Modeling Trans-Pacific Transport using Hemispheric CMAQ during April 2010: Part 1. Model Evaluation and Air Mass Characterization for the Estimation of Stratospheric Intrusion on Tropospheric Ozone

Abstract. Trans-Pacific transport has been recognized as a potential source of air pollutants over the U.S.A. The state-of-the-science Community Multiscale Air Quality (CMAQ) Modeling System has recently been extended for hemispheric-scale modeling applications (referred to as H-CMAQ). In this study, H-CMAQ is applied to study the trans-Pacific transport during April 2010. The results will be presented in two continuous papers. In this part 1 paper, model evaluation for tropospheric ozone (O3) is presented. Observations at the surface, by ozonesondes and airplane, and by satellite across the northern hemisphere are used to evaluate the model performance for O3. H-CMAQ is able to capture surface and boundary layer (defined as surface to 750 hPa) O3 with a normalized mean bias (NMB) of −10 %; however, a systematic underestimation with an NMB up to −30 % is found in the free troposphere (defined as 750–250 hPa). The surface and aloft relative humidity (RH) showed a positive bias around NMB of +10 % or greater. In addition, a new air mass characterization method is developed to distinguish influences of stratosphere-troposphere transport (STT) from the effects of photochemistry on O3 levels. Potential vorticity (PV) is used to diagnose air masses of stratospheric origin and related to RH in order to characterize stratospheric air masses. The tropopause location is determined using a PV threshold value of 2.0 PVU (1 PVU = 10−6 m2 K kg−1 s−1). The constructed PV-RH relationship indicates that PV of 2.0 PVU generally corresponds to RHs of 30–40 %. The air mass characterization method is then developed based on the ratio of O3 and an inert tracer indicating stratospheric O3 to examine the importance of photochemistry, and the PV-RH relationship is used to determine stratospheric intrusions. Over the U.S.A., STT impacts show large day-to-day variations, and STT impacts can either originate from the same air mass over the entire U.S.A. with an eastward movement, or stem from different air masses at different locations. The relationship between surface O3 mixing ratios and estimated stratospheric air masses in the troposphere show a negative slope, indicating that high surface O3 values are primarily affected by other factors (i.e., emissions), whereas this relationship shows an almost flat slope at elevated sites, indicating that STT has a near constant impact at elevated sites. Based on this newly established air mass characterization technique, this study can contribute to understand the role of STT, and also the implied importance of emissions leading to high surface O3. Further research focused on emissions is discussed in a subsequent part 2 paper.