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Atmospheric Environment 46 (2012) 354e364

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Atmospheric Environment

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Ozone variability in the atmospheric boundary layer in Marylandand its implications for vertical transport model

Xiao-Ming Hu a,*, David C. Doughty a, Kevin J. Sanchez a, Everette Joseph b, Jose D. Fuentes a,**

aDepartment of Meteorology, The Pennsylvania State University, University Park, PA 16802, USAbDepartment of Physics and Astronomy, Howard University, Washington, DC 20059, USA

a r t i c l e i n f o

Article history:Received 14 April 2011Received in revised form24 September 2011Accepted 26 September 2011

Keywords:OzonesondeBeltsvilleWRF/ChemResidual layerNocturnal boundary layer

* Corresponding author. Current address: Center foStorms, University of Oklahoma, Norman, OK 73072,** Corresponding author.

E-mail addresses: xhu@ou.edu (X.-M. Hu), jdfuent

1352-2310/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.atmosenv.2011.09.054

a b s t r a c t

Although much research has focused on daytime ozone (O3) distribution in the atmospheric boundarylayer, there remain many unresolved processes related to O3 transport in the residual layer. To addresssuch unresolved questions, a field study was conducted in Beltsville, MD during the summer of 2010 tostudy the spatial and temporal distribution of O3 and other pollutants using ground-based gas analyzersand ozonesondes. During elevated pollution events in the daytime, the convective boundary layer, whichreached a maximum depth of about 2 km, had nearly uniform O3 levels of almost 100 parts per billion(ppbv). Due to intermittent and intense vertical turbulent motion, the residual layer became “leaky” andpermitted vertical transport to enhance ground-level O3 mixing ratios by as much as 10e30 ppbv ina span of 0.5e3 h. Model simulations, using the Weather Research and Forecasting model with Chemistry(WRF/Chem), were carried out to investigate the impact of different treatments of vertical mixing on thesimulation of O3 in the nocturnal boundary layer and residual layer. WRF/Chem model simulationsprovided realistic O3 vertical distribution during the daytime. During the nighttime, in the residual layer,model outputs resulted in higher O3 levels compared with the in-situ observations. Model sensitivityanalyses showed that increasing the turbulent length scales and improved stability functions yieldedimprovements in the vertical transport of O3 within the residual layer. One key conclusion of this study isthat models such as WRF/Chem require improved numerical algorithms to properly account for thenocturnal vertical transport of O3 in the residual region of the atmospheric boundary layer.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Uncertainties exist in the manner that numerical modelsdetermine vertical transport of pollutants within the atmosphericboundary layer (Neu, 1995; Seigneur, 2001; Yerramilli et al., 2010).Such uncertainties remain one of the sources of inaccuracies inmodel simulations (Pleim, 2007a,b; Hu et al., 2010). While muchprogress has been made in simulating pollutant vertical transportin the convective boundary layer (CBL), additional attention is stillneeded to improve transport processes in the nighttime boundarylayer (NBL) and the residual layer (RL) (Salmond and McKendry,2005; Beare et al., 2006; Brown et al., 2008; Hong, 2010;Fernando and Weil, 2010).

r Analysis and Prediction ofUSA.

es@psu.edu (J.D. Fuentes).

All rights reserved.

Previous studies, related to the nighttime variability of ozone (O3),have focusedon thenocturnal boundary layer (the air layer extendingfrom the surface up to 200e400 m above the ground) and only fewstudies (e.g., Salmond andMcKendry, 2002) have investigated the O3exchange between the NBL and the RL. Due to the limited mixing inthe NBL, O3 mixing ratios typically reach minimum levels at theground due to dry deposition and nitric oxide (NO) titration. There-fore, strong and positive O3 gradients develop throughout the NBL(Stutz et al., 2004; Geyer and Stutz, 2004; Brown et al., 2007; Pughet al., 2011). In contrast, O3 mixing ratios are often considerednearly invariant with time within the RL (e.g., Vecchi and Valli, 1999;Neu et al., 1994). However, recent studies (e.g., Salmond andMcKendry, 2002) suggest that this picture is incomplete as O3 levelscan be highly variable in the RL. Ozone can be transported down tothe surface from the RL, thereby contributing to the maximum O3levels observed near the surface during daytime (Neu et al., 1994;Zhang and Rao, 1999; Yorks et al., 2009; Morris et al., 2010). Addi-tional studies are required to estimate the nighttime O3 verticalvariability in the residual layer and the free troposphere so that theforecasting of air quality during the daytime can be improved.

