Photochemical Grid Modeling in the Midwest: A RetrospectiveA Retrospective A brief review of photochemical grid modeling efforts in the Midwestern states over the past 20 years. In

14 em january 2015 awma.org

by Rob Kaleel

Rob Kaleel is executive director of the Lake Michigan Air Directors Consortium (LADCO), Rosemont, IL. E-mail: [email protected].

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Photochemical Grid Modeling in the Midwest:A Retrospective

A brief review of photochemical grid modeling efforts in the Midwestern states over the past 20 years.In the past 25 years, photochemical grid mod-els have become a widely used by the U.S. Environmental Protection Agency (EPA), states, and regional organizations such as the Lake Michigan Air Directors Consortium (LADCO). In fact, LADCO was established in 1989 to develop just such a tool for the states border-ing Lake Michigan: Illinois, Indiana, Michigan,

and Wisconsin. This article focuses on the development and use of these models over the past two decades in the Midwestern states represented by LADCO (the above-mentioned states, plus Ohio and Minnesota), recognizing that similar modeling efforts and similar expe-riences have been on-going in other parts of the country as well.

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Copyright 2015 Air & Waste Management Association

LADCO Is FormedSince the early 1990s, the Midwestern states have worked cooperatively with other states and EPA to use photochemical grid models to evaluate the effectiveness of emission reduction strategies needed to meet national air quality standards and goals. Initially the states’ focus was to gain a better understanding of the formation and trans-port of ozone (O3) and its precursors over Lake Michigan. Prior efforts to evaluate and imple-ment control strategies in the region using EPA’s then-preferred method, the Empirical Kinetics Modeling Approach (EKMA) were unsuccessful in meeting the 1-hour O3 standard in effect at that time. EKMA, which simulates O3 in a hypothetical, vertically well-mixed column of air that is “trans-ported” along a trajectory from the urban cen-ter to the downwind location of peak O3, proved to be an overly simplistic approach, even though it incorporated a sophisticated chemical mecha-nism (Carbon Bond IV). Given the complexities of the region’s geography (the interaction of urban plumes in the lakeshore environment), the Mid-western states signed a memorandum of agree-ment in 1989 to develop a photochemical grid model for the Lake Michigan region. LADCO was formed at that time to coordinate those efforts.

The Midwestern states retained Systems Appli-cations International (SAI), the developer of the Urban Airshed Model (UAM), to prepare a “scop-ing study” to guide the development of a mod-eling system for the Lake Michigan region. The development of the modeling system commenced on multiple tracks. Understanding that the ade-quacy of the model for eventual regulatory pur-poses depended on a thorough evaluation of the model’s ability to simulate episodic, high O3 concentrations in the region, and recognizing the inadequacy of ambient air quality and meteoro-logical measurements for such an evaluation, SAI recommended an intensive fi eld measurement program be conducted to obtain the needed data. The Lake Michigan Ozone Study (LMOS) was thus conducted during the summer of 1991. Concurrently, LADCO retained several contrac-tors for the development of model inputs, includ-ing multistate emissions inventories and gridded

meteorological parameters for several ozone epi-sodes, and application and evaluation of the UAM for the Lake Michigan region. The geographic scale of the resulting modeling domain, as devel-oped 20 years ago for application to the Lake Michigan region, is shown in Figure 1.

O3 Study Produces Key FindingsIn addition to establishing the infrastructure to sup-port photochemical grid modeling in the Midwest, the Lake Michigan states made two key policy-rel-evant fi ndings as a result of LMOS and the devel-opment of the UAM. First, the measurements and modeling showed that volatile organic compound (VOC) emission reductions were more effective than nitrogen oxide (NOX) reductions in lowering O3 concentrations in and immediately downwind of the Chicago metropolitan area. NOX reductions were, however, shown to be more effective than

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Photochemical Grid Modeling in the Midwest:A Retrospective Figure 1. Modeling domain used during the Lake Mich-

igan Ozone Study (LMOS). The domain incorporated two-way nested grids in the Urban Airshed Model (UAM-V) for the fi rst time in a modeling application in the Midwest. Grid A had a grid resolution of 36 km; Grid B was 12 km; and Grid C was 4 km.Source: Systems Applications International



VOC reductions in lowering O3 concentrations at locations farther downwind of the metropolitan area. This finding was reaffirmed by recent mod-eling performed by LADCO (using the CAMx model developed by Environ). Figure 2, which depicts O3 differences (increases and decreases) due to a hypothetical 30% domain-wide NOX emission reduction, shows O3 increases (disben-efits) within the urban core and O3 decreases at locations downwind. Conversely, Figure 3 shows only O3 decreases in the urban area resulting from an assumed 30% VOC emission reduction, but with relatively small downwind O3 benefi s.

