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Atmospheric Environment 45 (2011) 3192e3202

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Atmospheric Environment

journal homepage: www.elsevier .com/locate/atmosenv

Lake Michigan air quality: The 1994e2003 LADCO Aircraft Project (LAP)

Theresa Foley a, Eric A. Betterton a,*, P.E. Robert Jacko b, John Hillery c

aDepartment of Atmospheric Sciences, University of Arizona, 1118 E 4th St, PO Box 210081, Tucson, AZ 85721-0081, USAb Purdue University, School of Civil Engineering, 550 Stadium Mall Drive, West Lafayette, IN 47907-2051, USAcWisconsin Department of Natural Resources, 2300 N. Dr. Martin Luther King Jr Drive, Milwaukee, WI 53212, USA

a r t i c l e i n f o

Article history:Received 11 March 2010Received in revised form11 February 2011Accepted 15 February 2011

Keywords:OzoneOzone trendsLake breezePhotochemical clockPhotochemical age

* Corresponding author. Tel.: þ1 520 621 6831; faxE-mail address: betterton@atmo.arizona.edu (E.A.

1352-2310/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.atmosenv.2011.02.033

a b s t r a c t

The goal of the 1994 to 2003 LADCO Airplane Project (LAP) was to study ozone formation over LakeMichigan so that equitable regional control strategies could be devised. This paper for the first timedocuments LAP in the peer-reviewed literature.

Dye et al. (1995) found that the atmosphere over Lake Michigan is stable in the summer due to theairwater temperature difference, which creates an efficient reaction chamber for ozone formation. Theyalso hypothesize that the southwest winds characteristic of ozone-conducive conditions transport ozonefurther north over the lake before it crosses the shoreline onto land. This statistical analysis of LAP datasupport the hypothesis of Dye et al.

Below 200 m above the lake, ozone formation is VOC-limited in the morning and becomes NOx limitedin the afternoon. Above 200 m, ozone formation is NOx-limited throughout the day. The onshore NOx andVOC diurnal cycles peak during the early morning rush hour and are clearly linked to traffic patterns.Over the lake, VOC and NOy concentrations peak during the mid-morning rather than the early morning,supporting the hypothesis that the land breeze transports VOC and NOy over the lake. The diurnal NOx

pattern over Lake Michigan is less clearly defined than the VOC pattern possibly as a result of emissionsfrom five coal-burning power plants located on the Lake Michigan shoreline.

Using a “photochemical clock” model, we estimate the climatological average hydroxyl radicalconcentration over the lake to be (9.43 � 5.88) � 106 molecule cm�3 near Chicago and (8.43 � 3.68) �106 molecule cm�3 near Milwaukee.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

There is great interest in better understanding the factorsaffecting air quality in the Great Lakes region because of the long,ongoing history of ozone exceedances in this densely populatedpart of the country. During the course of the study described here,the 1990 Clean Air Act Amendments created a 1 h ozone standard of120 ppbv; and then in 1997, the EPA created a new 8 h standard of80 ppbv (US EPA, 1997). In 2004 the 1 h standard was revoked (USEPA, 2004). The four states surrounding Lake Michigan (Illinois,Wisconsin, Indiana and Michigan) have not met either ozonestandard in spite of control measures. Between 1994 and 2003, theLake Michigan region exceeded the 1 h ozone standard an averageof three times a year and the 8 h standard an average of 17 timesa year (Adamski, 2007).

: þ1 520 621 6833.Betterton).

All rights reserved.

The steady-state ozone concentration is dependent upon theNO2/NO ratio because O3 formation is initiated by the photolysis ofNO2 to NO (Seinfeld, 1989), which generates an oxygen atom that inturn combines with an oxygen molecule to form ozone. Thehydroxyl radical, HO, indirectly increases the O3 concentration byreacting with hydrocarbons to produce the HO2 radical, which inturn converts NO to NO2, without consuming O3.

The HO concentration over Lake Michigan has not previouslybeen measured or estimated, partly because it is difficult to do(Platt et al., 2002), although HO measurements have been made inBerlin (Platt et al., 2002), Mexico City and Los Angeles (Dusanteret al., 2009).

To a first approximation, the amount of ozone formed dependsupon the relative concentrations of volatile organic compounds(VOC) and nitrogen oxides (NOx), which in turn dictate whichemission control strategies would be the most effective (Seinfeld,1989). In NOx-limited regions, NOx abatement is expected tolower ozone more effectively than VOC abatement. On the otherhand, if an excess of NOx is present; than the area is consideredVOC-limited and VOC abatement is expected to be more effective

T. Foley et al. / Atmospheric Environment 45 (2011) 3192e3202 3193

for reducing ozone (National Academy of Sciences, 1991). In tran-sitional regimes controlling both pollutants would be considerednecessary.

Synoptic meteorology is an important factor for ozone forma-tion in the Lake Michigan region. Elevated ozone levels often occurwhen a strong high pressure system is located east of the region(Hanna and Chang,1995; Lennartson and Schwartz,1999; Eshel andBernstein, 2006). Similar synoptic conditions are associated withelevated ozone levels in eastern cities such as Pittsburgh (Comrieand Yarnal, 1992).

High pressure systems conducive to ozone formation createclear skies and relatively weak pressure gradients. These conditionsare also conducive to the formation of lake and land breezes, whichare caused by differential heating of Lake Michigan and thesurrounding coastline (Moroz, 1967; Lyons and Cole, 1976; Keenand Lyons, 1978). The lake breeze (Fig. 1a) occurs when the landis warmer than the lake water and typically begins to penetrateinland at about 8e9 AM CST (Lyons and Olsson, 1973). The landbreeze (Fig. 1b) appears to develop at about 10e11 PM CST andremains until after the early morning rush hour traffic (Lyons et al.,1990).

