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

Abstract

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 (O 3 ) 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 O 3 . H-CMAQ is able to capture surface and boundary layer (defined as surface to 750 hPa) O 3 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 O 3 levels. This method is developed based on the ratio of O 3 and an inert tracer indicating stratospheric O 3 to examine the importance of photochemistry, and sequential intrusion from upper layer. During April 2010, on a monthly average basis, the relationship be-tween surface O 3 mixing ratios and estimated stratospheric air masses in the troposphere show a slight negative slope, indicating that high surface O 3 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 O 3 . Further research focused on emissions is discussed in a subsequent paper (Part 2).

Comprehensive analyses of source sensitivities and apportionments of PM<sub>2.5</sub> and ozone over Japan via multiple numerical techniques

“…Horizontal resolutions of d01, d02, d03, and d04 are 45 × 45, 15 × 15, 5 × 5, and 5 × 5 km, respectively. The top height of the model was lifted from 10 000 to 5000 Pa to explicitly treat transport in the lower stratosphere (Itahashi et al, 2020). The vertical layer heights were adjusted to be consistent with those of the Chemical Atmospheric Global Climate Model for Studies of Atmospheric Environment and Radiative Forcing (CHASER) (Sudo et al, 2002), which was used to provide boundary concentrations, to avoid numerical diffusions to adjacent layers.…”

“…Various improvements were applied to the original emission inputs used in the first phase of J-STREAM described in Chatani et al (2018b) based on the outcomes of the model inter-comparisons. Hemispheric Transport of Air Pollution (HTAP) emissions version 2.2 (Janssens- Maenhout et al, 2015) was used for anthropogenic sources and international shipping for Asian countries except for Japan. While the target year of HTAP v2.2 is 2010, the ratios of sectoral annual emissions reported by Zheng et al (2018) were multiplied for China, and those reported by the Clean Air Policy Support System (CAPSS) (Lee et al, 2011) were multiplied for South Korea, to represent the changes in the precursor emissions of recent years.…”

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

“…How about the trans-Pacific transport? Anthropogenic emissions in China grew during the 2000s (Itahashi et al, 2014) and reached the highest levels in the world in 2010; however, substantial reductions have been measured by satellites since then (Irie et al, 2016;Krotkov et al, 2016;van der A et al, 2017;Itahashi et al, 2018). In addition, bottom-up emission inventories indicate that Chinese NO x emissions were reduced as a consequence of clean air actions (Zheng et al, 2018).…”

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

“…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.…”