The AQMEII is an unprecedented effort focused on the evaluation of regional air quality (AQ) models that are used to predict concentrations and deposition of a number of pollutants over the North American and European continents. More than 20 modeling groups participated in the first AQMEII two-continent model evaluation activity, in which more than 10 regional-scale AQ models were applied retrospectively to calculate concentration and deposition fields for the year 2006. The performance of the AQ models was evaluated using routine AQ and meteorological surface monitoring network measurements, as well as sets of upperair measurements obtained by ozonesondes and instrumented commercial aircraft, and results have been published in a recent series of articles. This article describes the 2006 North American and European surface AQ networks and data sets that were used in the AQMEII evaluation process and some issues encountered in the use and interpretation of these data sets. A companion article describes the upper-air measurements that were used in the AQMEII for model evaluation.