In the early morning, the land breeze is believed to transportozone precursor compounds created by rush hour traffic over the

Typical Lake Breeze Characteristics

30 km 2010 0 10 3020 40 km

Lake Land

inflow layer300-1000 m 4-7 m s-1

updraft>1 m s-1

frontalzone1-2 km wide

2-4 m s-1return flow layer500-2000 m

subsidence inversion

Typical Land Breeze Characteristics

Synoptic Inversion Layer

30 km 2010 0 10 3020 40 km

Lake Land

Outflow layer50-400 m<4 m s-1

Frontal zone 0.5-1.5 km wide

Updraft40-70 cm s-1

subsidence inversion

Return flow layer100-1000 m< 1 m s-1

a

b

Fig. 1. Schematic diagrams of the Lake Michigan (a) lake and (b) land breeze (fromAdamski, 2003).

lake where stable air favors the formation of ozone. The stable airover the lake consists of two layers: a shallow conduction layer50e200 m thick and another stable layer above the conductionlayer. The higher layer is formed as inland air from the convectiveboundary layer is advected over the lake and cools (Dye et al.,1995). Afternoon lake breezes are believed to transport theozone back over land and cause high levels of ozone along theLake Michigan shoreline. Lennartson and Schwartz (2002) foundthat 82% of ozone exceedances in Wisconsin were correlated withthe lake breeze. Dye et al. also hypothesized that south to south-west winds characteristic of high ozone days transport the ozonenorth over the lake before being blown back over land by anafternoon lake breeze.

The 1991 Lake Michigan Ozone Study (Dye et al., 1995) made thefirst systematic measurements over Lake Michigan and analyzedfour ozone episodes. In other groundbreaking work, Sillman et al.(1993) modeled ozone formation in the shallow layer over LakeMichigan, including an O3/NOx/VOC sensitivity study. The LakeMichigan Air Directors Consortium (LADCO) spearheaded a secondLake Michigan study to analyze additional ozone episodes anddetermine if the model of Dye et al. is applicable to a more generalmodel of ozone formation over a large, relatively cold lake such asLake Michigan.

In spite of the enormous effort that went into the ten-yearLADCO Aircraft Project (LAP), it has never been documented in thepeer-reviewed literature. Thus a major purpose of this paper issimply to describe the LAP field campaign and to address a few ofthe key scientific questions. Other questions will be addressed insubsequent papers as we further analyze the vast LAP data archive.

�What are the concentrations and diurnal patterns of ozone andozone precursor compounds over the lake?� Does a statistical analysis of the LAP data support the model ofDye et al.?� Is ozone formation over Lake Michigan VOC-limited or NOx-limited?� What is the climatological average hydroxyl radical concen-tration over Lake Michigan?

2. LAP Experimental protocol

LAP was a collaborative effort involving the states of Wisconsin,Illinois, Michigan and Indiana as well as the United States Environ-mental Protection Agency (US EPA). Seven public agencies and oneconsulting firm participated in LAP (Table 1). Two airplanes wereused in the study. The Wisconsin Department of Natural Resources(WDNR) operated a Cessna Skymaster C-337 based in Oshkosh,Wisconsin. Jacko and Associates (JAS) operated a Cessna 210 TurboCenturion based inWest Lafayette, Indiana at the Purdue UniversityAirfield. The study period was every summer (June 1 to August 31)from1994 to 2003, except for the lastweek in July during the annualExperimental Aircraft Association (EAA) Convention.

The decision on whether to fly was made jointly by LADCO andthe four Lake Michigan states. The meteorologists based theirdecision on whether the synoptic weather conditions wereconducive to ozone formation, as discussed in the introduction.During the ten-year LAP campaign, a total of 328 flights were flownon 81 days over Lake Michigan and its southern and westernboundaries. The 8 h ozone standard of 80 ppbv was not promul-gated until after the LAP campaignwas over. However, the airplanesflew on 37 days that would have been 8 h exceedance days. LAPachieved an “event” capture of 27%, which helps to ensure that thedata are representative of conditions conducive to ozone formation.The airplanes also flew on 45 non-exceedance days.

Table 1Agencies and responsibilities for the LADCO Project.

Agency Responsibility

Lake Michigan Air Directors Consortium (LADCO) Overall project managerWisconsin Department of Natural Resources (WDNR) Ozone forecasting, operate Cessna airplane, calibrate WDNR and JAS analyzers, audit

VOC samplers from both airplanes, quality assurance of all WDNR and JAS VOC dataJacko and Associates, West Lafayette, Indiana (JAS) Operate Cessna airplane, quality assurance of ozone and meteorological data collectedMichigan Department of Natural Resources Ozone forecastingIndiana Department of Environmental Management Ozone forecasting, audit ozone instrument on JAS airplaneIllinois Environmental Protection Agency Ozone forecasting, audit ozone and nitrogen oxide instruments on both airplanesWisconsin State Laboratory of Hygiene Analyze all VOC samples collected by both airplanesUnited States Environmental Protection Agency (USEPA) Maintain reference ozone and nitrogen oxide analyzers

T. Foley et al. / Atmospheric Environment 45 (2011) 3192e32023194

2.1. Aircraft instrumentation

Air intake probes were installed halfway down the right wingand the sampling lines were approximately 2 m long. A 9.5 mm i.d.stainless steel tube was used for the collection of VOC samples. A9.5 mm i.d. Teflon line was used for the NOx or NOy and ozoneanalyzers. During flight, ram air brought the ambient air into theTeflon intake manifold and a bellows pump (MB-41 from MetalBellows Company) pulled the air from a T-fitting through Teflontubing to the various instruments inside the airplanes.

TheWDNR platformmeasured O3 using ultraviolet photometry;NOx or NOy using chemi-luminescence; GPS position; and meteo-rological parameters (temperature, barometric pressure and rela-tive humidity). The JAS platform measured O3; GPS position; andair temperature. In 2002, the Vaisala HMP-35 meteorologicalsensor on the WDNR airplane was replaced with an AIMS-10meteorological pod, which had the capacity to measure the u and vcomponents of wind direction aloft. See Table 2 for more infor-mation on the aircraft instrumentation.

