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One-Day Ozone Event Along Central I-95 Corridor on June 11, First 90 °F Plus Day of Summer: We Got It Half Right

Ozone exceeded the 70 ppbv NAAQS on Saturday, June 11 along a swath of the central I-95 Corridor, including northeastern Maryland, the eastern shore of Maryland, northern Delaware, northeastern metro Philadelphia, and central and southern New Jersey (Figure 1). A warm front lifted across the northern Mid-Atlantic region in the morning, ushering in a hot, humid, and modified air mass (Figure 2). It was the first 90 °F plus day across the I-95 Corridor, with a high temperature of 91 °F at KPHL and KDOV. The air mass was modified, with 24-hr back trajectories for KDOV ending at 12 UTC on June 11 (Figure 3) showing transport of air from locations where ozone reached the USG range on June 10 across NC and OH (Figure 4). As a result, we issued an AQA alert for a USG ozone forecast for Delaware on Saturday; all four of the northern Delaware ozone monitors exceeded, with a maximum 8-hr average observation of 74 ppbv. We kept the forecast in the upper Moderate range for Philadelphia because convection was expected to develop in the afternoon (Figure 5). This convection never materialized, so Philadelphia received full strong June sun all day. The surface winds were breezy, but they were southwesterly, so all they ended up doing was pushing anthropogenic emissions from I-95 to the three northeastern Philadelphia monitors, which all exceeded, with a maximum 8-hr average observation of 75 ppbv.

There are a couple of really interesting things to note for the June 11 exceedances. First, for both Philadelphia and Delaware, the previous day’s (June 10) observed ozone was in the Good range (8-hr average max of 46 ppbv in Philly and 52 ppbv in Delaware) due to the influence of a very clean air mass under northerly flow. Historically, it is extremely rare to go from Good to USG the next day. The second aspect of note is that all of the air quality models missed this event. The NOAA model guidance (Figure 6) is a representative example – the locations of highest ozone aren’t even correct, let alone the ozone magnitudes. For this event, upwind persistence was the most useful forecast variable.

So, the good news is that our hit rate for Delaware is 1.0 – we have correctly forecasted all three ozone exceedance days in Delaware (May 25-26 and June 11), with health alerts issued to the public. The bad news is that our hit rate is only 0.5 in Philadelphia – we missed two of the four observed exceedance days so far (June 1 and 11). In retrospect, the “forecast of least regret” for Philadelphia on June 11 should have been USG – we probably should not have counted on convection to develop, since the model guidance had been inconsistent leading up to the event. But the 12 UTC hi-res guidance was compelling enough (Figure 5) to convince me to keep ozone in the upper Moderate for Philadelphia.

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Figure 1.
 Observed ozone AQI values for Saturday, June 11.

WPC_CONUS_21UTC_20160611
Figure 2.
 WPC surface analysis with observations and fronts for Saturday, June 11 at 21 UTC.

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Figure 3.
 24-hour back trajectories ending at KDOV on Saturday, June 11 at 12 UTC.

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Figure 4.
 Observed ozone AQI values for Friday, June 10.

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Figure 5.
 Precipitation guidance from the 12 UTC run of the 4 km NAM model for Saturday, June 11 at 22 UTC.

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Figure 6.
 8-hr average maximum ozone guidance for Saturday, June 11 from the 12 UTC run of the NOAA model on June 10.

Recent Trends in Observed Ozone in the Mid-Atlantic and a Look Ahead to the 2016 Ozone Season

Beginning in 2013 and continuing into 2015 and 2015, there has been a “step-down” in peak observed ozone (O3) across the Mid-Atlantic region. As shown in Figure 1, the number of O3 exceedance days (observed 8-hour average O3 ≥ 76 ppbv) in the southeastern Pennsylvania forecast region (SEPA, which includes Philadelphia) dropped substantially in 2013-2015 compared to 2003-2012. The black horizontal lines in Figure 1 indicate the average number of days when observed 8-hour O3 was ≥ 76 ppbv during the periods 1997-2002 (39.3 days), 2003-2012 (19.0 days), and 2013-2015 (5.0 days). The initial “step-down” during the period 2003-2012 has been attributed to reductions in O3 precursors, primarily emissions of nitrogen oxides (NOx) from energy generating units (EGUs) in the eastern U.S associated with the so-called “NOx SIP Rule.” Another, temporary “step-down” in O3 precursor emissions and concentrations occurred during the Great Recession in 2009, when only 3 O3 exceedance days were observed in SEPA.

