WESTAR Council & the University of NevadaWESTAR Council & the University of Nevada

Conference onConference onWestern Ozone TransportWestern Ozone Transport

October 10-12, 2012October 10-12, 2012Reno, NVReno, NV

• Increase our understanding of the science of ozone background and transport in the West

•Show how the science can help state regulatory agency decision making for:

•Nonattainment area planning •Interstate transport assessments

Conference Objectives

•What are the source areas of ozone and what is the relevance of each to surface ozone in the western U.S.?

• Local/regional• Western regional transport• Long-range transport from Asia• Stratospheric intrusions• Wildfire

Questions addressed at the Conference

•What assessment techniques are being used to determine ozone?

• Ambient monitoring• Ozonesondes• Aerosol tracers

• Lidar• Satellite• Modeling

• Global• Regional

Questions addressed at the Conference

• Clean Air Scientific Advisory Committee (CASAC)

• 2010 - Recommended 60 to 70 ppb• EPA proposed 60-70 ppb • but retained 2008 standard (75 ppb)

Ozone NAAQS Background

• EPA Ozone NAAQS 2012 Policy Assessment:•“strong support” for tightening Ozone NAAQS

•CASAC review of the EPA Policy Assessment:• “provides a strong rationale for consideration of ozone standards (8 hour averages) of 60 ppb and 70 ppb”• “provides adequate justification for considering concentrations below 60 ppb, in the 50 to 60 ppb range.”

Ozone NAAQS Background

The Problem

120 ppb 1979 1-hr avg

84 ppb1997 8-hr

75 ppb 2008 8-hr

40 60 80 100 120O3 (ppbv)

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U.S. National Ambient Air Quality Standard for O3 has evolved over time

Future?(proposed)

typical U.S.“background” (model estimates)[Fiore et al., 2003;Wang et al., 2009;Zhang et al., 2011]

Allowable O3 produced from U.S. anthrop. sources (“cushion”)

Lowering thresholds for U.S. O3 NAAQS implies thinning cushion between regionally produced O3 and background

Some challenges for WUS O3 air quality management

Asia Pacific

stratosphere lightning

“Background” ozone

Wildfire, biogenic

Western USA

Rising Asian emissions [e.g., Jacob et al., 1999; Richter et al., 2005; Cooper et al., 2010]

Natural events e.g., stratospheric [Langford et al [2009];

fires [Jaffe & Wigder, 2012]

Warming climate+in polluted regions [Jacob & Winner, 2009 review]

+ natural sources [recent reviews: Isaksen et al., 2009; Fiore et al., 2012]

? Transport pathways

Need process-level understanding on daily to multi-decadal time scales

Today’s talk: 1) Intro: satellites + models 2) NASA AQAST Overview 3) Results for WUS from two collaborative AQAST projects 4) Air quality projections in a warming climate

X

GFDL AM3 simulation with N. American anthrop. emissions shut off (1981-2007)

MEAN OVER 27 YEARS STANDARD DEVIATION

ppb ppb

Variability in springtime O3 background

Characteristics of background O3

• Wildfires:– Emery et al. (2012) found that modeled day-specific fires in the West can

increase 8-hour ozone concentrations by 10-50 ppb.– Jaffe et al. (2008) determined that WUS seasonal mean ozone

concentrations increased by ~ 9 ppb in years with high wildfire activity.

• Stratospheric intrusions:– Lin et al. (2012) estimated that 8-hour ozone contributions from strong

stratospheric intrusion events could range up to ~ 55 ppb in the WUS. – Langford et al. (2010) noted that strong intrusion events are relatively

straightforward to identify, but harder to quantify.

• International transport:– Zhang et al. (2011) estimated that the worldwide anthropogenic methane

plus intercontinental aNOX/aVOC contributions to 8-hour ozone is ~ 9 ppb at low-elevation sites and ~ 13 ppb at sites in the West. Also estimated mean contributions of 1-3 ppb from Canada and Mexico, with larger / more variable impacts in border regions.

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Free tropospheric ozone trend above western North America

An extension of the 1995-2008 ozone trend, adding years 2009, 2010 and 2011.

All available data above western North America, regardless of transport history.