TheWDNR aircraftmeasuredNOx in 1994 and 1995 and switchedto measuring NOy beginning in 1996. Here, NOx ¼ NO þ NO2, whileNOy ¼ all oxides of N � þII, including NOx. NOz ¼ NOy � NOx,including nitric acid (HNO3), nitrous acid (HONO), organic nitratessuch as peroxyl acetyl nitrate (PAN), and particulate nitrates(Cavender, 2005).

LAP quality control protocols for chain of custody, operatortraining and instrument calibrationwere very similar to those usedin the 1991 Lake Michigan Ozone Study (Rendahl, 1992). Theinstruments aboard each aircraft were calibrated according to USEPA standards for ambient air quality monitoring. The accuracy ofeach instrument was also checked before and after every flight day.If an instrument was operating outside US EPA Standards, the datawere flagged as invalid. Additional information on the LAP qualitycontrol program can be found in the Supplemental Material.

Typically the airplanes maintained a constant speed of about72 m s�1, resulting in an average flow rate through the sampling

Table 2Instruments aboard the two aircraft. The first two entries are for the JAS aircraft and the

Parameter Year Instrument Ze

Ozone JAS 1994e7/7/02 Teco 49 �1Ozone JAS 7/8/02e2003 2B Tech model 202 �1Temperature JAS 1994e2003 Cu-Constantan N/O3 1994e2003 Teco 49 �1O3 1996e2003 Teco 49C 0.5NOx 1994 Teco 42 25NOx 1995 API 200A 0.2NOy 1996e2003 Teco 42CY 0.2Temperature, relative humidity 1994e2001 Vaisala HMP-35 N/Pressure 1994e2001 Davis PTB 100A N/Light scattering coefficient 2002e2003 M903 Integrating Nephelometer N/Temperature, relative humidity,

wind speed2002e2003 Vaisala AIMS-10 N/

lines of 9 L min�1 and an estimated residence time in the samplinglines of about 2 s. All of the analyzers and instruments recordeda value every 10 s, corresponding to a spatial interval of about 20 m.

2.2. Flight routes

The planes flew four times on each flight day so that the landbreeze, lake breeze, earlymorning rush hour and evening rush hourcould all be captured. TheWDNR airplane flew south from Oshkoshalong the boundary of the Urban Airshed4 grid and turned eastsouth of the lake, landing in Fort Wayne, Indiana about 3 h later.After refueling, the airplane returned to Oshkosh over Lake Mich-igan, arriving in Oshkosh about two and a half hours later. Themorning flight left Oshkosh at about 3:30 AM local time (CDST) andagain at 11:00 AM local time. The JAS airplane flew in a triangularflight path south of Lake Michigan (Fig. 2).

In 1999, the flight paths were changed so that more data couldbe collected over the lake (Fig. 3). Each WDNR flight originatedfrom Oshkosh, departing at approximately 5:00, 9:00, 13:00 and17:00 local time (CDST). The JAS airplane flew directly from thePurdue University Airport in West Lafayette, IN to Bradford, IL, thenNNW to Lacrosse, Wisconsin and back. Since the JAS flights wereover land, and not over the lake, we chose to defer analysis of thosedata to another paper, in the interests of space.

2.3. VOC and carbonyl sampling

The WDNR designed and built the combination VOC/carbonylsamplers for both aircraft (Allen et al., 1994, 1996). VOC compoundswere collected in 1.8 L Sumapassivated canisters and analyzed bygaschromatography according to US EPA method TO-12. Carbonylcompounds were sampled by pumping air through silica gelcartridges coated with 2, 4-dintitrophenylhydrazine which wereanalyzedbyhigh performance liquid chromatography (Tejada,1986).During theLAP campaign719VOCsamples and737 carbonyl sampleswere collected over the lake and the surrounding area.

rest of the entries are for the WDNR aircraft.

ro Noise (ppbv) Lower Detection Limit Response Time (sec) Precision (ppbv)

2 ppbv 10 21.5ppbv 10 1.5

A N/A 5 N/A2 ppbv 20 21.0 ppbv 20 (modified to 4) 1.02.0 ppbv 10 �0.5

5 0.5 ppbv <60 0.5% reading0 0.40 ppbv 40 (modified to 10) �0.4A N/A 5 N/AA N/A N/A N/AA 1 � 10�6 m�1 20 N/AA N/A N/A N/A

Table 3GPS Coordinates of VOC and carbonyl airborne sampling locations.

Site Name Location Latitude Longitude Altitude(m AGL)

Years

Location1 SW cornerof grid

41.23e40.83 88.83e88.17 152 and610

1994e1998

Location 2a(carbonylonly)

S of lake 41.03(∼constant)

88.00e∼85.3 Nearsurfaceto 610

1995e1998

Location 5 Near Chicago 40.83e42.00 87.70e87.13 30 and300

1994e2003

Location 6 NearMilwaukee

42.88e42.45 87.65e87.07 30 1994e2003

Location 7 N ofMilwaukee

43.40e42.98 87.83e87.23 304 1994e2000

Fig. 2. The WDNR and JAS 1994 to 1998 routes, plotted with the following flight data.WDNR: 7/14/1995 at 2:37e9:00 CST and JAS, 8/18/1994 at 10:55e15:04 CST.

T. Foley et al. / Atmospheric Environment 45 (2011) 3192e3202 3195

VOC and carbonyl samples were collected at a height of 30 and300 m over the lake near Chicago. Thirty meter samples were alsocollected over the lake near Milwaukee and 61 km north of Mil-waukee near Belgium, Wisconsin (Table 3).

2.4. Onshore monitoring sites

LAP had four onshore sites which were part of the State andLocal Air Monitoring Network (SLAMS) and operated according to

Fig. 3. The WNDR and JAS 1999 to 2003 WDNR and JAS routes, plotted with thefollowing data. WDNR: 6/9/1999, 7:57e10:45 CST, and JAS: 6/25/02, 11:05e15:45 CST.

