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Zhang L.,Harvard University | Jacob D.J.,Harvard University | Downey N.V.,Earth System science LLC | Wood D.A.,BP America Production Company | And 6 more authors.
Atmospheric Environment

The policy-relevant background (PRB) ozone is defined by the US Environmental Protection Agency (EPA) as the surface ozone concentration that would be present over the US in the absence of North American anthropogenic emissions. It is intended to provide a baseline for risk and exposure assessments used in setting the National Ambient Air Quality Standard (NAAQS). We present here three-year statistics (2006-2008) of PRB ozone over the US calculated using the GEOS-Chem global 3-D model of atmospheric composition with 1/2° × 2/3° horizontal resolution over North America and adjacent oceans (2° × 2.5° for the rest of the world). We also provide estimates of the US background (no anthropogenic US emissions) and natural background (no anthropogenic emissions worldwide and pre-industrial methane). Our work improves on previous GEOS-Chem PRB estimates through the use of higher model resolution, 3-year statistics, better representation of stratospheric influence, and updated emissions. PRB is particularly high in the intermountain West due to high elevation, arid terrain, and large-scale subsidence. We present for this region a detailed model evaluation showing that the model is successful in reproducing ozone exceedances up to 70 ppbv. However, the model cannot reproduce PRB-relevant exceptional events associated with wildfires or stratospheric intrusions. The mean PRB estimates for spring-summer are 27 ± 8 ppbv at low-altitude sites and 40 ± 7 ppbv at high-altitude sites. Differences between the PRB simulation and the natural simulation indicate a mean enhancement from intercontinental pollution and anthropogenic methane of 9 ppbv at low-altitude sites and 13 ppbv at high-altitude sites. The PRB is higher than average when ozone exceeds 60 ppbv, particularly in the intermountain West. Our PRB estimates are on average 4 ppbv higher than previous GEOS-Chem studies and we attribute this to higher lighting, increasing Asian emissions, and improved model resolution. Whereas previous studies found no occurrences of PRB exceeding 60 ppbv, we find here some occurrences in the intermountain West. The annual 4th-highest PRB values in the intermountain West are typically 50-60 ppbv, as compared to 35-45 ppbv in the East or on the West Coast. Such high PRB values in the intermountain West suggest that special consideration of this region may be needed if the ozone NAAQS is decreased to a value in the 60-70 ppbv range. © 2011 Elsevier Ltd. Source

Fujita K.,Nagoya University | Sakai A.,Nagoya University | Takenaka S.,Earth System science LLC | Nuimura T.,Nagoya University | And 2 more authors.
Natural Hazards and Earth System Sciences

Glacial lakes are potentially dangerous sources of glacial lake outburst floods (GLOFs), and represent a serious natural hazard in Himalayan countries. Despite the development of various indices aimed at determining the outburst probability, an objective evaluation of the thousands of Himalayan glacial lakes has yet to be completed. In this study we propose a single index, based on the depression angle from the lakeshore, which allows the lakes to be assessed using remotely sensed digital elevation models (DEMs). We test our approach on five lakes in Nepal, Bhutan, and Tibet using images taken by the declassified Hexagon KH-9 satellite before these lakes experienced an outburst flood. All five lakes had a steep lakefront area (SLA), on which a depression angle was steeper than our proposed threshold of 10 before the GLOF event, but the SLA was no longer evident after the events. We further calculated the potential flood volume (PFV); i.e., the maximum volume of floodwater that could be released if the lake surface was lowered sufficiently to eradicate the SLA. This approach guarantees repeatability to assess the possibility of GLOF hazards because it requires no particular expertise to carry out, though the PFV does not quantify the GLOF risk. We calculated PFVs for more than 2000 Himalayan glacial lakes using visible band images and DEMs of ASTER data. The PFV distribution follows a power-law function. We found that 794 lakes did not have an SLA, and consequently had a PFV of zero, while we also identified 49 lakes with PFVs of over 10 million m3, which is a comparable volume to that of recorded major GLOFs. This PFV approach allows us to preliminarily identify and prioritize those Himalayan glacial lakes that require further detailed investigation on GLOF hazards and risk. © Author(s) 2013. Source

Jaffe D.A.,University of Washington | Wigder N.,University of Washington | Downey N.,Earth System science LLC | Pfister G.,U.S. National Center for Atmospheric Research | And 2 more authors.
Environmental Science and Technology