Fig. 1. Map of model domains.

1 UTC ¼ Eastern Daylight Time (EDT) þ 4 h.

X.-M. Hu et al. / Atmospheric Environment 46 (2012) 354e364 355

The goals of the present study are to investigate the temporaland spatial variability of O3 in the nocturnal boundary layer and toestimate the O3 vertical transport in the residual layer usinga regional air quality model. Results obtained from the presentinvestigation may reduce uncertainties in regional O3 forecastingand thus improve the fidelity of air quality models.

2. Methods

The present investigation relied on the integration of airbornepollutant data within a regional air quality model.

2.1. In-situ measurements

During summer 2010 a research field campaign was conductedat Howard University’s Atmospheric Research Site in Beltsville, MD(39.06�N, 76.88�W). The study site is located in suburban Marylandon 30 ha of land and is approximately 20 km northeast of the mainHoward University campus. The suburban setting contains minimaldevelopment with not more than 5% of the land area occupied bybuilding structures, making it an ideal environment for studyinga broad range of air quality processes. Forests dominate the locallandscape with both deciduous and coniferous tree species. A 31-mwalkup tower serves as the platform to mount meteorologicalinstruments such as platinum resistance thermometers, humidityprobes, anemometers, and radiometers. A sonic anemometer(model CSAT-3, Campbell Scientific Inc., Logan, UT) measured high-frequency wind speed and temperature at 28 m above the surface.During summer 2010 several intensive observation periods werecarried out to obtain profiles of O3, temperature, humidity, andwinds using balloon-borne meteorological and O3 sondes. Ozone-sondes were based on the electrochemical concentration cell (ECC),which are known to have accuracy in the troposphere of 5e10%. Thegoal of the ozonesonde intensives was to determine the temporaland spatial distribution of O3 during regional air pollution episodes.An intensive field campaign included at least four O3 profiles in24 h. In addition, air pollutants were continuously measuredthroughout the summer 2010. Ambient levels of O3 (model 49i,Thermo Environmental Instruments, Inc., Franklin, MA), sulfurdioxide (model 43i-TLE, Thermo Environmental Instruments, Inc.),carbon monoxide (model 48i, Thermo Environmental Instruments,Inc.), nitric oxide and nitrogen dioxide (model 42i, Thermo Envi-ronmental Instruments, Inc.) were recorded every minute. Airintakes were located at 5 m above the surface.

2.2. Three-dimensional simulations

To study the O3 vertical transport in the stable boundary layer,the Weather Research and Forecasting model with Chemistry(WRF/Chem) version 3.2.1 (Grell et al., 2005) was used in this study.Three one-way nested domains (Fig. 1) were employed with gridspacing of 36, 12, and 4 km, respectively. Each domain had 44vertical layers extending from the surface to 100 hPa. The lowestmodel sigma levels were at 1.0, 0.996, 0.99, 0.98, 0.97, 0.96, 0.95,0.94, 0.93, 0.92, 0.91, 0.895, 0.88, 0.865, 0.85, 0.825, and 0.8 (cor-responding heights above ground were 0, 32, 81, 162, 244, 327, 411,495, 580, 666, 752, 882, 1015, 1149, 1284, 1513, and 1747 m). Allmodel domains used the Dudhia shortwave radiation algorithm,the rapid radiative transfer model (RRTM) for longwave radiation,theWRF Single-Moment 6-class (WSM6) microphysics scheme, theNoah land-surface scheme. The GrelleDévényi ensemble convec-tive parameterizationwas employed in domains 1 and 2, but turnedoff in domain 3. Three simulations were conducted with the YonseiUniversity (YSU), MelloreYamadaeJanjic (MYJ), and the asym-metric convective model version 2 (ACM2) planetary boundary

layer (PBL) schemes, respectively. The National Centers for Envi-ronmental Prediction (NCEP) Final (FNL) Global Forecast System(GFS) operational analyses were used for the initial and boundaryconditions of all meteorological variables.