US EPA standards. All four monitoring sites werewithin three km ofthe Lake Michigan shoreline. NOx or NOy analyzers were located atGary, the Jardine Water Treatment Plant near downtown Chicagoand the University of Wisconsin-Milwaukee. Ozone analyzers werelocated at Gary, Jardine, the University of Wisconsin-Milwaukeeand the WDNR Southeast Region Headquarters in Milwaukee.

VOC data were collected by PerkineElmer automated gaschromatographs located at the WDNR Southeast Region Head-quarters, Jardine and Gary. Illinois, Indiana andWisconsin followeda similar quality control program so that the gas chromatographdata would be comparable. When canister data were compared todata from the gas chromatographs, the results were similar exceptfor acetylene, which the automated gas chromatograph consis-tently measured as 50% lower than the canisters. The reason for thedifference remains unclear. Other details of the gas chromatographquality control program can be found in the Supplemental Material.

3. Methods used to analyze LAP data

A statistical analysis was conducted to evaluate two keycomponents of the Dye et al. model; the existence of stable layers ofair over the lake and the northward advection of ozone over thelake. To accomplish this goal, the O3, NOy and NO data that corre-sponded to the 15 min VOC and carbonyl sampling periods wereaveraged. NOy was measured beginning in 1996, but NO was notmeasured until 1999 so extent of reaction could not be calculatedfor the 1994 to 1998 data.

To eliminate bias from meteorology, the data were matched byflight date and time. The matched data from Location 5 (Chicago),Location 6 (Milwaukee) and Location 7 (Belgium,Wisconsin)were allcollected within an hour of each other. The Location 5 samples at30mAGL and 300mAGLwere collectedwithin 20min of each other.

3.1. Observation-based methods

This paper also analyzes data from the WDNR aircraft using thefollowing observation-based methods which have the advantage ofusing actual measurements rather than modeled data and are alsouseful for evaluating the performance of photochemical models(Sillman,1999). A constraint to observation-based algorithms is thatthey cannot be used to predict the result of changes in futureemissions (Hidy, 2000). Another constraint is that observation-basedmethods are sensitive to theVOCcomposition (Sillman,1999).

Indicator species,whichmark the transition fromVOC-limited toNOx-limited, are calculated using three dimensional Eulerianmodels (Sillman, 1999 and Sillman and He, 2002). The O3/NOy ratiowas used as an indicator species in this study so that the observa-tions can be compared to themodeling results of Sillman. SomeNOyspecies such as HNO3 are water soluble and deposition remains anarea of uncertainty for over-water measurements (Sillman, 1999).

T. Foley et al. / Atmospheric Environment 45 (2011) 3192e32023196

Over Lake Michigan, Sillman (1999) found that an O3/NOy ratioof less than 8 indicated VOC-limited conditions. Sillman con-structed the indicator valuesmodelingmidday and afternoon hoursand there is some uncertainty when applying the results tomorning data (Sillman, 2007). Ratios of 6e8 will be consideredtransitional in this study to take into account variations in the VOCconcentrations which can change the O3/NOy ratio.

The extent of reaction or Smog Production algorithm ofBlanchard et al. (1999) is defined as how far an area has proceededtowards its maximum possible ozone production. It is an empiricalmethod based on smog chamber data and requires ambientmeasurements of O3, NO and either NOx or NOy. An extent ofreaction value of 0.0e0.6 is considered VOC-limited, 0.6 to 0.8transitional or NOx responsive, and greater than 0.8 NOx-limited.

Using the Smog Production algorithm, Blanchard et al. (1999)analyzed data from June 26, 1991 data collected during the LakeMichigan Ozone Study. They found that metropolitan Chicago wasVOC-limited, which is typical of urban areas. North of Chicago onboth sides of the lake and over the lake, conditions were NOx-limited.

NOy, ozone and NO were all measured directly in this study,which permitted the extent of reaction at time t, E(t), to be calcu-lated directly from Eqs. (9) and (14) of Blanchard et al. (1999).

EðtÞ ¼ O3ðtÞ þ DO3 � O3ð0Þ þ NOðiÞ � NOðtÞb½NOXðiÞ�a

(1)

NOXðiÞ ¼ NOyðtÞ þ DNOyðtÞ (2)

NOðiÞ ¼ F � NOyðtÞ (3)

O3(t), NO(t) and NOy(t) are the measured concentrations (ppbv);DO3(t) ¼ ozone deposition (ppbv); DNOy(t) ¼ NOy deposition(ppbv). The Blanchard et al. default values of for O3(0) ¼ 40 ppbv;a ¼ 0.67, b ¼ 19, and F ¼ 0.95; were used in this study.

For measurements in the boundary layer, DO3(t) and DNOy(t)were calculated using Eq. (11) of Blanchard et al. (1999).

DO3ðtÞ ¼36PtJ

vvO3ðJÞ

zðtÞ

DNOyðtÞ ¼36PtJ

vvNOyðJÞ

zðtÞ

(4)

Measurements made above the boundary layer were not cor-rected for deposition.

The deposition velocity (vd) over Lake Michigan is a function ofthe static stability, which in turn is a function of the air and watertemperature difference. It varies seasonally, with the highest valuesin the winter and lowest values in the summer when the air/watertemperature is the greatest. Instead of calculating the depositionvelocity using Eq. (12) of Blanchard et al., a deposition velocity of0.25 cm s�1 was used for both O3 and NOy in this study (Hicks,1977). The maximum height of the boundary layer over the lake z(t) on days conducive to ozone formation is about 200 m (Dye et al.,1995). To estimate O3(J) and NOy(J), the 1999e2003 over-waterdata are used to estimate average ozone and NOy concentrations foreach flight time.

