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Atmospheric Environment 45 (2011) 1138e1144

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Atmospheric Environment

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Measurements of carbon dioxide in an Oregon metropolitan region

Andrew Rice*, Gregory BostromPortland State University, Department of Physics, Portland, OR 97207-0751, United States

a r t i c l e i n f o

Article history:Received 27 July 2010Received in revised form15 November 2010Accepted 17 November 2010

Keywords:Carbon dioxideUrban carbon budgetUrban areaGreenhouse gas emissionTraffic emissionPortland

* Corresponding author. Tel.: þ1 503 725 3095; faxE-mail address: arice@pdx.edu (A. Rice).

1352-2310/$ e see front matter � 2010 Elsevier Ltd.doi:10.1016/j.atmosenv.2010.11.026

a b s t r a c t

Ambient concentrations of atmospheric carbon dioxide (CO2) are reported for the Portland, Oregon (USA)metropolitan region for the late July through December, 2009 period. Three stationary locations wereestablished: a downtown location on the campus of Portland State University; a residential site insoutheast Portland; and a rural station on Sauvie Island, located w30 km northwest of Portland in theColumbia River Gorge. Continuous measurements of CO2 at each site average 403e408 ppm and showconsiderable variability at each site (360e610 ppm) due to CO2 sources, sinks and meteorological vari-ability. Within this variability, a marked 20e30 ppm diurnal cycle is observed due to photosyntheticactivity and variations in the planetary boundary layer. In-city CO2 concentrations are on average enhancedby 5e6 ppm over the Sauvie Island site during upgorge wind conditions, a difference which is greatest inthe afternoon.Measurements of the 13C/12C ratio of CO2 in downtownPortland are significantly depleted in13C relative to 12C comparedwith background air and suggest that regional CO2 is dominated by petroleumsources (75e80%). High degrees of relationship between CO2 variability and primary air pollutants CO andNO (r2 ¼ 0.80 and 0.77), measured by the Oregon Department of Environmental Quality at the SoutheastPortland location, corroborate this finding and illustrate the importance of traffic emissions on elevatedambient CO2 concentrations.

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1. Introduction

Metropolitan regions are responsible for a large fraction ofanthropogenic carbon dioxide (CO2) emissions globally (Andreset al., 2010). Urban centers, in particular, are dense in populationand support extensive commercial activity resulting in high rates offossil fuel consumption in energy and transportation sectors. Theapportionment by fuel source in cities varies widely around theworld depending on regional petroleum, natural gas, and coal use(Decker et al., 2000).

Due to a lack of federal regulatory framework in the UnitedStates, a patchwork of regional and statewide approaches toreducing CO2 emissions has emerged. The City of Portland, Oregontogether with Multnomah County established itself as an earlyfrontier in this regard by creating greenhouse gas emissionsinventories in 1990 and adopting a regional plan to reduce emis-sions in 1993. The most recent work-product of this effort isa Climate Action Plan, released in 2009, which details an ambitiousproposal to decrease greenhouse gas emissions by 40% below 1990levels by 2030 and 80% below by 2050 (Adams et al., 2009).

: þ1 503 725 9525.

All rights reserved.

The Portland Metropolitan region, located in the Columbia RiverGorge, is themost populous region in Oregon. Greenhouse gas (GHG)emissions inventories for Multnomah County (population w710 k,Proehl, 2008) estimate 2008 emissions to be 8.5 million metric tonsCO2 equivalent (MMTCO2-e), of which CO2 is the dominant source(Adams et al., 2009). The County accounts for w14% of Oregonstatewide GHG emissions (Duncan et al., 2009). Despite this largefraction of state GHG, per capita emissions in theMultnomah Countyare 12 MTCO2-e, which are lower than the statewide average of17 MTCO2-e. Recent emissions inventories suggest that County-wideGHG emissions are near 1990 levels, despite a 22% growth inpopulation (Proehl, 2008). This contrasts with the US National GHGemissions, which have grownw14% since 1990 (EPA, 2010). Despitethese efforts, there has been no validation of either emissionsinventories in the Portland metropolitan region or their trendsin time.

A number of studies in the past decade have characterized CO2concentrations and fluxes in urban areas, most of which can beattributed to fossil sources (George et al., 2007; Grimmond et al.,2002; Idso et al., 2001; Newman et al., 2008; Pataki et al., 2009;Velasco et al., 2005). These studies make progress towards betterunderstanding urban CO2 sources and ultimately improving emis-sions inventories. Prior to this study, little work has taken place incities of the northwestern United States. Yet this region warrantscomparison because its fossil fuel intensity may be lower than

A. Rice, G. Bostrom / Atmospheric Environment 45 (2011) 1138e1144 1139

other regions of the United States due to efforts in urban planningas well as the benefit of a relatively moderate climate. Here, wereport methods and first results of a study to characterize CO2variability and its 13C isotopic composition in Portland, Oregon. Wefocus this paper on measurements from three regional monitoringstations in themetropolitan area during themonths of July throughDecember, 2009.

2. Methods

2.1. Study area

TheColumbia Gorge is one of thewindiest areas in the northwestdue to the unique topography as the only near-sea level gap throughthe Cascade Mountains. The consistently strong winds haveattracted significant investment in wind energy and is the reasonthe Columbia Gorge is one of the premier windsurfing locations inthe continental United States. During Summer months and into Fallmonths upgorge winds dominate due to a strong eastewesttemperature gradient across the Cascade Range. In the Portlandregion, mean wind direction during this time of year is dominatedby northwesterlies. In Winter months a reversal in eastewesttemperature and density gradients cause thewind direction to shiftdirection to downgorge. These Winter easterlies are regularlyenhanced by synoptic scale pressure differences which set upa condition for downgradient movement of air through theColumbia Gorge known as “gap flow” (Sharp and Mass, 2004).