What is responsible for the recent “step-down” in observed O3? Meteorology likely plays a role. The Mid-Atlantic had a somewhat atypical weather pattern during the summers of 2013-2015, which limited the development of heat waves in June-August. Historically, most high O3 days in the Mid-Atlantic region were characterized by a “heat wave” synoptic weather pattern, featuring a “ridge” of high pressure aloft, slowly eastward migrating surface high pressure, westerly transport aloft from the Ohio River Valley, sunny skies, and maximum temperature (Tmax) ≥
90 °F. Fewer incidences of “heat wave” synoptic patterns in June-August for the past three summers have presumably impacted peak observed O3 levels. But a quick comparison of the total number of hot days (Tmax ≥ 90 °F), days with measureable precipitation, and O3 exceedances for the periods 1997-2002, 2003-2012, and 2013-2015 in SEPA suggests that weather is not the sole contributing factor to the recent “step-down” in observed O3. Figure 2 shows that the average number of days with Tmax ≥ 90 °F was essentially constant for the three periods (25-27 days), while the average number of days with measureable precipitation was roughly the same in 1997-2002 (45) and 2013-2015 (47) – slightly less than the average for 2003-2012 (52). In contrast, the average number of O3 exceedance days dropped from 39 in 1997-2002 to 19 in 2003-2012 (attributed to reductions in regional NOx associated with the NOx SIP Rule) to only 5 in 2013-2015. Thus, even though the overall synoptic weather pattern was somewhat atypical during June-August 2013-2015, there were a comparable number of hot and rain-free days compared to previous years. Based on these weather conditions alone, observed O3 during 2013-2015 would have been expected to be similar to previous years. But it was not – and research shows that upwind NOx emissions have been steadily declining in recent years. Thus, lower regional NOx emissions seem to be the primary driver of the recent “step-down” in observed O3.

So what can we expect from the 2016 O3 season? One thing seems certain – we will have more O3 exceedance days than previous years because U.S. EPA lowered the O3 National Ambient Air Quality Standard (NAAQS) from 75 to 70 ppbv. We estimate that this change will increase the number of exceedance days in SEPA by 2.5 to 3 times, relative to 2013-2015. So for example, in 2015, there were 18 days with observed 8-hour O3 ≥ 71 ppbv, compared to 8 days with observed 8-hour O3 ≥ 76 ppbv. If it’s an unusually hot and dry summer, we may see even more O3 exceedance days. So far, after a rash of early season exceedances across parts of western Pennsylvania and Virginia during late April, it’s been cool, rainy, and cloudy during the first three weeks of May. Climatologically, that should change as we close out May and move into the warmer days of June, when we expect O3 will be on the rise.

SEPA_O3_1997-2015
Figure 1.  Number of days when maximum observed 8-hour O3 exceeded thresholds of 115 ppbv (purple bars), 95 ppbv (red bars), and 75 ppbv (orange bars) in SEPA for 1997-2015.  The black lines indicate the average number of days with observed 8-hour O3 ≥ 76 ppbv for the period 1997-2002 (39.3 days), 2003-2012 (19.0 days), and 2013-2015 (5.0 days).

SEPA_Historical_Wx-O3_1997-2015
Figure 2.  Average number of days with Tmax ≥ 90 °F and measureable precipitation at KPHL, and observed 8-hour O3 ≥ 76 ppbv in SEPA for May 1 to September 30 during the periods 1997-2002 (blue bars), 2003-2012 (red bars), and 2013-2015 (green bars).

2016 Medium Range Discussions Will Begin May 23

Every year, we begin issuing our 5-day medium range air quality forecast discussions for the Mid-Atlantic region in the mid-May time period.   The medium range discussions are a great learning tool for our summer forecasting intern(s), so we usually wait to start issuing the discussions until the intern(s) begin working with us.  We also like to wait until the ozone season gets going, so we have something to discuss.  Our 2016 forecast intern, Matt Brown (check out his bio in the About Us section), began working with me today.  It’s looking like a fairly quiet week for air quality, so I’m going to get Matt oriented, and we’ll begin the forecast discussions next week.  The discussions will be similar to previous years, except we won’t be issuing them on the weekends this year, just on Mondays through Fridays.

A Weather Forecaster’s Perspective on Air Quality Forecasting

Since my first day as an undergraduate meteorology student at Penn State, I was exposed to weather forecasting through the Penn State Campus Weather Service club. There are two branches of the club: the communications branch and the forecasting branch. The communications branch provides daily radio broadcasts for clients and video forecasts that are uploaded online. The forecasting branch prepares five day forecasts for various regions across Pennsylvania, which are also uploaded online for the public to access. Included in the five day forecasts are high and low temperatures, precipitation, a brief description of the forecasted weather conditions, and a forecast discussion. I spent most of my time as a member of the forecasting branch. I learned the basics of weather forecasting from upperclassmen who had been a part of the club for a few years. By my junior year, I became a shift manager. As a shift manager, I supervised students to make sure all of the forecasting zones were covered. I also helped teach new members the basics of forecasting, as the upperclassmen did when I was a new member.

