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Elementa (Wash D C) ; 6(1): 56, 2018.
Artigo em Inglês | MEDLINE | ID: mdl-30364819


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 O3 levels to protect public health and welfare, but unraveling the origins of surface O3 is complicated by the presence of contributions from multiple sources including background sources like stratospheric transport, wildfies, 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 O3 over the continental U.S., and evaluate how it inflattainment of the air quality standards. We conclude that spring and summer seasonal mean U.S. background O3 (USB O3), or O3 formed 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 4th highest MDA8s exceeding 60 ppb, at some locations. At lower elevation sites, e.g., along the West and East Coasts, seasonal mean MDA8 USB O3 is in the range of 20-40 ppb, with generally smaller contributions on the highest O3 days. The uncertainty in U.S. background O3 is around ±10 ppb for seasonal mean values and higher for individual days. Noncontrollable O3 sources, such as stratospheric intrusions or precursors from wildfires, can make significant contributions to O3 on some days, but it is challenging to quantify accurately these contributions. We recommend enhanced routine observations, focused fi 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 O3 in the U.S.

Atmos Chem Phys ; 18(5): 3839-3864, 2018.
Artigo em Inglês | MEDLINE | ID: mdl-30079085


This study analyzes simulated regional-scale ozone burdens both near the surface and aloft, estimates process contributions to these burdens, and calculates the sensitivity of the simulated regional-scale ozone burden to several key model inputs with a particular emphasis on boundary conditions derived from hemispheric or global-scale models. The Community Multiscale Air Quality (CMAQ) model simulations supporting this analysis were performed over the continental US for the year 2010 within the context of the Air Quality Model Evaluation International Initiative (AQMEII) and Task Force on Hemispheric Transport of Air Pollution (TF-HTAP) activities. CMAQ process analysis (PA) results highlight the dominant role of horizontal and vertical advection on the ozone burden in the mid-to-upper troposphere and lower stratosphere. Vertical mixing, including mixing by convective clouds, couples fluctuations in free-tropospheric ozone to ozone in lower layers. Hypothetical bounding scenarios were performed to quantify the effects of emissions, boundary conditions, and ozone dry deposition on the simulated ozone burden. Analysis of these simulations confirms that the characterization of ozone outside the regional-scale modeling domain can have a profound impact on simulated regional-scale ozone. This was further investigated by using data from four hemispheric or global modeling systems (Chemistry - Integrated Forecasting Model (C-IFS), CMAQ extended for hemispheric applications (H-CMAQ), the Goddard Earth Observing System model coupled to chemistry (GEOS-Chem), and AM3) to derive alternate boundary conditions for the regional-scale CMAQ simulations. The regional-scale CMAQ simulations using these four different boundary conditions showed that the largest ozone abundance in the upper layers was simulated when using boundary conditions from GEOS-Chem, followed by the simulations using C-IFS, AM3, and H-CMAQ boundary conditions, consistent with the analysis of the ozone fields from the global models along the CMAQ boundaries. Using boundary conditions from AM3 yielded higher springtime ozone columns burdens in the middle and lower troposphere compared to boundary conditions from the other models. For surface ozone, the differences between the AM3-driven CMAQ simulations and the CMAQ simulations driven by other large-scale models are especially pronounced during spring and winter where they can reach more than 10 ppb for seasonal mean ozone mixing ratios and as much as 15 ppb for domain-averaged daily maximum 8 h average ozone on individual days. In contrast, the differences between the C-IFS-, GEOS-Chem-, and H-CMAQ-driven regional-scale CMAQ simulations are typically smaller. Comparing simulated sur face ozone mixing ratios to observations and computing seasonal and regional model performance statistics revealed that boundary conditions can have a substantial impact on model performance. Further analysis showed that boundary conditions can affect model performance across the entire range of the observed distribution, although the impacts tend to be lower during summer and for the very highest observed percentiles. The results are discussed in the context of future model development and analysis opportunities.

Nat Commun ; 6: 7105, 2015 May 12.
Artigo em Inglês | MEDLINE | ID: mdl-25964012


Evidence suggests deep stratospheric intrusions can elevate western US surface ozone to unhealthy levels during spring. These intrusions can be classified as 'exceptional events', which are not counted towards non-attainment determinations. Understanding the factors driving the year-to-year variability of these intrusions is thus relevant for effective implementation of the US ozone air quality standard. Here we use observations and model simulations to link these events to modes of climate variability. We show more frequent late spring stratospheric intrusions when the polar jet meanders towards the western United States, such as occurs following strong La Niña winters (Niño3.4<-1.0 °C). While El Niño leads to enhancements of upper tropospheric ozone, we find this influence does not reach surface air. Fewer and weaker intrusion events follow in the two springs after the 1991 volcanic eruption of Mt. Pinatubo. The linkage between La Niña and western US stratospheric intrusions can be exploited to provide a few months of lead time during which preparations could be made to deploy targeted measurements aimed at identifying these exceptional events.

Sci Total Environ ; 530-531: 471-482, 2015 Oct 15.
Artigo em Inglês | MEDLINE | ID: mdl-25548133


Ozone (O3) has been measured at Great Basin National Park (GBNP) since September 1993. GBNP is located in a remote, rural area of eastern Nevada. Data indicate that GBNP will not comply with a more stringent National Ambient Air Quality Standard (NAAQS) for O3, which is based upon the 3-year average of the annual 4th highest Maximum Daily 8-h Average (MDA8) concentration. Trend analyses for GBNP data collected from 1993 to 2013 indicate that MDA8 O3 increased significantly for November to February, and May. The greatest increase was for May at 0.38, 0.35, and 0.46 ppb yr(-1) for the 95th, 50th, and 5th percentiles of MDA8 O3 values, respectively. With the exception of GBNP, continuous O3 monitoring in Nevada has been limited to the greater metropolitan areas. Due to the limited spatial detail of O3 measurements in rural Nevada, a network of rural monitoring sites was established beginning in July 2011. For a period ranging from July 2011 to June 2013, maximum MDA8 O3 at 6 sites occurred in the spring and summer, and ranged from 68 to 80ppb. Our analyses indicate that GBNP, in particular, is ideally positioned to intercept air containing elevated O3 derived from regional and global sources. For the 2 year period considered here, MDA8 O3 at GBNP was an average of 3.1 to 12.6 ppb higher than at other rural Nevada sites. Measured MDA8 O3 at GBNP exceeded the current regulatory threshold of 75 ppb on 7 occasions. Analyses of synoptic conditions, model tracers, and air mass back-trajectories on these days indicate that stratospheric intrusions, interstate pollution transport, wildfires, and Asian pollution contributed to elevated O3 observed at GBNP. We suggest that regional and global sources of ozone may pose challenges to achieving a more stringent O3 NAAQS in rural Nevada.

Poluentes Atmosféricos/análise , Monitoramento Ambiental , Ozônio/análise , Poluição do Ar/estatística & dados numéricos , Nevada