Entrainment of stratospheric air and Asian pollution by the convective boundary layer in the southwestern U.S.

Langford, A. O.; Alvarez, R. J.; Brioude, J.; Fine, Rebekka; Gustin, Mae Sexauer; Lin, Meiyun; Marchbanks, Richard D.; Pierce, R. Bradley; Sandberg, Scott P.; Senff, Christoph J.; Weickmann, A. M.; Williams, E. J.

doi:10.1002/2016jd025987

Cited by 50 publications

(58 citation statements)

References 69 publications

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“…Studies based on chemical transport models (CTMs) have shown that air quality in the U.S. can be considerably influenced by pollutants beyond the U.S. boundaries, such as through intercontinental transport, and through stratosphere-totroposphere exchange (Zhang et al, 2011;Lin et al, 2012;Nopmongcol et al, 2016;Langford et al, 2017;Lin et al, 2017;Hogrefe et al, 2017;Mathur et al, 2017). Similar findings have also been reported based on routine observations and field 5 campaign measurements (e.g.…”

Section: Introductionsupporting

confidence: 65%

Multi-Model Comparison in the Impact of Lateral Boundary Conditions on Simulated Surface Ozone across the United States Using Chemically Inert Tracers

Liu

Hogrefe

Christensen

et al. 2018

Preprint

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Abstract. This study represents an inter-comparison of four regional-scale air quality simulations, focused on understanding similarities and differences in the simulated impact of ozone lateral boundary conditions (LBCs) on the ground-level ozone predictions across the U.S. The chemically inert tracers were implemented in the simulations as a diagnostic tool to 15 understand the similarities and differences between models at process level. For all simulations, three chemically inert tracers (BC1 BC2 and BC3) are used to track the impact of ozone specified at different altitudes along the lateral boundaries of the modeling domain encompassing the contiguous U.S. The altitude ranges specified for BC1, BC2, and BC3 broadly represent the planetary boundary layer (PBL), free troposphere, and upper troposphere-lower stratosphere, respectively.The four simulations, namely WRF/CMAQ, WRF/CAMx, WRF/DEHM and COSMO-CLM/CMAQ, can have considerable 20 differences in the simulated inert tracers at surface, indicating their different estimates in the impact of lateral boundary on surface ozone within the U.S. due to the physical processes alone in chemical transport models. WRF/CMAQ is used as a base case, and the differences between WRF/CMAQ and the other three models are examined, respectively. The model pair of COSMO-CLM/CMAQ and WRF/CMAQ shows the smallest differences in inert tracers, with the difference in BCT (sum of BC1, BC2 and BC3) peak in winter to be 1.6 ppb averaged across all sites. The model pair of WRF/DEHM and 25 WRF/CMAQ shows the largest differences, with difference in BCT peak in summer to be 8.1 ppb averaged across all sites. Furthermore, the model differences in inert tracers are discussed with respect to the physical processes that inert tracers undergo. It is found that the process of vertical turbulent mixing between the PBL and the free troposphere is the main cause of the model differences in the simulated inert tracers, especially the relative contributions of BC1 and BC2 to the total inert tracers, in most seasons and regions of the U.S., although the processes of sub-grid cloud mixing and dry deposition can also 30 be important drivers for specific regions and seasons.Atmos. Chem. Phys. Discuss., https://doi

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Section: Introductionsupporting

confidence: 65%

Multi-Model Comparison in the Impact of Lateral Boundary Conditions on Simulated Surface Ozone across the United States Using Chemically Inert Tracers

Liu

Hogrefe

Christensen

et al. 2018

Preprint

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“…There are generally insufficient data for characterizing these distributions for other parts of Asia, (Figure 4c-e). Higher ozone levels in the western US have been attributed to a number of factors, including intercontinental transport of Asian pollution, stratospheric intrusions and wildfires, as well as the combination of high elevations and an exceptionally deep convective boundary layer which allows high altitude ozone plumes to reach the surface (Lin et al, 2017;Langford et al, 2017). For SOMO35, there are more high values in non-urban (SOMO35 > 7000 ppb day = 45) compared to urban (SOMO35 > 7000 ppb day = 7) locations across the globe, and the percentage of SOMO35 values > 7000 is also higher for non-urban than urban sites (1.53% versus 0.50%).…”

Section: Distribution Of Present-day Ozone Metricsmentioning

confidence: 99%

Tropospheric Ozone Assessment Report: Present-day ozone distribution and trends relevant to human health

