Modeling stratospheric intrusion and trans-Pacific transport on tropospheric ozone using hemispheric CMAQ during April 2010 – Part 2: Examination of emission impacts based on the higher-order decoupled direct method

Abstract

Abstract. The state-of-the-science Community Multiscale Air Quality (CMAQ) modeling system, which has recently been extended for hemispheric-scale modeling applications (referred to as H-CMAQ), is applied to study the trans-Pacific transport, a phenomenon recognized as a potential source of air pollution in the US, during April 2010. The results of this analysis are presented in two parts. In the previous paper (Part 1), model evaluation for tropospheric ozone (O3) was presented and an air mass characterization method was developed. Results from applying this newly established method pointed to the importance of emissions as the factor to enhance the surface O3 mixing ratio over the US. In this subsequent paper (Part 2), emission impacts are examined based on mathematically rigorous sensitivity analysis using the higher-order decoupled direct method (HDDM) implemented in H-CMAQ. The HDDM sensitivity coefficients indicate the presence of a NOx-sensitive regime during April 2010 over most of the Northern Hemisphere. By defining emission source regions over the US and east Asia, impacts from these emission sources are examined. At the surface, during April 2010, the emission impacts of the US and east Asia are comparable over the western US with a magnitude of about 3 ppbv impacts on monthly mean O3 all-hour basis, whereas the impact of domestic emissions dominates over the eastern US with a magnitude of about 10 ppbv impacts on monthly mean O3. The positive correlation (r=0.63) between surface O3 mixing ratios and domestic emission impacts is confirmed. In contrast, the relationship between surface O3 mixing ratios and emission impacts from east Asia exhibits a flat slope when considering the entire US. However, this relationship has strong regional differences between the western and eastern US; the western region exhibits a positive correlation (r=0.36–0.38), whereas the latter exhibits a flat slope (r < 0.1). Based on the comprehensive evaluation of H-CMAQ, we extend the sensitivity analysis for O3 aloft. The results reveal the significant impacts of emissions from east Asia on the free troposphere (defined as 750 to 250 hPa) over the US (impacts of more than 5 ppbv) and the dominance of stratospheric air mass on upper model layer (defined as 250 to 50 hPa) over the US (impacts greater than 10 ppbv). Finally, we estimate changes of trans-Pacific transport by taking into account recent emission trends from 2010 to 2015 assuming the same meteorological condition. The analysis suggests that the impact of recent emission changes on changes in the contribution of trans-Pacific transport to US O3 levels was insignificant at the surface level and was small (less than 1 ppbv) over the free troposphere.

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

“…Potential vorticity (PV) has been shown to be a robust indicator of air mass exchange between the stratosphere and the troposphere. The value of PV itself generally increases with altitude, and previous studies suggested that a value of 2 PVU (1 PVU = 10 −6 m 2 K kg −1 s −1 ) is an indicator of stratospheric air (Hoskins et al, 1985;Wernli and Bourqui, 2002;Itoh and Narazaki, 2016). Through this study, the tropopause is diagnosed by 2 PVU.…”

Incorporation of volcanic SO<sub>2</sub> emissions in the Hemispheric CMAQ (H-CMAQ) version 5.2 modeling system and assessing their impacts on sulfate aerosol over Northern Hemisphere

“…All available observation data were used in this study. As we have discussed in previous work (Itahashi et al, 2020b), Asian air pollution can impact on air quality over the U.S.A.; however, the magnitude of volcanic emissions on tropospheric SO4 2levels over North America has not been well studied. To evaluate the impact of volcanic SO2 emissions located in the northern hemisphere To evaluate model performance with ground-based observations, the Pearson's correlation coefficient (R) with student's t-test is used for assessing the statistical significance level.…”

“…The Community Multiscale Air Quality (CMAQ) modeling system version 5.2 extended for hemispheric applications (H-CMAQ) (Sarwar et al, 2015;Xing et al, 2015a;Mathur et al, 2017;Itahashi et al 2020aItahashi et al , 2020b) is used to incorporate and assess the impacts from volcanic SO2 emissions. H-CMAQ is configured to cover the entire Northern Hemisphere, utilizing a polar stereographic horizontal discretization of 187×187 grid points with a grid spacing of 108 km and a terrain-following vertical coordinate system with 44 layers of variable thickness from the surface up to 50 hPa.…”

“…Finally, the impacts caused by including the degassing volcanoes were investigated throughout the troposphere. In subsequent analysis, the boundary layer is defined from the surface to 750 hPa, and the free troposphere is defined from 750 to 250 hPa and pressure levels of 750, 500, and 250 hPa are used to refer as the bottom, middle, and top of the free troposphere similar to our previous study (Itahashi et al, 2020b). The vertical profile of simulated SO4 2concentration averaged along the edge of western U.S.A. (defined in Fig.…”

Incorporation of volcanic SO<sub>2</sub> emissions in the Hemispheric CMAQ (H-CMAQ) version 5.2 modeling system and assessing their impacts on sulfate aerosol over the Northern Hemisphere

“…To evaluate model performance with ground-based observations, the Pearson's correlation coefficient (R) with Student's t test is used for assessing the statistical significance level. The normalized mean bias (NMB) and the normalized mean error (NME) are also calculated as follows (e.g., Zhang et al, 2006;Itahashi et al, 2020b).…”