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. Stratospheric intrusion and trans-Pacific transport have been recognized as apotential source of tropospheric ozone over the US. Thestate-of-the-science Community Multiscale Air Quality (CMAQ) modeling systemhas recently been extended for hemispheric-scale modeling applications(referred to as H-CMAQ). In this study, H-CMAQ is applied to study thestratospheric intrusion and trans-Pacific transport during April 2010. Theresults will be presented in two companion papers. In this Part 1 paper,model evaluation for tropospheric ozone (O3) is presented. Observationsat the surface, by ozonesondes and airplane, and by satellite across theNorthern Hemisphere are used to evaluate the model performance for O3.H-CMAQ is able to capture surface and boundary layer (defined as surface to750 hPa) O3 with a normalized mean bias (NMB) of −10 %; however, asystematic underestimation with an NMB up to −30 % is found in the freetroposphere (defined as 750–250 hPa). In addition, a new air masscharacterization method is developed to distinguish influences ofstratosphere–troposphere transport (STT) from the effects of photochemistryon O3 levels. This method is developed based on the ratio of O3and an inert tracer indicating stratospheric O3 to examine theimportance of photochemistry, and sequential intrusion from upper layer.During April 2010, on a monthly average basis, the relationship betweensurface O3 mixing ratios and estimated stratospheric air masses in thetroposphere show a slight negative slope, indicating that high surfaceO3 values are primarily affected by other factors (i.e., emissions),whereas this relationship shows a slight positive slope at elevated sites,indicating that STT has a possible impact at elevated sites. STT shows largeday-to-day variations, and STT impacts can either originate from the sameair mass over the entire US with an eastward movement found during earlyApril, or stem from different air masses at different locations indicatedduring late April. Based on this newly established air mass characterizationtechnique, this study can contribute to understanding the role of STT andalso the implied importance of emissions leading to high surface O3.Further research focused on emissions is discussed in a subsequent paper (Part 2).

atmospheric chemistry and physics

Modeling stratospheric intrusion and trans-Pacific transport on tropospheric ozone using hemispheric CMAQ during April 2010 – Part 1: Model evaluation and air mass characterization for stratosphere–troposphere transport