Reconciling the differences between OMI-based and EPA AQS in situ NO<sub>2</sub> trends

Abstract

Abstract. With the improved spatial resolution than earlier instruments and more than ten years of service, tropospheric NO2 retrievals from the Ozone Monitoring Instrument (OMI) have led to many influential studies on the relationships between socioeconomic activities and NOx emissions. This study focuses on how to improve OMI NO2 retrievals for more reliable trend analysis. We retrieve OMI tropospheric NO2 vertical column densities (VCDs) and obtain the NO2 seasonal trends over 15 the United States, which are compared with coincident in situ surface NO2 measurements from the Air Quality System (AQS) network. The Mann-Kendall method is applied to derive the NO2 seasonal and annual trends for four regions at coincident sites during 2005-2014. The OMI-based NO2 seasonal relative trends are generally biased high compared to the in situ trends by up to 3.7% yr -1 , except for the underestimation in the Midwest and Northeast during Dec-Jan-Feb (DJF). We improve the OMI retrievals for trend analysis by removing the ocean trend, using the MODerate-resolution Imaging Spectroradiometer 20 (MODIS) albedo data in air mass factor (AMF) calculation, and applying a lightning flash filter to exclude lightning affected OMI NO2 retrievals. These improvements result in close agreement (within 0.3% yr -1 ) between in situ and OMI-based NO2 regional annual relative trends. Thus, we recommend future studies to apply these procedures to ensure the quality of satellitebased NO2 trend analysis, especially in regions without reliable long-term in situ NO2 measurements. We derive optimized OMI-based NO2 regional annual relative trends using all available data for the West (-2.0%±0.3 yr ); the difference is mainly due to the fact that the locations of AQS sites are concentrated in urban and suburban regions.