To determine gas-phase chemical reactions, the Regional Atmo-spheric Chemistry Mechanism (RACM) (Stockwell et al., 1997) wasused. Heterogeneous reactions were not included in the simulations,which might be important for nighttime NO3 and N2O5 (Geyer et al.,2001). Such omission might introduce biases in the O3 simulationduring nighttime. Anthropogenic emissions of chemical species camefrom the national emission inventory (NEI) for 2005. Biogenic emis-sions were calculated using established algorithms (Guenther et al.,1994). Initial and boundary conditions for the chemical specieswere extracted from the output of the global model MOZART4(Emmons et al., 2010). The focus of the modeling study was onregional high-O3 episodes observed in the mid-Atlantic region of theUnited States during August 9e10, 2010. Model simulations wereconducted for the period ranging from 00:00 Universal Time Coor-dinated (UTC1) August 7, 2010 to 04:00 UTC August 11, 2010.

Previous investigations (Ha and Mahrt, 2001) illustrated thatvertical mixing in the residual layer was an important source ofmodel uncertainties. Vertical resolution was shown to affect themodeled vertical mixing strength (Ha and Mahrt, 2001). Therefore,sensitivity WRF/Chem simulations with higher vertical resolutionwere conducted to investigate how much uncertainties could beattributed to coarse vertical model resolution. Sensitivity simula-tions with different treatments of vertical mixing (i.e., differentstability functions and asymptotic lengths) in the residual layerwerealso conducted to identify the proper treatment. Such sensitivityinvestigation extended the previous studies (Hu et al., 2010;Nielsen-Gammon et al., 2010) that focused on the sensitivity of the WRFmodel for simulating the daytime convective boundary layer.

3. Results

3.1. General observation

During August 2010, average maximum O3 mixing ratiosroutinely exceeded 60 parts per billion (ppbv) during the afternoon(approximately 15:00 Eastern Daylight Time (EDT)) (Fig. 2). Ozone

Fig. 2. (Top) Observed mean time series of O3 in Beltsville, Maryland during August2010 and (bottom) the time series of O3 on each day.

Fig. 3. Observed (top to bottom) O3, standard deviation of the vertical wind speed (sw), NOx

right) August 18 and 22, 2010.

X.-M. Hu et al. / Atmospheric Environment 46 (2012) 354e364356

mixing ratios during 6:00e7:00 EDT remained below 10 ppbv on37% days. During 18% of the days, the O3 mixing ratios during6:00e7:00 EDT reached above 25 ppbv. In general, themean surfaceO3 mixing ratios decreased from about 22 to 12 ppbv from 00:00 to6:00 EDT with a depletion rate of 1.8 ppbv h�1. There were severalcases, however, when this pattern of depletion did not occur.

3.2. Elevated surface ozone during the nighttime

During August 18 and 22, 2010, O3 mixing ratios exhibitedunusual patterns, particularly between 6:00 and 7:00 EDT when O3levels remained above 40 ppbv (Fig. 3). Turbulence data were ob-tained between August 18 and August 26, 2010. The standarddeviation of the vertical wind speed (sw) remained low(<0.2 m s�1) during the nighttime and reached higher values(>0.6 m s�1) during the daytime on clear days. However, on thenight of August 17 and 21, 2010, the sw showed relatively highvalues, >0.4 m s�1 (Fig. 3), indicating stronger turbulence andvertical mixing on those nights. Stronger vertical mixing trans-ported O3-rich air from the residual layer aloft to the surface,increasing the surface O3 mixing ratio to higher levels compared to

, temperature, relative humidity (RH), and wind speed at Beltsville, Maryland on (left to

X.-M. Hu et al. / Atmospheric Environment 46 (2012) 354e364 357

the other nights. However, higher O3 mixing ratios at dawn did notcontribute to daytime elevated O3 levels on August 18 and 22, 2010.This condition was associated with clouds that suppressed photo-chemistry. The precipitation during August 18 and 22, 2010 wasassociated with the passage of fronts. The sharp increase of O3 of37 ppbv and decrease of temperature around 3:00 EDT on August18, 2010were associatedwith a cold front that abruptly transportedO3-rich air from aloft to the surface.

