. In 2013, the system was further improved and expanded by carrying out two main tasks: Firstly the spatial component of the system was extended from previously a single year to a 3-year average, thus decreasing the impact of inter-annual variability. Secondly, a preliminary validation of the background estimates was carried out and an approximate uncertainty was calculated for four species in the dataset and is now communicated to the user on the project website. The validation results indicate that overall the average estimated concentrations follow the average observed concentration fairly well. This is particularly true for PM10 and PM2.5. The averaged estimates for O3 and NO2 also follow the averaged observations relatively closely, although NO2 is clearly underestimated at all four validation stations. In terms of quantitative uncertainty estimates for the hourly estimates at the level of individual species, O3 is the parameter which was found to have the lowest uncertainty of all four species. While the median absolute uncertainty for hourly O3 estimates was found to be 18.7 ¿g m-3, the median relative uncertainty was only 33.8 %. Both PM10 and PM2.5 had significantly higher uncertainties, with a median relative uncertainty for the hourly estimates of 90 % and 88 %, respectively. Finally, the hourly estimates for NO2 also showed quite high relative uncertainties with a value of 73 %.