3.2. Photochemical age and hydroxyl radical concentration

In this study, the photochemical age technique (McKenna, 1997;McKenna et al., 1995) is used to estimate the hydroxyl radicalconcentration. This technique depends upon the fact that reactive

VOC are consumed more rapidly than less reactive VOC duringphotochemical processing and that as an air mass “ages”, the ratiosof various VOC will change. The concept of photochemical age hasalso been described as a photochemical clock that is reset whenfresh emissions are added to an aged air mass (Kleinman et al.,2003).

The photochemical age method assumes that the generation ofozone occurs only through hydroxyl radical reactions, the hydroxylradical concentration is constant, diluent air contains negligibleconcentrations of hydrocarbons and all the hydrocarbons in the airmass have similar spatial and temporal emission patterns. Theconcentration of a species ci at time t can be written as follows:

ciðtÞ ¼ ci0FdðtÞexp� Zt

0

Ldt

!(5)

where ci0 is the initial concentration of species i, Fd(t) is the dilutionfactor and

R t0 Ldt is the loss rate of species i. Because the loss is

assumed to be due solely to the hydroxyl radical, L is equal to k[OH�] where k is the reaction rate constant. For constant [OH�], thisterm is constant and can be placed outside the integral. Eq. (5) canbe expressed as the ratio of two VOC species to cancel the unknowndilution factor Fd(t), which is assumed to be equal for all species.Eq. (5) becomes:

lnðc1=c2Þ ¼ �0@ðk1 � k2Þ½OH��

Zt0

dt

1Aþ lnðci1=ci2Þ (6)

where ci1 and ci2 are the initial concentrations of species 1 and 2.Evaluating the integral and rearranging results in Eq. (7).

y ¼ m12t þ lnðci1=ci2Þ (7)

where y ¼ ln(c1/c2) and m12 equals�(k1�k2) [OH�]. Thus a plot of yversus t is linear with a slope of (k1�k2) [OH�]avg, from which theaverage hydroxyl radical concentration can be calculated becausek1 and k2 are known.

In their Lagrangian study, McKenna, (1997) sampled a single airparcel at different points along its trajectory. At t ¼ 0, the air parcelcontained fresh VOCs emitted by the source then as the parceltraveled away from the source, photochemical aging changed theVOC composition. Conceptually, in our study a polluted air parceloriginating over the combined urban air sheds of Chicago, Mil-waukee and Gary at 7 AM local time (Fig. 4; adapted fromAdamski, 2004) is advected over the lake at approximately2 m sec�1 (Adamski, 2003) by the land breeze. At 10 AM local timethe air parcel reaches the airplane sampling line parallel to theshoreline, about 80 km offshore (Figs. 2 and 3), where it stagnateswhile the land breeze weakens and the lake breeze develops. Weassume that we sampled a single air parcel during each of thethree daily flights (10 AM, 2 PM and 6 PM local time). Althoughthis is not strictly true, it should be remembered that we areanalyzing a chemical climatology developed over a period ofdecade so time intervals will necessarily be imprecise. It shouldalso be noted that we are studying high pollution days whichcoincide with periods of weak dynamics (slow transport) undera high pressure system. This conceptual model is not valid on daysin which synoptic conditions inhibited the formation of lake andland breezes. However the model is a reasonable approximationbecause flights over Lake Michigan were by choice conducted ondays that were conducive to the formation of lake breezes as wellas ozone. We also note that an inherent ambiguity exists inseparating the effects of changing [HO�] from changing processingtime (Eq. (7)).

Table 5Statistical analysis comparing Chicago (Location 5) and Milwaukee (Location 6) at30 m AGL. The number of flights is n.

Average Concentration

Parameter n Chicago Milwaukee Statistical Test Significant?

Ozone (ppbv) 101 60.6 63.6 Paired Wilcoxon NoNOy (ppbv) 66 15.567 9.665 Paired Wilcoxon YesTotal Non-methane

HC (ppbC)101 75 65 Paired Wilcoxon No

VOC/NOy Ratio 55 10.2 20.5 Paired Wilcoxon NoExtent of Reaction 49 0.49 0.54 Paired Wilcoxon No

Fig. 4. Median hourly total non-methane hydrocarbon concentrations at (a) Chicagoand (b) Milwaukee.

T. Foley et al. / Atmospheric Environment 45 (2011) 3192e3202 3197

4. Results and discussion

4.1. Statistical analysis

All of the VOC, NOy and O3 distributions were evaluatedfor normality using the Lilliefors test at a probability of 0.05. Thematched ozone datasets for Locations 5, 6 and 7 were normallydistributed but the other distributions were not. Because all thedata were matched by flight date and flight time, ambientconcentration was the only variable and a one factor analysis ofvariance (ANOVA) could be performed on the Locations 5, 6 and 7

Table 4Statistical analysis comparing Chicago (Location 5), Milwaukee (Location 6) andBelgium, Wisconsin (Location 7) sampling locations at 30 m AGL. The number offlights is n.

Average Concentration

Parameter n Chicago Milwaukee Belgium,WI

StatisticalTest

Significant?

Ozone (ppbv) 44 49.4 54.9 58.0 ANOVA YesNOy (ppbv) 30 17.11 11.12 9.60 Kruskale

WallisNo

Totalnon-methaneHC (ppbC)

44 114 97 92 KruskaleWallis

No

VOC/NOy Ratio(dimensionless)

27 8.3 9.3 10.8 KruskaleWallis

No

ozone data. The non-parametric KruskaleWallis ANOVA was per-formed on the NOy and VOC data. The non-parametric Wilcoxonsigned rank sum test was used to evaluate the paired data.

When data from all three sampling locations are compared,ozone concentrations increase as the plane travels northwardover the lake and the trend is statistically significant (Table 4),which supports the Dye et al. hypothesis. NOy and VOC decreasefurther northward, but the trends are not statistically significant.When only Locations 5 and 6 are compared, the trend ofdecreasing NOy concentration further north is statisticallysignificant (Table 5). Ozone and the extent of reaction increasefrom Chicago to Milwaukee, but the trends are not statisticallysignificant.