On August 11, 2010 during the period from 1:00 to 4:00 EDTsurface O3 mixing ratio increased by about 30 ppbv while NOxmixing ratio decreased byw25 ppbv (Fig. 4). Thereafter, surface O3mixing ratio decreased to 24 ppbv before the daytime O3 produc-tion began. During the night of August 10 and most of August 11,2010 the air mass came from the north. A cold front passed theresearch site, traveling from north to south on August 12, 2010. If O3increases resulted from the advection of an upstream, pollutedplume then mixing ratios of other pollutants such as CO would behigher. However, CO level decreased as O3 increased. The duration(several hours) of the nocturnal O3 increase was similar to the“leaky inversion” observed in New Hampshire (Talbot et al., 2005),which was caused by the vertical exchange of air between thesurface layer under the nocturnal thermal inversion and theresidual layer above it. Since the residual layer had higher O3 andlower mixing ratios of other pollutants, the vertical exchange oftrace gases allowed decreases in surface NOx and CO and increasesin surface O3. During the nighttime, the calculated vertical gradientof O3 near the surface was 0.06e0.08 ppbv m�1. Since surface O3

increased by 30 ppbv, this implied that air was mixed downwardfrom at least 400 m above ground. Similar effects were observed on

Fig. 4. Observed (top to bottom) O3, NOx, wind vector, temperature at Beltsville,Maryland on August 11, 2010.

the night of August 13e14, 2010 when a rapid event of the “leakyinversion” was associated with surface O3 mixing ratio increasesranging from 9 to 19 ppbv around 3:00 EDT on August 14, 2010(Fig. 5).

3.3. Residual layer ozone: case study and simulations

A regional episode of elevated O3 occurred on August 9e10,2010, with maximum O3 mixing ratios exceeding 80 ppbv. In thelower atmosphere (<1 km), O3 mixing ratios showed positive andstrong vertical gradients in the afternoon (Fig. 6). Ozone needs timeto be formed through a complex chemistry from primarily pollut-ants, which are usually emitted at or near the ground, so O3 mixingratios are higher away from the ground (e.g., Sillman, 1999). On thenight of August 8 and 9, 2010 surface O3 reached about 10 ppbv(Fig. 6). During the nighttime, the measured profile of O3 showeda strong vertical gradient below 1 km (Fig. 6), indicating theoccurrences of dry deposition and NO titration of O3 near thesurface. The O3 mixing ratio in the RL (w60 ppbv) resembled thatin the free troposphere, which was lower than that in thedaytime boundary layer (w80 ppbv at 13:54 EDT on August 10and w100 ppbv at 13:46 EDT on August 9, 2010).

Model simulations using the YSU, MYJ, and ACM2 PBL schemeswere conducted to investigate the air chemistry over the mid-Atlantic region of the United States. During the afternoon ofAugust 9 and 10, 2010, simulated patterns of surface wind fields byWRF/Chem with the YSU PBL scheme compared reasonably withobservations (Fig. 7). On the afternoon of August 9, easterly windsdominated in the eastern Virginia region whereas southerly/southwesterly winds prevailed in the northeastern United States.On the afternoon of August 10, westerly winds dominated in mostof Pennsylvania. Such wind patterns played an important role inregulating the air quality across the mid-Atlantic region.

Simulated O3 in the afternoon of August 9 and 10, 2010 werecompared with the measured O3 at the AIRNOW sites (Fig. 8). Themodel results captured the main regions of elevated O3 levels

Fig. 5. Observed (top to bottom) O3, temperature, and specific humidity (SH) atBeltsville, Maryland on August 14, 2010.

Fig. 6. Observed vertical profiles of O3 at Beltsville, Maryland on August 9e10, 2010.

X.-M. Hu et al. / Atmospheric Environment 46 (2012) 354e364358

during both days. On August 9, 2010 high O3 mixing ratios(>80 ppbv) occurred in the Washington DC (DC) to New York Citycorridor. On August 10, 2010 the region with high O3 mixing ratioextended to the west (southeast of Pennsylvania). Surface O3mixing ratios in the afternoon were correlated with O3 mixingratios in the upper layer (540 m above ground) in the morning(Fig. 8). On the morning of August 9, 2010 the plume of O3 in theupper layers was along the urban corridor of the eastern UnitedStates. Thus, in the afternoon, surface O3 mixing ratios in the urbancorridor were elevated. On the morning of August 10, 2010 the

Fig. 7. Simulated (top) and observed (bottom) surface wind field at 20:00 UTC on (from Leftdataset.

plume from the Midwest impacted the eastern United States. Thesurface O3 mixing ratios along the DC-to-New York City corridoralso exceeded 80 ppbv during the afternoon of August 10, 2010. Incontrast, in the morning of August 8, 2010 the upper layers wererelatively clean in the eastern United States. The surface O3 mixingratio was relatively low in the eastern United States. Therefore, theO3 in the residual layer contributed to the surface O3 duringdaytime. The residual layer also acted like a reservoir of otherpollutants, which likely contributed to surface O3 formation oncetransported to the surface. Note that the downward transport of O3and other pollutants from the residual layer is only one of manyfactors that contribute to high surface O3.