The second key policy-relevant finding was that the 1-hour O3 National Ambient Air Quality Stan-dards (NAAQS) could not feasibly be met with just VOC reductions strategies imposed in the metropolitan area. High levels of O3 observed by aircraft measurements upwind of the Chicago area, represented as boundary conditions in the UAM, would prevent the area from achieving attainment of the NAAQS with just local emission

reductions. Clearly a strategy that incorporated both local VOC emissions reductions coupled with reductions in transported O3 from upwind sources was needed.

Similar Efforts Underway in Other RegionsSimilar modeling and data collection efforts were underway in other parts of the country. In 1991, for example, the Northeast states and EPA com-pleted the Regional Ozone Modeling for North-east Transport (ROMNET) study, which evaluated O3 control strategies for meeting the 1-hour O3 NAAQS in the Northeast corridor. One of the goals of the study, which employed the Regional Oxidant Model (ROM), was to evaluate the impor-tance of city-to-city transport within the Northeast corridor. Among the major findings of the study, it was determined that unfeasibly stringent emis-sion reductions (as much as 91% VOC reductions) would need to be implemented to reduce pre-dicted O3 concentrations below the level of the 1-hour NAAQS.


Figure 2 (left). Model sensitivity of an assumed 30% reduction of NOX emissions domain-wide. Areas shown in blue represent areas of O3 improvement; areas shown in yellow and red indicate O3 increases from reduced NOX emissions (NOX disbenefit).Source: Alexander Cohan, LADCO

Figure 3 (right). Model sensitivity of an assumed 30% reduction of anthropogenic VOC emissions domain-wide. Areas shown in blue represent areas of O3 improvement from reduced VOC emissions.Source: Alexander Cohan, LADCO

Alexander Cohah, LADCO Alexander Cohah, LADCO

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Another effort that was particularly important for understanding the regional nature of the O3 prob-lem was the Southern Oxidant Study (SOS), which was initiated in 1990. SOS was a multiyear cooper-ative effort involving researchers from universities, state agencies, EPA, and other stakeholder organi-zations. One of the key contributions of the SOS was an improved understanding of the role of bio-genic (primarily VOC) emissions in the formation of O3. Biogenic VOC emissions, which are ubiqui-tous in the southern states, can create NOX-limited conditions, which favor the effectiveness of NOX controls for reducing O3 concentrations. Thus, the states in the Northeast, Southeast, and Midwest, were converging on the growing understanding of the regional nature of O3 and that emission reduc-tions from sources beyond the boundaries of des-ignated non-attainment areas would be needed to reach attainment of the O3 NAAQS.

OTAG Is FormedClearly a new approach was needed to achieve the O3 standard. In 1995, the Ozone Transport Assessment Group (OTAG) was formed to assess the transport of O3 on a regional basis and to determine the benefits from national or regional control strategies. OTAG was as much a collab-orative process as it was a technical study. Much has been written about the lessons learned from OTAG, and will therefore not be repeated here, but from a modeling perspective, it was perhaps the largest effort ever conducted. (Editor’s Note: See article on OTAG by Theresa Pella elsewhere in this issue.)

Figure 4 depicts the OTAG modeling domain, which covered virtually the entire eastern United States. Key among the major conclusions reached by OTAG was that regional NOX reductions are effective in producing O3 benefits, but that the benefits decrease with distance from the source. Figure 5, which depicts the regional O3 benefits from an assumed 30% domain-wide NOX reduc-tion (recent LADCO modeling using CAMx), illus-trates substantial O3 benefits across the domain, with the greatest benefits in key source regions, such as the Ohio River Valley. OTAG was ulti-mately successful in that it provided the techni-cal justification for EPA’s regulatory approach for reducing O3 transport, the NOX SIP Call.

The Midwestern states had key roles in completing the technical tasks required during the OTAG pro-cess. At LADCO, the Midwest Modeling Center was established to conduct some of the modeling required during the OTAG process. Similar model-ing centers were established in the Northeast and Southeast states and these centers, in conjunction


Figure 4. OTAG modeling domain. The inner grid represents the fine grid (12-km grid resolution), and the outer grid represents the coarse grid (36-km grid) used during OTAG.Source: Ozone Transport Assessment Group

Alexander Cohah, LADCO

Figure 5. Model sensitivity of an assumed 30% reduction of NOX emissions domain-wide. Areas shown in blue represent areas of ozone improvement; areas shown in yellow and red indicate O3 increases from reduced NOX emissions (NOX disbenefit).Source: Alexander Cohan, LADCO



with EPA, successfully and cooperatively managed one of the largest public-private air quality proj-ects ever conducted. The infrastructures created at the modeling centers were forerunners of the Regional Planning Organizations (RPOs) estab-lished years later to coordinate and conduct the studies needed to address fi ne particulate matter (PM2.5) and regional haze.