The atmosphere changes from VOC-limited at 30 m AGL to NOx-limited at 300 m AGL. The ambient ozone concentration is higher at300 m than at 30 m while the ambient concentrations of VOC andNOy are lower (Table 6). These differences are all statisticallysignificant which support the Dye et al. hypothesis of stratified airlayers over the lake.

4.2. Diurnal patterns over Lake Michigan

Maps to illustrate the daily diurnal pattern were constructed.Fig. 5 illustrates the increase of ozone over Lake Michigan and thesurrounding land on June 13, 2001, which was a 1 h ozoneexceedance day. The pattern of ozone buildup observed on this dayis typical of an ozone exceedance day. The over-land portion of theroute has an approximate elevation of 300e400 m, which at 6 AMlocal time would likely be above the boundary layer. High-concentrations of ozone are observed during the over-land portionof the 6 AM flight. Over the lake, the plane flew at altitudes of30e300 m. Ozone levels began to build over southern portion ofthe route by about 10 AM, with many values exceeding the 1 hstandard of 120 ppbv. By 2 PM, high ozone levels have movedfurther north which is in agreement with the Dye et al. model. Onthis day, the extent of reaction peaks at midday and becomes lowerin the evening (Fig. 6) while the NOy concentration remained fairlyconstant at 2 ppbv (supplementary material). Earlier researcherspredicted this as an explanation for high ozone, as pointed out byan anonymous reviewer.

Table 6Statistical analysis comparing Chicago (Location 5) at 30 and 300 m AGL.The number of flights is n.

Average Concentration

Parameter n 30 m AGL 300 m AGL Statistical Test Significant?

Ozone (ppbv) 107 62.5 72.4 Paired Wilcoxon YesNOy (ppbv) 73 15.26 6.34 Paired Wilcoxon YesTotal Non-methane

HC (ppbC)107 73 47 Paired Wilcoxon Yes

VOC/NOy Ratio 63 8.7 15.3 Paired Wilcoxon NoExtent of Reaction 51 0.50 0.85 Paired Wilcoxon Yes

Fig. 5. Ozone concentrations on June 13, 2001, a 1 h ozone exceedance day. a) 4:15e6:51 CST b) 8:00e10:45 CST c) 12:00e14:39 CST d) 16:00e18:45 CST.

T. Foley et al. / Atmospheric Environment 45 (2011) 3192e32023198

The VOC diurnal pattern at Location 5 (Chicago) offers strongevidence of the land breeze influence. At an elevation of 30 m, VOCconcentrationspeakduring the10:00flight (Fig. 7) rather thanduringthe earlymorning rushhour (Fig. 4). The delay inpeak concentrationsis probably due to the 2e4 h it takes the land breeze to transport VOCover the lake to the sampling locations 8 km from the shoreline.

Motor vehicle emissions are the largest source of VOC onshore(LADCO, 1995). Gasoline exhaust typically contains the BETXcompounds (benzene, ethylbenzene, toluene and xylenes), acety-lene and the olefins propylene and ethylene. In a downtown Chi-cago tunnel study, Doskey et al. (1992) found these compoundswhich are typical of the gasoline signature.

The VOC speciation over the lake also shows a clear gasolinesignature (Fig. 8), which is strong evidence that VOC is transportedover the lake by the land breeze. Commercial diesel boats andgasoline-powered recreational boats make only a small contribu-tion to the regional VOC emissions inventory (Koerber, 2007 usingdata from Lindhjem, 2004).

Box plots of the 1993 to 2003 ozone and NOy data were con-structed using the results of our statistical analysis as a guideline. Thestatistical analysis demonstrated that ambient ozone and NOyconcentrations are strongly influencedbyelevation. BecauseDyeet al.

identify 200 m AGL as the maximum height of the conduction layer,the data were sorted according to elevation above the lake. Thestatistical analysis showed a trend of decreasing VOC and NOyconcentrationsand increasingozone concentrationsnorth of Chicago,but except for ozone, these trends were not statistically significant.Therefore as a first approximation, the horizontal location is omittedin the box plots so that the diurnal patterns can be better visualized.

The onshore diurnal pattern for NOx is very similar to the VOCpattern of Fig. 4 and is clearly tied to rush hour traffic (not shown).The land breeze signal can also be found in the NOy diurnal pattern(Fig. 9). Below 200 m AGL, NOy increases mid-morning anddecreases in the afternoon as NOy is oxidized into water-solublespecies such as HNO3 which deposit onto the water surface. Theland breeze signal is less distinct in the NOY diurnal pattern,possibly due to emissions from five coal-burning power plantssouth of Milwaukee along the Lake Michigan shoreline. PleasantPrairie (1233 MW) and South Oak Creek (1192 MW) are locatedsouth of Milwaukee in Wisconsin. Waukegon (689 MW) is locatedin Illinois north of Chicago while Fisk (326 MW) and Crawford(542MW) are located south of Chicago. The flight crew occasionallyobserved brown power plant plumes, which tend to have highlevels of nitrogen oxides (Ryerson et al., 2001). Lyons and Pease

Fig. 6. Extent of reaction on June 13, 2001, a 1 h ozone exceedance day. a) 4:15e6:51CST b) 8:00e10:45 CST c) 12:00e14:39 CST d) 16:00e18:45 CST.

Fig. 8. Top fifteen VOC at Location 5 (Chicago) using 1995e2002 data. The box lengthis the 25th to 75th percentile (inter-quartile range). The line inside of each box is thesample median. The whisker length is 1.5 times the inter-quartile range and valuesoutside this range are represented by asterisks. (a) 30 m AGL and (b) 300 m AGL.

T. Foley et al. / Atmospheric Environment 45 (2011) 3192e3202 3199

(1973) found that power plant plumes transported over the lake onozone-conducive days can travel for long distances over the lakewith little dilution because of the stable atmosphere.