On August 9 and August 10, 2010 we noticed two differentregional transport mechanisms. On August 9, 2010 the air masseswith accumulated pollutants (including O3) were transportedalong the urban corridor region of the eastern United Stateswhereas on August 10, 2010 air masses were transported from themid-west to the eastern United States. Air parcel trajectory anal-yses (Figure not shown) confirmed such transport paths. Along thecorridor from Richmond, Va to New York City, there are heavyemissions of O3 precursors. So if transport occurred along thiscorridor, O3 might accumulate to a high level in the afternoon.Transport from the Midwest (northern Ohio River Valley) alsocontributed to the O3 episodes in the eastern United States (Hainset al., 2008).

To determine whether numerical models realistically representvertical transport of pollutants, model-simulated O3 mixing ratiowas contrasted with the field observations made at Beltsville(Fig. 9). The model captured the daytime high O3 mixing ratios, butoverestimated the nighttime O3 (except on the night of August 10).Air quality models tend to overestimate ground-level O3 during

to Right) August 9, and 10, 2011. The observation on the bottom row is from the MADIS

Fig. 8. Simulated O3 with the YSU PBL scheme at (top) layer 1 at 20:00 UTC and (bottom) layer 8 (w540 m above the ground) at 14:00 UTC on (from Left to Right) August 9, and 10,2010. The observed O3 values at the AIRNOW sites are indicated by shaded circles.

X.-M. Hu et al. / Atmospheric Environment 46 (2012) 354e364 359

the nighttime and this discrepancy is partially attributed tounderestimation of dry deposition (Mao et al., 2006; Chen et al.,2006, 2008; Zhang et al., 2009; Lin and McElroy, 2010).

In WRF/Chem vertical mixing of chemical species and meteoro-logical variables are handled separately. Vertical mixing of meteo-rological variables is handled in the PBL scheme, which is explainedin detail in Hu et al. (2010) and Nielsen-Gammon et al. (2010). Theturbulent diffusion coefficient is diagnosed in some PBL schemes(e.g., YSU and MYJ but not ACM2) and passed to a separate subrou-tine to compute the vertical mixing of chemical species. As a result,only local mixing is considered for chemical species while non-localmixing and entrainment processes are not considered for chemicalspecies included inWRF/Chem even though they are considered formeteorological variables in the YSU and ACM2 schemes. Carbonmonoxide is relativelywellmixed in the boundary layer based on the

Fig. 9. Observed and simulated O3 mixing ratios by WRF/Chem at Beltsville, Maryland.

MYJ andYSUschemes,while it is accumulatednear the surface basedon the ACM2 scheme (Fig. 10). Carbon monoxide is a primarypollutant,which ismostly released near the surface. Accumulation ofCO near the surface indicates that the ACM2 scheme lacks verticalmixing in the atmospheric boundary layer during daytime. This isbecause in the current WRF/Chem model, the turbulent diffusion

Fig. 10. Simulated mean vertical profile of CO at 18:00 UTC on August 9, 2010 indomain 3 by the WRF/Chem model with the MYJ, YSU, and ACM2 scheme.

X.-M. Hu et al. / Atmospheric Environment 46 (2012) 354e364360

coefficient is not diagnosed in the ACM2 scheme and a minimumvertical diffusion coefficient (1.0 � 10�6 m2 s�1) is used for verticalmixing of chemical species when the ACM2 scheme is chosen(Jonathan E. Pleim, United States EPA, personal communication). Inthe MYJ and YSU schemes, the diagnosed turbulent diffusion coef-ficient (w102 m2 s�1 in the daytime boundary layer) is used forvertical mixing of chemical species. Because of this prescription ofthe turbulent exchange coefficient, the weak vertical mixing doesnot transport surface CO to the upper boundary layer when ACM2 ischosen (Fig. 10). This illustrates that the current implementation ofACM2 in WRF/Chem does not properly simulate the vertical mixingof chemical species. This result explains the difference betweensurface O3 predicted by WRF/Chem with ACM2, and the other PBLschemes reported in Yerramilli et al. (2010). Thus, the current WRF/Chemmodel needs revision tomore realistically estimate theverticaltransport of chemical species. TheMYJ scheme predictedmore CO inthe lower 1 km and less CO above 1.5 km than the YSU schemebecause of weaker vertical mixing from the MYJ scheme (Hu et al.,2010). Weaker vertical mixing from the MYJ scheme was also re-ported in Niwano et al. (2007), which caused overestimation of O3precursors andO3 below1e1.5 km. The non-localmixing of chemicalspecies implemented in a global chemical-transport model (i.e.,GEOS-Chem) appeared to improve the vertical distribution ofchemical species (Lin and McElroy, 2010).

