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 NO x -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 3ppbv impacts on monthly mean O3 all-hour basis, whereas the impact of domestic emissions dominates over the eastern US with a magnitude of about 10ppbv 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 250hPa) over the US (impacts of more than 5ppbv) and the dominance of stratospheric air mass on upper model layer (defined as 250 to 50hPa) over the US (impacts greater than 10ppbv). 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 1ppbv) over the free troposphere.
Highlights
Tropospheric ozone (O3) is a secondary air pollutant produced through photochemical reactions including nitrogen oxides (NOx) and volatile organic compounds (VOCs) (Haagen-Smit and Fox, 1954)
Sensitivity coefficients towards domain-wide emissions calculated by higher-order decoupled direct method (HDDM) are shown in Fig. 1; these values represent monthly means and in turn are computed from hourly sensitivity coefficient output by the Community Multiscale Air Quality (CMAQ) model configured with HDDM
Each point in the scatter plots is shaded by time zone, and it can be seen that high O3 mixing ratios over the Central Standard Time (CST) and Eastern Standard Time (EST) zones are not linked to the impacts of east Asian emissions (R of 0.06 and −0.03, respectively, and not significant), while moderately higher O3 mixing ratios found over Pacific Standard Time (PST) and Mountain Standard Time (MST) appear to be linked to higher impacts from east Asian emissions (R values were 0.36 and 0.36, respectively, and p < 0.001)
Summary
Tropospheric ozone (O3) is a secondary air pollutant produced through photochemical reactions including nitrogen oxides (NOx) and volatile organic compounds (VOCs) (Haagen-Smit and Fox, 1954). Literature estimates of the contributions of these two factors are summarized our previous study (see Table 1 of Itahashi et al, 2020; hereafter referred to as Part 1) The occurrence of this trans-Pacific transport and stratospheric intrusion can be related to the midlatitude jet stream, and this is controlled by La Niña and El Niño. As illustrated in the Part 1 paper, the objective of this sequential research is to better understand the relative contributions of precursor emissions from the US and east Asia and the impacts of STT on air quality in the US during springtime. A brief summary of the findings from that analysis and the motivation for this study is presented subsequently
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