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1 Contributors: Arlene Fiore 1, Lee Murray 1, Luke Valin 1, Olivia Clifton 1, Jean Guo 1 Author: Melissa Seto 1 Analysis of Wisconsin 2007 High-Ozone event Section 1A: Can satellite NO 2 columns inform us about the spatial scale and regional transport of ozone pollution episodes such as the one identified by the Wisconsin Department of Natural Resources from June 10-18, 2007? Nitrogen dioxide (NO 2 ) is relevant to understanding ozone pollution because nitrogen oxides (NO x NO + NO 2 ) are precursors to ozone. That is, the photolysis of NO 2 leads to the production of ozone. Consequently, high NO 2 concentrations may lead to high ozone concentrations. While NO 2 stays in the polluted boundary layer for less than a day, ozone typically remains for a few days in the Eastern U.S. boundary layer during summer. Consequently, ozone regional transport can be inferred from ground-based data. However, it remains unclear whether NO 2 stays in the air long enough to use the NO 2 columns retrieved from satellite data as a proxy for regional transport. Below, we explore whether satellite NO 2 data can inform us about the spatial scale of ozone pollution episodes. For each day of the WI episode, we have included (1) a map of Environmental Protection Agency (EPA) Air Quality System (AQS) maximum daily 8-hour average (MDA8) surface ozone 2 in parts per billion (ppb), (2) a map of NO 2 anomalies in units of molecules/cm 2, and (3) a National Oceanographic and Atmospheric Association (NOAA) weather map 3. Before we analyze the maps in detail, we provide a brief overview of the information provided by these maps and the data collection techniques. Ozone concentrations in parts per million are sampled every hour and the EPA AQS MDA8 metric is calculated by computing twenty-four over-lapping 8-hour averages (i.e., 12-7am, 1-8am) and taking the highest 8-hour average per day. The NO 2 anomalies are calculated by subtracting mean NO 2 during May, June, July and August (MJJA) from the daily NO 2 at each location. Therefore, the more positive (red) the anomaly, the greater it exceeds the summer average. The instrument that measures NO 2 is called the Ozone Monitoring Instrument (OMI), which flies aboard National Aeronautics and Space Administration s Aura satellite. The OMI has a spatial sampling resolution as fine as 13 km x 24 km. The satellite overpass time (i.e. when the satellite passes over the location on the map) is 1:30 PM. While the current NO 2 anomaly plots do not differentiate lack of observations from zero anomaly, we will update this by plotting zero anomalies in grey in future maps. 1 Columbia University/ Lamont-Doherty Earth Observatory 2 We provide animations of AQS MDA8 ozone data for each year from 2006 to 2013 at: 3 Downloaded from 1

2 The NOAA weather maps help us visualize the meteorological conditions that may contribute to the regional movement of high ozone concentrations across the eastern U.S. The weather maps used in this report are taken from 7 AM of each particular day. In general, high pressure systems (labeled with High and red lines) are associated with fewer clouds and higher ambient air pollution concentrations. Low pressure systems are associated with more clouds and precipitation and lower ambient air pollution concentrations. Section 2A: GEOS-Chem simulations for source attribution during the June eastern U.S. ozone event We are interested in answering five questions with the GEOS-Chem simulations: (1) To what extent is the model capturing the AQS observations? (2) How does the North American Background (NAB) simulation compare with the Base and Observed (AQS)? (3) How is the NAB contributing to the observed pollution event? (4) What is the Wisconsin contribution to the event? (5) How much are other states, Canada, and Mexico contributing to the observed event? GEOS-Chem is a three-dimensional, global model of atmospheric composition. The version we use is driven by meteorology from the Goddard Earth Observation System (GEOS) of the National Aeronautics and Space Administration s (NASA) Global Modeling Assimilation Office. Emissions are from the global EDGAR and RETRO (for anthropogenic VOC) inventories, but replaced over the USA by NEI95 except for biofuels which use NEI99. (Other regional replacements include: Canada by CAC; Mexico by BRAVO; Europe by EMEP; East Asia by Streets et al. (2006)). Different scaling factors are applied for weekday versus weekend, month, and year. We use this model at 2º latitude x 2.5º longitude horizontal resolution to generate simulations of the Wisconsin 2007 event, including: (1) Base, (2) North American Background (NAB), and (3) No WI contribution (No-WI). The base case simulation includes all relevant processes to try to match the observed ozone concentrations during the nine-day event. The better the base GEOS-Chem simulation matches the EPA AQS data, the more reliable the model. We note that GEOS-Chem has been extensively evaluated in numerous prior publications ( The NAB simulation represents ozone levels that would exist if anthropogenic emissions from North American countries such as Canada, U.S. and Mexico were set to zero. Thus, the NAB simulation includes natural sources of ozone, ozone produced by countries outside of North America, and ozone transported from the stratosphere (EPA, 2006). The No-WI simulation sets the Wisconsin anthropogenic emissions to zero 4. By subtracting the No-WI simulation from the base case, we can deduce the WI ozone contribution to the 2007 episode: Base case (No-WI) = WI contribution. 4 We could run these zero-out simulations relatively quickly for a first look for air quality agencies if it would be useful before they run their high resolution regional models with their preferred emissions inventories. 2