During daytime, elevated O3 due to photochemical productionwas well mixed in the boundary layer (Fig. 11). There was a strong

Fig. 11. Simulated (dashed lines) and observed (solid red lines) vertical profiles of (left tobottom) different times on August 9, 2010.

gradient of O3 between the boundary layer and the free tropo-sphere above. The model captured the vertical structure of O3during daytime. The WRF/Chem model with the YSU scheme pre-dicted higher PBL height thanwith the MYJ scheme due to strongervertical mixing strength (Hu et al., 2010). However, the currentmodel did not properly represent the vertical distribution of O3during nighttime. In the observed O3 profile, there was evidencethat O3 was well mixed between the residual layer and the abovefree troposphere. In the stable boundary layer, O3 exhibited a strongvertical gradient due to deposition, NO titration, and suppressedturbulence near the surface. However, in the simulated profiles, O3showed strong gradients between the residual layer and the freetroposphere. These results (Fig. 11) indicated that the prescribedvertical mixing was weak in the model. Weak vertical transportbetween the residual layer and the free troposphere in the modelcould also be observed from the vertical profiles of potentialtemperature and water vapor mixing ratio. In the observed profiles,potential temperature and water vapor were well mixed to higheraltitude than model-simulated profiles. The comparison of thevertical profile of O3 on the early morning of August 10, 2010 (notshown) also displayed similar differences as in August 9, 2010: themodel simulated higher O3 mixing ratio in the RL than that in thefree troposphere while observations showed similar O3 in bothregions. Thus, the present study implies that the current WRF/Chem model may need modification to more realistically estimatevertical mixing in the residual layer.

right) potential temperature, O3, water vapor mixing ratio and wind speed at (top to

Table 1Model configurations in the WRF/Chem sensitivity simulations with the YSU PBLscheme.

Simulation code Wind shearenhancement factor

Stability function l (m)

YSU control 1 f ðRiÞ ¼ 1

ð1þ 5RiÞ230

YSU SS1 2 f ðRiÞ ¼ 11þ 10Ri

30

YSU SS2 2 f ðRiÞ ¼ 11þ 10Ri

50

YSU SS3 2 f ðRiÞ ¼ 11þ 10Ri

80

X.-M. Hu et al. / Atmospheric Environment 46 (2012) 354e364 361

Nighttime vertical profiles of potential temperature, O3 andwater vapor are shown in Fig. 12. In the lower free troposphere, O3ranged between 40 and 70 ppbv. The O3 levels in the RL resembledthose found in the free troposphere in all the nights, except thenight of July 5, 2010 (the profiles are bolded on this night). Theresemblance indicated therewas sufficient vertical mixing betweenthe RL and the free troposphere during most of the nights. Thenighttime O3 vertical profiles between 2006 and 2009 measured atBeltsville were also checked (not shown here). During most of thenights, the O3 in the RL resembled that in the free troposphere. Forthe night of July 5, 2010 the elevated O3 in the boundary layerremained in the RL. During this night, strong gradients of O3remained in the top of the RL. Vertical profiles of potentialtemperature and water vapor at the same time demonstrated thestrong stability during this night. There was a strong inversion layerat the top of the RL and there was a strong gradient of water vaporbetween the RL and the free troposphere above.

Modeled vertical mixing strength can be sensitive to the verticalresolution (Ha and Mahrt, 2001). Models with poor vertical reso-lution may overestimate the Richardson number (Ri), thus intro-ducing errors in the computation of vertical mixing (Beare et al.,2006). To assess the potential for this effect, sensitivity WRF/Chem simulations with higher vertical resolution were conducted.In the high-resolution simulations, from the surface to 3000 mabove the ground the number of model layers increased from 20 to36. No significant improvement in the simulated O3 profiles wasachieved in the high-resolution simulations (Figure not shown). Forthis case, the vertical resolution was of secondary importance.Cuxart et al. (2006) also showed that the single-column boundarylayer models were insensitive to the vertical resolutions (verticalgrid space ranging between 25 and 100 m) for an Arctic stableboundary layer case.

