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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
Citation:
Itahashi, S., R. Mathur, C. Hogrefe, AND Y. Zhang. 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. Atmospheric Chemistry and Physics. Copernicus Publications, Katlenburg-Lindau, Germany, 20(6):3373–3396, (2020). https://doi.org/10.5194/acp-20-3373-2020
Impact/Purpose:
Ozone concentrations at the surface are affected not only by local emissions but also by ozone concentrations aloft through vertical mixing. The levels of ozone concentrations aloft are controlled by an interplay of transport processes and photochemistry, particularly the exchange of air masses between the stratosphere and troposphere and the long-range transport of ozone formed photochemically in polluted upwind regions and lofted into the free troposphere. This study applies the H-CMAQ modeling system to study these processes during several pollution events in April 2010. Simulated surface concentrations and vertical ozone profiles are compared against observations North America, Europe, and Asia. A new scheme is introduced to identify events when ozone concentrations are strongly affected by stratosphere-troposphere air mass exchange. Better identifying such events and quantifying the contribution of non-anthropogenic sources to surface ozone concentrations is needed to design more effective and efficient control strategies aimed at reducing surface ozone concentrations.
Description:
Stratospheric intrusion and trans-Pacific transport have been recognized as a potential source of tropospheric ozone over the US. 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 stratospheric intrusion and trans-Pacific transport during April 2010. The results will be presented in two companion 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). 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. This method is developed based on the ratio of O3 and an inert tracer indicating stratospheric O3 to examine the importance of photochemistry, and sequential intrusion from upper layer. During April 2010, on a monthly average basis, the relationship between surface O3 mixing ratios and estimated stratospheric air masses in the troposphere show a slight negative slope, indicating that high surface O3 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 large day-to-day variations, and STT impacts can either originate from the same air mass over the entire US with an eastward movement found during early April, or stem from different air masses at different locations indicated during late April. Based on this newly established air mass characterization technique, this study can contribute to understanding 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 paper (Part 2).