My weather forecasting experience also stems from my participation in WxChallenge, the national collegiate weather forecasting competition, as a member of the Penn State team during my junior year. The Penn State team has won the competition the past four years. The competition consists of making forecasts four days a week for ten cities across the United States. Each forecast includes four variables: high and low temperatures, maximum sustained wind speed, and the amount of precipitation to the nearest inch. In the 2014-2015 forecasting season, I placed in the top 50 forecasters in the competition out of a total of 1900 forecasters.

In the summer of 2015, I transitioned to working as an Air Quality Forecasting Intern in the Penn State Air Quality Forecasting Office, learning about operational ozone and PM2.5 forecasting. I had talked to a few of my professors about how to incorporate chemistry into my meteorology degree, and they guided me toward a focus on air quality. I took a few classes that concentrated on air quality and environmental policy, and I found them very interesting. I knew that focusing in air quality was the path that I wanted to take. I found the air forecasting internship through a classmate, fellow intern Lexie Herdt. This internship was the first experience I had dealing with air quality outside of a classroom setting. I found that air quality forecasting is similar to weather forecasting, but there are small differences that distinguish them.

One key difference I noticed was not having to decide on an exact value for temperature or the amount of expected precipitation. As a weather forecaster, most of my time was spent trying to decide on a single value to forecast for temperature or precipitation. For example, to make a perfect forecast in WxChallenge, every variable had to be narrowed down to a single value. If the temperature was off by a few degrees Fahrenheit or the amount of precipitation was off by a few hundredths of an inch, then error points were assigned to the forecast. The most accurate forecasts were given the fewest error points. So getting the forecast exactly right was essential to doing well in the competition. When transitioning to air quality forecasting, I found that narrowing down the temperature or precipitation forecast to a single value is not as essential as it is in weather forecasting. Knowing a range of expected temperatures (e.g., upper 80s °F to low 90s °F) or intensity of rainfall (e.g., light or heavy) is sufficient to make an accurate air quality forecast. For example, if a storm system moving through an area was expected to produce widespread rainfall for an entire day, then I would expect that the atmosphere would be cleaned out and that clouds would block ozone formation. Knowing the total amount of rainfall, whether it’s only 1 inch or 5 inches, is not essential. Dealing with a temperature forecast in a small area can be difficult as it can vary greatly in a very small distance. Determining a range of temperatures works well to make up for this issue when making an air quality forecast.

Having the public affect the air quality forecast is another difference I found. Typically, the public is impacted by the weather forecast. For example, if there is going to be a heat wave or a crippling snowstorm, then the public will have to adjust their plans accordingly, whether it involves travel or outdoor activities. With air quality forecasting, the public can impact the forecast through, for example, holiday travel and fireworks. Typically, the highest volume of traffic and vehicle emissions occur during the work week, Monday through Friday (and sometimes on Saturday, too). As a result, the highest ozone levels are seen during the work week, all things being equal. On average, ozone levels are lower on the weekends due to the lower vehicle emissions. However, on holiday weekends, such as Labor Day or Memorial Day, more people are likely to travel and vehicle emissions are higher than on a corresponding average weekend day. The higher concentrations of pollutant precursors from increased holiday travel emissions can lead to higher ozone levels than what is typically expected. We saw this in Philadelphia this past Independence Day holiday weekend, when ozone exceeded the NAAQS by 1 ppb at one monitor on Sunday, July 5; this exceedance was almost certainly attributable to the higher holiday emissions.

Fireworks celebrations can impact an air quality forecast by causing increased concentrations of particles. When a firework explodes, fine particles are expelled into the air from the smoke associated with the firework. During a typical fireworks show there are hundreds of explosions. This can cause a buildup of particles, especially if the wind is very calm and the smoke plume is stagnant. If the wind is light, it can blow the smoke plume away and cause a buildup of particle concentrations downwind. The impact of fireworks tends to be more localized rather than region wide, but they still have to be taken into account as part of the air quality forecast.

In addition to learning a new skill with air quality forecasting, I improved my weather forecasting skills considerably. Before my internship, I would only have to look at all of the weather observations and models a few days a week, especially on a synoptic scale. During my internship, I looked at observations and model guidance every day. My communications skills improved as well from writing the technical 5-day medium range air quality forecast discussion and the short 3-day air quality discussion for the public. Despite there being small differences, my improvement shows that weather forecasting and air quality forecasting have a strong connection.

PSAQFO Student Meteorologist Lexie Herdt in the News Again

Lexie Herdt, recent PSU Meteorology graduate and summer forecasting intern with the PSAQFO, was featured in today’s edition of the Philadelphia Inquirer newspaper. Lexie (below) worked with Dr. Amy Huff and Bill Ryan during summer 2014, conducting research to update an ozone statistical model for Philadelphia. Her work was funded through a fellowship with the National Science Foundation’s prestigious Research Experience for Undergraduates (REU) program. This summer, she has been verifying the performance of her model as guidance for operational ozone forecasts for the Philadelphia metropolitan area. Lexie is off to Texas Tech University in few weeks to begin a MS program. Congratulations Lexie!

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