In the MYJ scheme, the equation system for vertical mixing isstrongly coupled, which makes it difficult to test the sensitivity tothe vertical mixing strength without modifying the equationsystem substantially (Hu et al., 2010). On the other hand, sucha sensitivity investigation is easier in the YSU scheme. In the YSUscheme, the vertical diffusivity coefficients (K) in the residual layerand the free troposphere are computed as a function of mixinglength (l), vertical wind shear (S), and the stability function (f(Ri)):

K ¼ l2S f ðRiÞ (1)

in which:

1l¼ 1

kzþ 1

l(2)

where k is the von Karman constant (0.4), z is the height above thesurface, and l is the asymptotic length scale. Such first-order

Fig. 12. Vertical profiles of nighttime (from left to right) potential temperature, O3, and watenight of July 5 are marked using thicker lines.

parameterizations of turbulent vertical mixing are widely used inoperational numerical weather prediction (NWP) and climatemodels (Pleim, 2007b; Cuxart et al., 2006; Beare et al., 2006). Muchof the improvement of this parameterization in NWP and climatemodels involved adjusting the stability functions and the asymp-totic lengths (Cuxart et al., 2006). Various forms of the stabilityfunctions were proposed (Cuxart et al., 2006). In the current YSUmodel, the stability function is given by:

f ðRiÞ ¼ 1

ð1þ 5RiÞ2(3)

Which givesmuch less vertical mixing under stable conditionwhenRi > 0.1 than the “long-tails” stability function, given by:

f ðRiÞ ¼ 11þ 10Ri

(4)

The “long-tails” function worked well over land in coarseresolution NWP and climate models (Beare et al., 2006; Brownet al., 2008). The requirement of enhanced vertical mixing (likethat produced by the “long-tails” function) by the coarse resolu-tion NWP and climate models is related to the unresolvedheterogeneity at the surface and/or unresolved terrain drag(McCabe and Brown, 2007; Steeneveld et al., 2008, 2009). Thus,sensitivity simulations with the YSU scheme and the “long-tails”stability function were conducted. Also, the current operationalmodels always underestimate wind shear (see the fourth columnof Fig. 11), so they overestimate Richardson number. Thus, in thesensitivity WRF/Chem simulations, wind shear was increased bya factor of 2. The l parameter is adjustable and varies between 30and 250 m in numerical models (Cuxart et al., 2006; Nielsen-Gammon et al., 2010). The l is set to 30 m in the YSU schemewhile it is set to 80 m in the ACM2 scheme. Nielsen-Gammon et al.(2010) tested the sensitivity of the WRF model with the ACM2

r vapor mixing ratios measured at Beltsville, Maryland in 2010. The profiles during the

a b

dc

Fig. 13. Simulated and observed vertical profiles of (a) potential temperature, (b) wind speed, (c) water vapor mixing ratio, and (d) O3 at 06:00 UTC on August 9, 2010.

X.-M. Hu et al. / Atmospheric Environment 46 (2012) 354e364362

scheme to the value of l between 40 and 120 m. The WRF simu-lations showed less sensitivity to l in the daytime boundary layerand NBL, but exhibited larger sensitivity in the residual layer.Therefore, in this study, to determine the sensitivity of WRF/Chemsimulations to l in the residual layer, the l values of 30, 50, and80 m were tested. The configurations of the three sensitivitysimulations are listed in Table 1 in addition to that of the controlWRF/Chem simulation with the YSU PBL scheme. The simulatedprofiles of potential temperature, wind speed, water vapor mixingratio, and O3 by the sensitivity WRF/Chem simulation are shownin Fig. 13. In the WRF/Chem sensitivity simulations (i.e., YSU SS1,YSU SS2, and YSU SS3) variables are better mixed in the residuallayer than in the YSU control simulation. In the sensitivity simu-lations, simulated wind speed showed better agreement comparedto the observation made in the lower 2 km (Fig. 13b). However,wind speed was still underestimated and the wind shear waslower than the observation, which likely resulted in the under-estimation of turbulent mixing. In the YSU SS3, with the largest ltested, vertical gradients of scalars (e.g., water vapor and O3) werethe least in the residual layer, thus providing the best agreementwith observations. However, O3 in the residual layer was stilloverestimated by the simulation of YSU SS3 and simulated O3 inthe lowest w400 m was less sensitive to the model treatment ofvertical mixing. This result was likely due to errors from thetreatments of other processes in the model (Chen et al., 2006,2008; Brown et al., 2007).