Ozone increases in the afternoon, as expected (Fig. 10), and themedian afternoon ozone concentrations of the low and highelevation data are similar. But at higher altitudes the medianmorning concentrations are about 20 ppbv higher. This implies thatvertical mixing is inhibited over the lake in the morning and thereis more vertical mixing in the afternoon. It is possible that at lowerelevations the direct plume from Chicago was sampled more oftenwhile background air was sampled at higher altitudes.

4.3. VOC-limitedandNOx-limitedozone formationover LakeMichigan

Graphs of the ozone versus the O3/NOy ratio at various concen-tration ranges of ozone were constructed (not shown). Except forambientozone concentrations of less than40ppbv, no correlationwasfound between ambient ozone concentrations and the O3/NOy ratio.

Fig. 7. VOC diurnal pattern at Location 5 (Chicago), 30 and 300 m AGL using 1999 to 2002 daeach box is the sample median. The whisker length is 1.5 times the inter-quartile range an

Fig. 11 shows the diurnal pattern of the extent of reaction, whichwas calculated using the Smog Production Algorithm (Blanchardet al., 1999). An extent of reaction of 0.0e0.6 is VOC-limited,0.6e0.8 is transitional and greater than 0.8 is NOx-limited. At 30 mAGL over the lake, the median extent of reaction is VOC-limited inthe morning and NOx-limited in the afternoon and evening. Above200 m AGL, the median extent of reaction is NOx-limited in theearly morning, transitional at about 10:00 AM CDST and NOx-limited in the afternoon and evening. These results are consistentwith the statistical analysis, which suggests that the air mass athigher altitudes over the lake is more photochemically aged.

4.4. Photochemical clock

Ethane was selected as the “reference” VOC because of its lowreactivity compared to other high-concentration compounds. A poorcorrelation of any VOC with ethane is an indication of a dissimilarspatial and temporal pattern. Consequently, 2,2,4-trimethylpentane

ta. The box length is the 25th to 75th percentile (inter-quartile range). The line inside ofd values outside this range are represented by circles.

Fig. 9. Diurnal pattern for NOy using 1999 to 2003 data. The box length is the 25th to 75th percentile (inter-quartile range). The line inside of each box is the sample median.The whisker length is 1.5 times the inter-quartile range and values outside this range are represented by asterisks.

T. Foley et al. / Atmospheric Environment 45 (2011) 3192e32023200

(Location 5 and Location 6) and 2-methylpentane (Location 6, only)were not included in this analysis.

The ratios of high-concentration compounds to ethanewere plotted against the sampling times and the hydroxyl radicalconcentrations were calculated using Eq. (7) (Fig. 12). The

Fig. 10. Diurnal pattern for ozone using 1999 to 2003 data. The box length is the 25th to 7The whisker length is 1.5 times the inter-quartile range and values outside this range are r

Fig. 11. Diurnal pattern for the extent of reaction using 1999 to 2003 data. The box lengthsample median. The whisker length is 1.5 times the inter-quartile range and values outside

estimated Location 5 hydroxyl radical concentration is(9.4 � 5.9) � 106 molecule cm�3 and the estimated Location 6hydroxyl radical concentration is (8.3 � 3.7) � 106 molecule cm�3.The results are summarized in Table 7 and Table 8, which areanalogous to Table 12 of McKenna, 1997. The hydroxyl radical

5th percentile (inter-quartile range). The line inside of each box is the sample median.epresented by asterisks.

is the 25th to 75th percentile (inter-quartile range). The line inside of each box is thethis range are represented by asterisks.

Fig. 12. Ratio of toluene to ethane at Location 5, 30 m altitude, using data collectedfrom 1999 to 2002.

T. Foley et al. / Atmospheric Environment 45 (2011) 3192e3202 3201

concentrations estimates of this study are consistent with the fieldmeasurements obtained in other studies, which are in the range of(0.5e15) � 106 molecule cm�3 (Dusanter et al., 2009).

5. Summary

The VOC diurnal pattern and VOC speciation data support thehypothesis that the land breeze transports ozone precursorcompounds over the lake. Offshore, the peak VOC concentrationoccurs at about 10:00 AM CDST as compared to 7:00 AM onshore.The time lag is probably due to the time required for the landbreeze to transport the VOC to the sampling location. The NOy

Table 7Data from photochemical clock analysis of Location 5 (30 m) using Eq. (3) and 1999 to 2

Hydrocarbon Rethane Intercept Rage n

Ethaneb

Propanea 0.906d 0.488 � 0.075 �0.474d 67Butanea 0.779d 0.023 � 0.141 �0.349d 64n-Pentanea 0.829d �0.317 � 0.141 �0.482d 56Isopentaneb 0.592d 0.974 � 0.241 �0.400d 662-Methylpentaneb 0.787d �0.746 � 0.190 �0.284e 48Ethylenec 0.769d �0.110 � 0.167 �0.523d 57Acetylenec 0.876d �0.315 � 0.091 �0.442d 67Benzenec 0.711d �0.507 � 0.109 �0.470d 65Toluenec 0.417d �1.232 � 0.274 �0.396d 67

a DeMore and Bayes, 1999. Adjusted for average temperature of 296 K.b Atkinson, 1997. Ethane adjusted for average temperature of 296 K. Only 298 K datac Kleinman et al., 2003. 298�K data for all compounds.d Statistically significant at p ¼ 0.01.e Statistically significant at p ¼ 0.05.