3 Section 1B: Day-by-day explanations of observed ozone, NO 2 anomalies, and weather maps The following pages are detailed explanations of the ozone, NO 2, and weather maps for each day in the Wisconsin 2007 episode. Please note that our section on source attribution during the June Eastern U.S. ozone event using GEOS-Chem simulations begins on page 13. 3

4 June 10, Sunday Since this day is during the weekend, there is relatively low diesel truck and automobile activity. Therefore, the NO 2 and ozone concentrations are on the low side as shown by the mostly yellow, green or blue dots in Figures 1 and 2. Figure 1 suggests that the ozone pollution event in Wisconsin started locally around the coast of Lake Michigan. The weather map in Figure 3 shows that there is a high pressure system centered over Lake Erie. Figure 2: NO2 anomalies (molecules/cm 2 ) Figure 1: EPA AQS MDA8 Ozone Figure 3: NOAA Weather Map 4

5 June 11, 2007: Monday On Monday, a business day, NO 2 emissions are higher. The regional extent of the high pressure system, now centered over eastern Michigan, extends southward and westward. As a result, we see a band of orange and red dots indicating higher concentrations in the NO 2 map in Figure 5. The NO 2 positive anomalies (Fig. 5) seem to spread west as the high pressure system moves in the same direction (Fig. 6). Furthermore, the local ozone event has now spread to include most of Mid-western U.S. Figure 5 Figure 4 Figure 6 5

6 June 12, 2007: Tuesday The high pressure system continues to move west and is now centered over northern Michigan (Fig. 9). The system seems to transport high concentrations of both ozone and NO 2 westward to states including Ohio, Kentucky, Indiana, Wisconsin, Minnesota, and Iowa. Both Figures 7 and 8 support the regional nature of this pollution event. Figure 8 Figure 7 Figure 9 6

7 June 13, 2007: Wednesday As the high pressure system progresses in the southwest direction (Figure 12), both ozone and NO 2 high concentration plumes move southward and westward (Figures 10 and 11). Yet, parts of the plumes also move north into northernmost Minnesota (Figures 10 and 11). Figure 11 Figure 10 Figure 12 7

8 June 14, 2007: Thursday The high pressure influence over the Midwest decreases (Figure 15). A cold front moving southeastward leads to low ozone along the Eastern seaboard (Fig. 15). Pollution accumulates along the stationary front located at approximately 100ºW (Figs. 13 and 14). Both ozone concentrations and NO 2 anomalies move westward and are highest from approximately 88-95ºW (Figs. 13 and 14). This marks the westernmost extent of the regional ozone episode. The high pressure system centered over Colorado (Fig. 15) likely contributes to the accumulation of ozone over Colorado and New Mexico as shown in Figure 13. The ozone enhancements are also associated with positive NO 2 anomalies over that region (Figure 14). Figure 13 Figure 14 Figure 15 8

9 June 15, 2007: Friday The high pressure system contributing to the eastern U.S. ozone episode is now centered over Pennsylvania (Fig 18). Consequently, the ozone plume is pushed eastward (Fig 16). The direction of movement of the NO 2 plume is unclear here, but we see enhancements over the Ohio River Valley source region (Fig 17). Figure 17 Figure 16 Figure 18 9

10 June 16, 2007: Saturday The high pressure system moves southeast (Fig 21). While the ozone plume generally moves southeast with a narrow band extending into the Central U.S., Michigan now shows higher ozone concentrations (Fig 19). Similarly, the plume of NO 2 enhancements appears to progress in the same direction (Fig 20). gure 19 Figure 20 Figure 21 10

11 June 17, 2007: Sunday On Sunday, the decrease in diesel truck and automobile activity during the weekend leads to more blue dots in the NO 2 plot, indicating negative anomalies (Fig 23) but the ozone concentrations remain high (Fig 24). As the high pressure system moves eastward and the cold front sweeps southward of the Great Lakes, the ozone plume moves eastward (Fig 22 and 24). Figure 42 Figure 33 Figure 24 11

12 June 18, 2007: Monday The cold front has led to cleaner air over the New England states (Figures 25 and 27). Ozone and NO 2 anomalies are now confined to a narrow band (i.e. Michigan, Ohio, western Pennsylvania, West Virginia, Virginia, and North Carolina) between the advancing cold fronts (Figs ). gure 25 Figure 26 Figure 27 12