Wildfires generate substantial emissions of nitrogen oxides (NO x) and volatile organic compounds (VOCs). As such, wildfires contribute to elevated ozone (O3) in the atmosphere. However, there is a large amount of variability in the emissions of O3 precursors and the amount of O3 produced between fires. There is also significant interannual variability as seen in median O3, organic carbon and satellite derived carbon monoxide mixing ratios in the western U.S. To better understand O3 produced from wildfires, we developed a statistical model that estimates the maximum daily 8 h average (MDA8) O 3 as a function of several meteorological and temporal variables for three urban areas in the western U.S.: Salt Lake City, UT; Boise, ID; and Reno, NV. The model is developed using data from June-September 2000-2012. For these three locations, the statistical model can explain 60, 52, and 27% of the variability in daily MDA8. The Statistical Model Residual (SMR) can give information on additional sources of O3 that are not explained by the usual meteorological pattern. Several possible O3 sources can explain high SMR values on any given day. We examine several cases with high SMR that are due to wildfire influence. The first case considered is for Reno in June 2008 when the MDA8 reached 82 ppbv. The wildfire influence for this episode is supported by PM concentrations, the known location of wildfires at the time and simulations with the Weather and Research Forecasting Model with Chemistry (WRF-Chem) which indicates transport to Reno from large fires burning in California. The contribution to the MDA8 in Reno from the California wildfires is estimated to be 26 ppbv, based on the SMR, and 60 ppbv, based on WRF-Chem. The WRF-Chem model also indicates an important role for peroxyacetyl nitrate (PAN) in producing O3 during transport from the California wildfires. We hypothesize that enhancements in PAN due to wildfire emissions may lead to regional enhancements in O3 during high fire years. The second case is for the Salt Lake City (SLC) region for August 2012. During this period the MDA8 reached 83 ppbv and the SMR suggests a wildfire contribution of 19 ppbv to the MDA8. The wildfire influence is supported by PM2.5 data, the known location of wildfires at the time, HYSPLIT dispersion modeling that indicates transport from fires in Idaho, and results from the CMAQ model that confirm the fire impacts. Concentrations of PM2.5 and O3 are enhanced during this period, but overall there is a poor relationship between them, which is consistent with the complexities in the secondary production of O3. A third case looks at high MDA8 in Boise, ID, during July 2012 and reaches similar conclusions. These results support the use of statistical modeling as a tool to quantify the influence from wildfires on urban O3 concentrations. © 2013 American Chemical Society. Source

Emery C.,ENVIRON International Corporation | Jung J.,ENVIRON International Corporation | Downey N.,Earth System science LLC | Johnson J.,ENVIRON International Corporation | And 3 more authors.
Atmospheric Environment

Policy Relevant Background (PRB) ozone, as defined by the US Environmental Protection Agency (EPA), refers to ozone concentrations that would occur in the absence of all North American anthropogenic emissions. PRB enters into the calculation of health risk benefits, and as the US ozone standard approaches background levels, PRB is increasingly important in determining the feasibility and cost of compliance. As PRB is a hypothetical construct, modeling is a necessary tool. Since 2006 EPA has relied on global modeling to establish PRB for their regulatory analyses. Recent assessments with higher resolution global models exhibit improved agreement with remote observations and modest upward shifts in PRB estimates. This paper shifts the paradigm to a regional model (CAMx) run at 12. km resolution, for which North American boundary conditions were provided by a low-resolution version of the GEOS-Chem global model. We conducted a comprehensive model inter-comparison, from which we elucidate differences in predictive performance against ozone observations and differences in temporal and spatial background variability over the US. In general, CAMx performed better in replicating observations at remote monitoring sites, and performance remained better at higher concentrations. While spring and summer mean PRB predicted by GEOS-Chem ranged 20-45. ppb, CAMx predicted PRB ranged 25-50. ppb and reached well over 60. ppb in the west due to event-oriented phenomena such as stratospheric intrusion and wildfires. CAMx showed a higher correlation between modeled PRB and total observed ozone, which is significant for health risk assessments. A case study during April 2006 suggests that stratospheric exchange of ozone is underestimated in both models on an event basis. We conclude that wildfires, lightning NO. x and stratospheric intrusions contribute a significant level of uncertainty in estimating PRB, and that PRB will require careful consideration in the ozone standard setting process. © 2011 Elsevier Ltd. Source

Downey N.,Earth System science LLC | Emery C.,ENVIRON International Corporation | Jung J.,ENVIRON International Corporation | Sakulyanontvittaya T.,ENVIRON International Corporation | And 3 more authors.
Atmospheric Environment

We use a photochemical grid model instrumented with the high-order Decoupled Direct Method (HDDM) to evaluate the response of ozone (O3) to reductions in US-wide anthropogenic emissions, and to estimate emission reductions necessary to meet more stringent National Ambient Air Quality Standards (NAAQS) for O3. We simulate hourly O3 response to nationwide reductions in nitrogen oxides (NOx) and volatile organic compound (VOC) emissions throughout 2006 and compare O3 responses in 4 US cities: Los Angeles, Sacramento, St. Louis, and Philadelphia. We compare O3 responses between NOx-rich, O3-inhibited urban core sites and NOx-sensitive, higher O3 suburban sites and analyze projected O3 frequency distributions, which can be used to drive health effect models. We find that 2006 anthropogenic NOx and VOC emissions must be reduced by 60-70% to reach annual 4th highest (H4) maximum daily 8-h(MDA8) O3 of 75ppb (the current US standard) in Sacramento, St. Louis, and Philadelphia, and by 80-85% to reach an H4 MDA8 of 60ppb. Los Angeles requires larger emissions reductions and achieves an H4 MDA8 of 75ppb with 92% reductions and 60ppb with 97% reductions. As emissions are reduced, hourly and MDA8 frequency distributions tend toward mid-level background distributions. Mid-level O3 exposure is an important driver of O3 health impacts calculated by epidemiological models. A significant fraction (at least 48%) of summertime integrated MDA8 O3 at all sites remains after complete elimination of US anthropogenic NOx and VOC emissions, implying that mid-level O3 exposure due to background will become more important as domestic precursor emissions are controlled. © 2014 The Authors. Source

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