Table 8Data from photochemical clock analysis of Location 6 (30 m) using Equation (3) and 199

Hydrocarbon Rethane Intercept Rage n

Ethaneb

Propanea 0.901d 0.402 � 0.073 �0.435d 63Butanea 0.459d 0.039 � 0.161 �0.410d 60n-Pentanea 0.505d �0.427 � 0.182 �0.356d 50Isopentaneb 0.343d 0.846 � 0.261 �0.371d 61Ethylenec 0.715d �0.296 � 0.176 �0.482d 52Acetylenec 0.837d �0.509 � 0.098 �0.251e 63Benzenec 0.821d �0.656 � 0.109 �0.438d 60Toluenec 0.277d 1.120 � 0.284 �0.372d 63

a DeMore and Bayes, 1999. Adjusted for average temperature of 296 K.b Atkinson, 1997. Ethane adjusted for average temperature of 296 K. Only 298 K datac Kleinman et al., 2003. 298� K data for all compounds.d Statistically significant at p ¼ 0.01.e Statistically significant at p ¼ 0.05.

diurnal pattern is less clearly defined, possibly because of NOx- richplumes from five large power plants on the Lake Michigan shore-line near the flight path.

The statistical analysis of this paper supports the hypothesis ofDye et al.We found that ambient ozone concentrations increased asthe plane traveled northward over the lake. The over-water ambientconcentrations at 30 and 300 m AGL are significantly different,which implies a stable atmosphere in which vertical mixing issuppressed. Over land in the afternoon, the height of thewell-mixedboundary layer increases to about 1500 m (Blanchard et al., 1999).

Over the lake at low altitudes, the air mass is VOC-limited in themorning and becomes NOx-limited in the afternoon and evening.We also find that the air mass over the lake becomes more NOx-limited with altitude because the extent of reaction increases,implying that air parcels at higher elevations are more photo-chemically aged. Therefore, controlling both VOC and NOx emis-sions in the Lake Michigan region will probably be necessary toreduce ambient ozone concentrations.

Amajor goal of this paper is to document the ten-year LAP studyin the peer-reviewed literature and to begin to evaluate thearchived data. Further work will be done on the vertical and hori-zontal variability of the data; the dynamics of the lake and landbreezes using the aloft wind vector data and mesoscale meteoro-logical models. The estimates of the hydroxyl radical concentrationand the aloft measurements obtained from LAP will be useful forevaluating the accuracy of photochemical modeling over the lake.Deposition of ozone and NOy over the lake remains uncertain andneeds to be evaluated together with the particulate matter samples

002 data.

kOH cm3 molec�1 s�1 Slope (sec�1) [OH] molec cm�3

0.25 � 10�12

1.10 � 10�12 �(1.16 � 0.27) � 10�5 (1.37 � 0.32) � 107

2.34 � 10�12 �(1.49 � 0.51) � 10�5 (7.14 � 2.43) � 106

3.65 � 10�12 �(2.18 � 0.54) � 10�5 (6.41 � 1.60) � 106

3.70 � 10�12 �(3.01 � 0.86) � 10�5 (8.72 � 2.18) � 106

5.30 � 10�12 �(1.55 � 0.77) � 10�5 (3.06 � 1.53) � 106

8.44 � 10�12 �(2.91 � 0.64) � 10�5 (3.55 � 0.78) � 106

0.91 � 10�12 �(1.29 � 0.32) � 10�5 (1.94 � 0.49) � 107

1.23 � 10�12 �(1.66 � 0.39) � 10�5 (1.70 � 0.39) � 107

5.96 � 10�12 �(3.38 � 0.97) � 10�5 (5.93 � 1.72) � 106

Average 9.43 � 106

Standard Deviation 5.88 � 106

available for isopentane and 2-methylpentane.

9 to 2002 data.

kOH cm3 molec�1 s�1 Slope (sec�1) [OH] molec cm�3

0.25 � 10�12

1.10 � 10�12 �(9.40 � 2.49) � 10�6 (1.11 � 0.29) � 107

2.33 � 10�12 �(1.89 � 0.55) � 10�5 (9.09 � 2.64) � 106

3.63 � 10�12 �(1.81 � 0.68) � 10�5 (5.34 � 2.03) � 106

3.70 � 10�12 �(2.78 � 0.91) � 10�5 (8.04 � 2.66) � 106

8.44 � 10�12 �(2.54 � 0.65) � 10�5 (3.10 � 0.81) � 106

9.12 � 10�13 �(6.78 � 3.35) � 10�6 (1.02 � 0.50) � 107

1.23 � 10�12 �(1.42 � 0.38) � 10�5 (1.45 � 0.39) � 107

5.96 � 10�12 �(3.05 � 0.98) � 10�5 (5.34 � 1.71) � 106

Average 8.34 � 106

Standard Deviation 3.68 � 106

available for isopentane and 2-methylpentane.

T. Foley et al. / Atmospheric Environment 45 (2011) 3192e32023202

collected over the lake. The JAS data collected south of LakeMichigan also need to be analyzed.

Acknowledgements

This paper is dedicated to the pilots whose skill made thisproject possible, especially John Sherman who died in 1998 whileperforming other natural resource work. The authors also wish toremember Heath Van Handel, whose plane crashed in 2009 whilefighting a Wisconsin wildfire. The majority of the northern routeswere flown by WDNR pilots Larry Waskow, Bob Clark and CyrilGriesbach. The JAS aircraft was piloted by Dr. Robert Jacko, P.E.,Professor of Environmental Engineering at Purdue University anda coauthor of this paper.

LAP was funded by LADCO and the states of Illinois, Michigan,Wisconsin and Indiana. The authors would like to thank MikeKoerber of LADCO for his unwavering leadership and support duringthe ten-year project. Dr. Andrew Comrie, Dr. William Conant andtwo anonymous reviewers provided many valuable suggestions tothe authors of this paper. This paper could not have been writtenwithout the help of the following people from the WisconsinDepartment of Natural Resources: William Adamski, Mary Mertes,Edward Miller, Mark Allen and Larry Bruss. Finally the authorswould like to express their gratitude to the dedicated meteorolo-gists, auditors and operators who made this project possible.

Appendix. Supplementary material

Supplementary material associated with this paper can befound, in the online version, at doi:10.1016/j.atmosenv.2011.02.033.

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