Fleming

Doherty

Schneidemesser³

et al. 2018

Elementa: Science of the Anthropocene

255

202

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This study quantifies the present-day global and regional distributions (2010–2014) and trends (2000–2014) for five ozone metrics relevant for short-term and long-term human exposure. These metrics, calculated by the Tropospheric Ozone Assessment Report, are: 4thhighest daily maximum 8-hour ozone (4MDA8); number of days with MDA8 > 70 ppb (NDGT70), SOMO35 (annual Sum of Ozone Means Over 35 ppb) and two seasonally averaged metrics (3MMDA1; AVGMDA8). These metrics were explored at ozone monitoring sites worldwide, which were classified as urban or non-urban based on population and nighttime lights data.Present-day distributions of 4MDA8 and NDGT70, determined predominantly by peak values, are similar with highest levels in western North America, southern Europe and East Asia. For the other three metrics, distributions are similar with North–South gradients more prominent across Europe and Japan. Between 2000 and 2014, significant negative trends in 4MDA8 and NDGT70 occur at most US and some European sites. In contrast, significant positive trends are found at many sites in South Korea and Hong Kong, with mixed trends across Japan. The other three metrics have similar, negative trends for many non-urban North American and some European and Japanese sites, and positive trends across much of East Asia. Globally, metrics at many sites exhibit non-significant trends. At 59% of all sites there is a common direction and significance in the trend across all five metrics, whilst 4MDA8 and NDGT70 have a common trend at ~80% of all sites. Sensitivity analysis shows AVGMDA8 trends differ with averaging period (warm season or annual). Trends are unchanged at many sites when a 1995–2014 period is used; although fewer sites exhibit non-significant trends. Over the longer period 1970–2014, most Japanese sites exhibit positive 4MDA8/SOMO35 trends. Insufficient data exist to characterize ozone trends for the rest of Asia and other world regions.

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“…Near the U.S. borders with Canada and Mexico, international pollution transport enhances USB O 3 relative to NAB O 3 (Wang et al, 2009;Guo et al, 2018). In the southwestern U.S., seasonal mean USB O 3 is higher than in other regions during both spring and summer, and NCOS play a more important role on high O 3 days (Fiore et al, 2014a;Langford et al, 2017), although stratospheric intrusions occasionally decrease surface O 3 in the heavily polluted Los Angeles Basin (Langford et al, 2012).…”

Section: Spatial and Temporal Distributions Of Usb Omentioning

confidence: 99%

“…These occur most frequently in winter when Rossby wave activity is at a maximum in the Northern Hemisphere, but the potential impact on surface O 3 is greater in late spring through early summer, when there is more O 3 in the lower stratosphere and deeper mixed layers can more easily entrain O 3 that reaches the lower troposphere (Langford et al, 2017). Descending stratospheric intrusions can also merge with biomass burning plumes (Brioude et al, 2007) or transported pollution (Cooper et al, 2004a, b;Lin et al, 2012b) and carry additional O 3 from these sources downward to the surface.…”

Section: Evidence For Ncos From Observations and Modelsmentioning

confidence: 99%

“…Furthermore, the O 3 lifetime is longer in the lower free troposphere than in near-surface air where it undergoes depositional loss to the surface and where chemical reaction rates may be enhanced in warmer, more humid air masses. The O 3 levels measured at mountain sites and nearby populated areas may be similar, however, if the boundary layers are sufficiently deep and well-mixed as is often the case in the Intermountain West (Langford et al, 2017).…”

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confidence: 99%

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Scientific assessment of background ozone over the U.S.: Implications for air quality management

Jaffe

Cooper

Fiore

et al. 2018

Elementa: Science of the Anthropocene

114

149

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Ozone (O3) is a key air pollutant that is produced from precursor emissions and has adverse impacts on human health and ecosystems. In the U.S., the Clean Air Act (CAA) regulates O3levels to protect public health and welfare, but unraveling the origins of surface O3is complicated by the presence of contributions from multiple sources including background sources like stratospheric transport, wildfires, biogenic precursors, and international anthropogenic pollution, in addition to U.S. anthropogenic sources. In this report, we consider more than 100 published studies and assess current knowledge on the spatial and temporal distribution, trends, and sources of background O3over the continental U.S., and evaluate how it influences attainment of the air quality standards. We conclude that spring and summer seasonal mean U.S. background O3(USB O3), or O3formed from natural sources plus anthropogenic sources in countries outside the U.S., is greatest at high elevation locations in the western U.S., with monthly mean maximum daily 8-hour average (MDA8) mole fractions approaching 50 parts per billion (ppb) and annual 4thhighest MDA8s exceeding 60 ppb, at some locations. At lower elevation sites, e.g., along the West and East Coasts, seasonal mean MDA8 USB O3is in the range of 20–40 ppb, with generally smaller contributions on the highest O3days. The uncertainty in U.S. background O3is around ±10 ppb for seasonal mean values and higher for individual days. Noncontrollable O3sources, such as stratospheric intrusions or precursors from wildfires, can make significant contributions to O3on some days, but it is challenging to quantify accurately these contributions. We recommend enhanced routine observations, focused field studies, process-oriented modeling studies, and greater emphasis on the complex photochemistry in smoke plumes as key steps to reduce the uncertainty associated with background O3in the U.S.

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Entrainment of stratospheric air and Asian pollution by the convective boundary layer in the southwestern U.S.

Cited by 50 publications

References 69 publications

Multi-Model Comparison in the Impact of Lateral Boundary Conditions on Simulated Surface Ozone across the United States Using Chemically Inert Tracers

Multi-Model Comparison in the Impact of Lateral Boundary Conditions on Simulated Surface Ozone across the United States Using Chemically Inert Tracers

Tropospheric Ozone Assessment Report: Present-day ozone distribution and trends relevant to human health

Scientific assessment of background ozone over the U.S.: Implications for air quality management

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