Median values provide a more robust indicator than higher values of the distribution for the purpose of examining and comparing the regional and seasonal variability in NAB. The NAB predictions toward the higher end of the distribution (e.g., 75 th and 95th percentiles) are more reflective of infrequent or atypical events. Due to the overall uncertainties and assumptions in the inputs to the two modeling systems, the higher percentage NAB predictions are likely to have a greater degree of uncertainty than the median values. (Henderson et al., 2012)

Region

Spring (GEOS-Chem/CAMx) Summer (GEOS-Chem/CAMx)

Median (ppb)

75th percentile (ppb)

95th percentile (ppb)

Median (ppb)

75th percentile

(ppb)

95th percentile

(ppb)

California 34/35 40/40 48/48 30/36 36/40 45/47North Central 28/33 33/37 40/42 24/33 28/36 39/41

Northeast 23/31 26/34 33/38 18/29 23/32 34/36Southeast 30/34 34/38 41/45 29/31 36/34 44/41

West(x-CA)

All sites 44/43 47/48 52/55 41/41 46/46 54/52Low-

elevation sites

43/41 46/44 51/51 40/39 45/44 52/52

High-elevation

sites45/48 48/52 53/57 42/43 47/48 54/53

Estimates of background O3 in the U.S.

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Values are presented across the range of GEOS-Chem and CAMx results at all monitoring site locations with simulated MDA8 ozone concentrations above 55 ppb.

O3 is only regulated pollutant with positive gradient with height. O3 is only regulated pollutant where background concentrations

are more than 50% of the health threshold.

Trinidad Head, N. California O3 profiles: 1997-2009

Average MDA8 at 10 Rocky Mtn CASTNET sites

Avg MDA8 at these sites significantly correlated with elevation. While there are site to site differences, interannual variations are

significantly correlated across most sites (Jaffe 2011).

Great Basin

Elevated Ozone Reservoir (2011 data May - September)

Elevated Ozone Reservoir

Frenchman Mt~4020 ft

Arden4301 ft

?

Elevated Ozone Reservoir

Methods for estimating background O3

• Few true measurements of background ozone– Most, if not all, rural / remote monitoring sites in the US are affected, at

least to some extent, by ozone from US anthropogenic emissions

• Current scientific understanding of NA and U.S. background ozone is largely based on global / hemispheric photochemical models. These models have limitations:

– Uncertainties in global emissions estimates– Uncertainties in chemistry important on global spatial and time scales– Uncertainty in mixing between the boundary layer and free troposphere– Limitations in capturing processes associated with episodic events

• Much of the modeling to estimate background has relied upon a “zero-out” approach which may not properly capture the contribution of background sources to ozone under current ambient conditions.

– Zero out runs are designed to answer a specific question. What would ozone be in the absence of these emissions? (Not “in-situ” contribution.)

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• Exceptional Events:– Existing rule establishes procedures and criteria by which air quality data

affected by exceptional events can be excluded from regulatory decisions.– Among the criteria for defining an exceptional event is the determination

that a violation would not have occurred “but for” the event.– Routine background contributions are not exceptional events.

• 179B/International Transport:– Allows special treatment if projected air quality or air quality on the

attainment deadline date would meet the NAAQS “but for” emissions from another country

• Attainment demonstration guidance:– Consideration of boundary condition impacts on relative response factors?– Consideration of separate future-year boundary conditions?– Consideration of exceptional events in establishing baseline DV?– Will be an opportunity for State/Local review.

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Use of background O3 in NAAQS Implementation

• As ozone standards have tightened over time, background ozone is becoming more important and will need to be carefully considered in our efforts to attain NAAQS.

• Recent modeling efforts have estimated that mean NAB contributions can range from 38-41 ppb in the intermountain Western U.S.

– Background can be larger during episodic events associated with stratospheric intrusions, wildfires, or plumes of international emissions.

• Recent modeling suggests U.S. anthropogenic sources are the largest contributor to most days with high O3 in most locations.

• EPA policy and eventual State efforts will need to carefully consider ozone background in developing efficient and effective attainment strategies.

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Summary

Cross section of isentropic potential vorticity in potential vorticity units at 18ZMay 24, 2010, for latitude 39 degrees North across Colorado showing atropopause fold descending to about 5 kilometers (MSL) over central Colorado.from the 18Z analysis run of the NAM12 on may 24, 2010.

21Patrick Reddy CDPHE

• Develop standardized technical methods for analysis of Stratospheric Intrusions.

• Promote collaboration and data sharing between the states, academics and federal researchers.

• Promote archiving of key data sets.

• The workgroup will NOT specify criteria for approval of EE or make determinations of whether flagged data can be approved by EPA.

• Workgroup can NOT make recommendations on policy or how EPA could use this information.

Stratospheric Ozone Workgroup