4. Discussion and conclusions

Field observations made during summer 2010 in the mid-Atlantic region of the United States revealed that, during daytimeregional pollution events, the air layer extending from the surfaceto 2 km above the ground had uniform O3 levels reaching nearly100 ppbv. Following the onset of the stable nocturnal boundarylayer, O3 levels close to the ground rapidly decreased in response tosurface deposition and titration processes. At times, the atmo-spheric stable boundary layer became decoupled from the O3-richair in the residual layer. During the nighttime and the transitionperiod from night to day, the residual layer became “leaky” inresponse to intermittent and intense turbulent vertical motions oronset of convection. Under nighttime conditions, due to verticaltransport associated with the “leaky” residual layer, ground-levelO3 mixing ratios increased by as much as 10e30 ppbv in timeperiods ranging from 1 to 3 h. In contrast, in the absence of the“leaky” residual layer, the mean ground-level O3 mixing ratiosdecreased from 22 to 12 ppbv during the period from 0:00 to 6:00EDT, representing an O3 removal rate of 1.8 ppbv per hour (sincemean NO2 showed little variation during this period, the removalrate of Ox, sum of O3 and NO2, was the same as the O3 removal rate).Throughout the course of the evening, the O3 content in theresidual layer gradually decreased in response to the downwardtransport to the surface and removal due to deposition and NOtitration. The vertical O3 redistribution eventually resulted in weak

X.-M. Hu et al. / Atmospheric Environment 46 (2012) 354e364 363

O3 gradients between the residual layer and the free atmospherealoft. Therefore, for “leaky” conditions, the residual layer did notrepresent a major reservoir of O3 for the daytime convectiveboundary layer.

Model evaluations revealed that, during the daytime, estimatedmaximum ground-level O3 compared reasonably well with fieldobservations. In contrast, during the nighttime, modeled O3 levelscompared poorly with measurements. At night, modeled ground-level O3 mixing ratios were overestimated within “non-leaky”residual layers and were underestimated during “leaky” residuallayers. Poor model performance was due in part to the inadequaterepresentation of verticalmixing.While themodel provided realistichorizontal patterns in wind direction and wind speed, the verticalvariation of the wind speed compared poorly with measurements.For the evaluation period, the WRF/Chem model underestimatedwind speed variations with altitude. As a result, estimated atmo-spheric diffusivities employed to determine the vertical transport ofO3 were underestimated. In addition, sensitivity model simulationswith different treatments of vertical mixing (i.e., different stabilityfunctions and different values of the turbulent scales) indicated thatthe turbulent scales included in the WRF/Chem model need to beimproved to more appropriately consider turbulent transportprocesses in the residual layer. For the cases considered in thepresent study, “long-tails” stability functions and longer turbulentlength scales improved the simulation of nighttime vertical profilesof meteorological variables and chemical species.

Although this manuscript focused on the importance of properrepresentation of the nighttime vertical mixing process to thesimulation of O3, other sources of model inaccuracies cannot beignored (e.g., dry deposition (Mao et al., 2006; Chen et al., 2006,2008; Zhang et al., 2009; Lin and McElroy, 2010) and the repre-sentation of the different chemical regimes at different near-surface layers (Geyer and Stutz, 2004; Stutz et al., 2004; Brownet al., 2007)). When O3-rich air is mixed down to the NBL, NO3and N2O5 occupy distinct chemical regimes as a function of altitudeand can serve as major sinks for O3 (Brown et al., 2007). To correctlysimulate the O3 in the NBL and the RL, proper representation of thenighttime O3 and NOx chemistry is also critical. Nevertheless,vertical mixing plays a role in the nocturnal ozone chemistry, andso is one area that may be improved in models.

Acknowledgments

The field research was partially supported by NASA ResearchCooperative Agreement NNX08BA42A, and a contract from Mary-land Department of the Environment. KS and JDF received supportfrom the National Science Foundation (NSF) to partake in the fieldstudy (award 0914597). XMH was supported by NSF (Award0939455). Twoanonymous reviewers provided excellent commentsto improve the manuscript.

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