In 1997, EPA tightened the O3 NAAQS and changed the averaging time to 8 hours from the previous 1-hour standard. At the same time, EPA established the PM2.5 NAAQS. Then in 1999, EPA adopted the Regional Haze Rule, which required the states to develop and implement plans to reduce the pollution that causes vis-ibility impairment in federal Class I areas. Five multi-state RPOs, including the Midwest RPO (aka LADCO) were established to develop the technical basis for these plans. Photochemical grid models were the foundation of the plans developed by the states in subsequent years to meet the air quality goals established by all three federal actions.

Enhancing the CAMx ModelAs described above, modeling efforts to address O3 had been proceeding for several years, and continue to this day. Modeling for PM2.5 and regional haze was relatively new at the time that the federal requirements were established, and early model evaluations revealed defi ciencies in the available modeling platforms. The Midwest RPO worked closely with consulting company Environ in the period 2003–2005, and in the years since, to enhance the CAMx photochemical model to better address these pollutants. Enhancements included improved treatment of wet deposition, inclusion of PM chemistry in the Plume-in-Grid module, updates to gas-phase chemistry in the Car-bon Bond chemical mechanism, and development of the source apportionment probing tool (PSAT) for PM2.5. Improved photochemical grid model algorithms, as well as improved inventories of pre-cursor emissions, yielded signifi cantly improved model performance, as shown in Figure 6.


Figure 6. Monthly mean average bias CAMx model performance for PM2.5 constituent species: elemental carbon (EC), organic carbon (OC), sulfate (SO4), nitrate (NO3), ammonium (NH4), and soil. Through three iterations of the 2007 Base Case using the CAMx photochemical model, LADCO was able to greatly improve model performance, although the OC component is still underpredicted (negative bias).

Kirk Baker, LADCO

Monthly Average Mean Bias

Abby Jaye, LADCO

2007 Monthly Average Mean Bias – 12km (baseCv7_v5.40)

Abby Jaye, LADCO2007 Monthly Average Mean Bias – 12km (baseA_MOGbiog)



The three panels in Figure 6 illustrate the progres-sion (from top to bottom) of the model’s ability to replicate ambient measurements of the various PM constituent—EC, OC, sulfates, nitrates, ammo-nia, and soils—after model and inventory enhance-ments were incorporated.

In summary, over the past 20 years, EPA and states, including the states in the Midwest, have devoted significant resources to develop, evaluate, and perform photochemical grid models for meet-

ing air quality planning requirements. Although the complexity and scale of the modeling exer-cises have increased, as shown by comparing LADCO’s initial modeling domain (Figure 1) with its current domain (Figure 7), such models have become generally accepted and routinely used for analyzing air quality and developing effective attainment demonstrations. It can be expected that photochemical grid models will continue to be relied upon by the states and EPA for planning to meet future air quality goals. em

Figure 7. Current LADCO modeling domain.Source: Alexander Cohan, LADCO


Sources1. Procedures for Applying City-Specific EKMA; EPA-450/4-89-012; U.S. Environmental Protection Agency, Research Triangle Park, NC, July

1989.2. Haney, J.; Douglas, S.; Chinken, L.; Souten, D.; Burton, C.; Roberts, P. Ozone Air Quality Scoping Study for the Lower Lake Michigan Air

Quality Region; Systems Applications International, San Rafael, CA, August 1989.3. Posseil, N.; Milich, L.; Goodrich, B. Regional Ozone Modeling for Northeast Transport (ROMNET); EPA-450/4-91-002a; U.S. Environmental

Protection Agency, Research Triangle Park, NC, June 1991.4. Scheffe, R.; Morris, R. A Review of the Development and Application of the Urban Airshed Model; Atmos. Environ. 1993, 27B, 23-29.5. Koerber, M. Major Findings and Results of the Lake Michigan Ozone Study; Lake Michigan Air Directors Consortium, Rosemont, IL,

December 1994.6. Russell, A. Regional Photochemical Air Quality Modeling: Model Formulations, History, and State of the Science; Annu. Rev. Energy Environ.

1997, 22, 537-588.7. Keating, T.; Farrell, A. Transboundary Assessment: Lessons Learned from the Ozone Transport Assessment Group; NCEDR/99-02; National

Center for Environmental Decision-Making Research, Knoxville, TN, October 1999.8. Koerber, M. Midwest Subregional Modeling: 1-Hour Ozone Attainment Demonstration for the Lake Michigan Area; Lake Michigan Air

Directors Consortium, Rosemont, IL, September 2000.9. Yarwood, G.; Emery, C.; Wilson, G.; Kemball-Cook, S.; Rao, S.; Koo, B.; Morris, R. Supporting the MRPO’s PM and Visibility Modeling: CAMx

Evaluation and Improvements; Environ International Corporation, Novato, CA, March 2005.



Documents

Photochemical Grid Modeling in the Midwest: A RetrospectiveA Retrospective A brief review of photochemical grid modeling efforts in the Midwestern states over the past 20 years. In