13 Section 2B: Analysis of Source Attribution with AQS Observations and GEOS-Chem Simulation Plots (1) To what extent is the model capturing the AQS observations? When we directly compare the AQS maximum daily 8-hour average (MDA8) ozone maps with the GEOS-Chem Base simulations, we find that the model approximately captures the observed ozone concentrations. For example, on Sunday, June 10, both the Base and AQS maps show relatively low ozone concentrations (blue and green) along the east coast with some medium concentrations (yellow and orange) in southeastern states, such as Mississippi and Alabama, and Midwestern states around Lake Michigan including Wisconsin. On Monday June 11, the AQS observations show an increase in ozone concentrations around Lake Michigan as indicated by the red dots; similarly, the Base simulation displays higher concentrations (red) around Lake Michigan. Between June 10 and 15, both AQS and Base show a progression of high ozone concentrations (orange and red) that eventually covers Wisconsin. Between June 16 and 18, the high concentration plume moves back east, as discussed in the context of weather patterns in the Figures in Section 1. (2) How does the North American Background (NAB) compare with the Base and Observed (AQS)? Whereas the Base and Observed maps show a regional high concentration (red) ozone plume around the Great Lakes and the Southeast region on June 10-11, the NAB maps suggest that NAB is highest over states to the west of where the highest total surface ozone concentration occur (including over Minnesota, Iowa, North Dakota, South Dakota, and Nebraska). NAB levels are also higher over northeastern Canada, possibly associated with wildfires (Figure 29). A smaller, medium concentration (yellow and orange) NAB plume occurs over the Southeastern states (Florida, Georgia, Alabama, Mississippi), possibly from lightning activity associated with the frontal systems over that region on these days (Figures 3 and 6). (3) How is the NAB contributing to the observed pollution event? During the event, the NAB levels range from about ppb (yellow, orange, and red) over the state of Wisconsin. The highest NAB ozone during the event occurs on June 16-17, which is when total surface ozone over WI is not particularly high. The highest values over WI are thus not driven by NAB, but rather by ozone produced from anthropogenic emissions. On the days when observed surface ozone is highest over WI (June 14-15), the NAB contribution is generally below ppb. (4) What is the Wisconsin contribution to the event? The GEOS-Chem WI maps indicate the Wisconsin contribution to the event is small. June 13 to 15 are the only days in the event when the concentrations directly above or near Wisconsin exceed 4 ppb (yellow, orange, or red), and these are also days when total ozone is 13

14 highest. On these days, the maximum contribution of WI emissions over WI is 10 ppb. On June 11 and 12 when total surface ozone is similarly high over WI, the WI contribution does not exceed 6-7 ppb (yellow). (5) How much are other states, Canada, and Mexico contributing to the observed event? The spatial patterns of high anthropogenic ozone contribution from U.S. states except WI (third column, Figure 30) correspond with high observed ozone (first column, Fig. 30). For example, during June 16-18, there is a band of medium to high ozone (yellow, orange, and red) extending from the East coast into the Midwest (third column, Figure 30) and a band of medium to high ozone over a similar region on the observed maps (first column, Figure 30). Because the model and observed spatial and temporal variability agree, we infer that other states add about ppb to the regional ozone event and ppb over WI specifically during June The combined Canadian and Mexican anthropogenic contribution (fourth column, Figure 30) shows the greatest influence in northeastern U.S, adding approximately ppb of ozone (green, yellow, orange, and red) to the northeastern U.S. (fourth column, Fig. 30). In contrast, Canada and Mexico contribute close to zero ppb (blue) to Wisconsin specifically, except for June when they add approximately 5-10 ppb to WI (fourth column, Fig. 30). We infer that Canadian rather than Mexican sources are contributing to ozone concentrations over New England and upper Midwest states. 14

15 Figure 28: Source attribution for ozone during WI 2007 event. This plot extends from pages WI contribution was calculated by: GEOS-Chem Base No WI contribution = WI contribution. In order to enhance readability, the color bars of the four columns of maps have different ranges. The AQS observed and GEOS-Chem Base maps have the same color bar range (10-90 ppb) because we wanted to evaluate the accuracy of GEOS-Chem. 15

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18 Figure 29: Northeastern Canada wildfire map. Obtained from 18

19 Figure 30: Source attribution for WI 2007 high-ozone event using GEOS-Chem simulations. This plot continues from page 19 to page 21. The anthropogenic contribution from all U.S. states except WI (third column from left) was calculated by subtracting US background from the No-WI contribution. The Canadian and Mexican contribution (rightmost column) was calculated by subtracting the NAB from the US background. In order to enhance readability, the color bars of the four columns of maps have different ranges. The AQS observed and GEOS-Chem Base maps have the same color bar range (10-90 ppb) because we wanted to evaluate the accuracy of GEOS-Chem. 19

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