Fig. 1. Map showing the location of the Portland metropolitan region in the Columbia Riv122.77�W, 6 m elevation), Portland State University (PSU, 45.51�N, 122.69�W, 63 m elevatio

In this study, we exploit this climatic feature of the ColumbiaGorge by establishing one rural and two in-city sites tomonitor CO2concentrations. In mid-Summer 2009, three sites were established:Sauvie Island w30 km northwest of Portland (SIS, 45.77�N,122.77�W, 6 m); downtown Portland on the campus of PortlandState University (PSU, 45.51�N, 122.69�W, 63 m); and southeastresidential Portland (SEL, 45.50�N, 122.60�W, 75 m) (Fig. 1).Working in collaboration with Oregon Department of Environ-mental Quality (DEQ) two of the three sites are in DEQ shelters forthe ongoing regional air quality monitoring program (SIS and SEL).The northwesterlies dominant during Summer and early Fallmonths, establish the Sauvie Island site as directly upwind ofPortland and a model location to monitor GHG concentrations inincoming air to the metropolitan region. Elevated levels of GHGswithin the metropolitan region can be attributed to sources, mostof which are anthropogenic.

2.2. Instrumentation

All three sites are equipped with a Li-Cor infrared gas analyzer(model 840) which measures CO2 and H2O concentrations at 4 Hz,smoothed with a 20-s moving average, which we sample at 1 minintervals. Ambient air is sampled at 0.5 l min�1 through 6.35 mm(0.2500) tubing (PFA or Synflex) from an inlet that is located at least1 m above roof level at each of the sites. Ambient air is drawn intothe optical cell of the Li-Cor analyzer using a KNF diaphragm pump(Model NMP830). Data are collected and stored on a Campbell

er Gorge. Sampling locations for this study are indicated: Sauvie Island (SIS, 45.77�N,n), Southeast Portland (SEL, 45.50�N, 122.60�W, 75 m elevation).

A. Rice, G. Bostrom / Atmospheric Environment 45 (2011) 1138e11441140

Scientific Datalogger (Model CR1000). At SIS and SEL, instrumen-tation is housed inside single story temperature controlled DEQshelters alongside air quality instruments. At PSU, instrumentsoperate in a laboratory on the second floor of Science Building 2. Atthe PSU site, we are also equipped with a Picarro 13CeCO2 analyzer(Model G1101-i) which measures the 13C/12C isotope ratio of CO2and atmospheric CO2 concentration. Data is acquired every 5e10 sand processed from this instrument at 5 min averaged intervals forstatistical purposes.

Li-Cor CO2 instruments are calibratedweeklywith high pressurealuminum cylinders containing CO2 gas mixtures in ultrapure airfrom Scott-Marrin Inc. The weekly calibration procedure includesa zero using ultra-high purity N2, a span measurement usinga 511 ppm CO2 mixture, and a near-ambient 393 ppm CO2 mixture.The calibration scale is based on a 394 � 4 ppm gas mixture whichis NIST-Traceable to SRM 1672a (355 ppm). Instrumental drift,checked prior to recalibration, is typically <1 ppm per week.Precision of measurement of the Li-Cor analyzer is <1 ppm aver-aged over a 1 min interval. The H2O channel of the Li-Cor instru-ments are calibrated using a dew point generator; CO2 values arecorrected for dilution and band broadening due to H2O vapor.

At PSU, calibration of the 13CeCO2 analyzer is automated andoccurs once every 12 h using a high pressure cylinder of calibratedwhole air on a timer-controlled valve manifold. Measurements ofthe 13C/12C ratio of atmospheric CO2 are expressed in parts perthousand (or ‘per-mil’) using the delta notation (d) relative to theVPDB scale, where d13C ¼ [(13C/12C)sample/(13C/12C)VPDB � 1] � 1000as established by the International Atomic Energy Agency (IAEA) inVienna, Austria (Coplen, 1995). Isotopic calibration of the Picarroinstrument is based on dual inlet isotope ratio mass spectrometryusing a suite of three gas phase 1-L CO2 reference gases acquiredfrom Oztech Trading LLC (d13C �3.60&, �40.68&, and �47.54&versus VPDB). These were calibrated independently at PSU usingNBS 19 (SRM 8544, d13C þ1.95& versus VPDB). Uncertainty ofcalibration is better than�0.02& for d13C. The high pressure wholeair cylinder used for the Picarro instrument (d13C �9.85& versusVPDB) was calibrated using techniques described previously(Schauer et al., 2005). Precision of measurement for the Picarro13CeCO2 analyzer is better than 0.5& averaged over 5min intervals.Instrumental drift between calibrations is typically less than 0.5&.A second analysis of CO2 concentrations at PSU was also possibleusing the Picarro analyzer, which has a precision of <50 ppb over5 min intervals. The Picarro and Li-Cor instruments were inagreement for CO2 concentrations (r2 ¼ 0.92).

Long term monitoring stations SEL and SIS are maintained byOregon DEQ for studying regional air pollution. Standard

Fig. 2. Time series of 1 min CO2 concentrations and 5 min mean d13C of CO2 measured(For interpretation of the references to colour in this figure legend, the reader is referred t

meteorological data is available at both sites (i.e., temperature,barometric pressure, humidity, wind speed, wind direction, andsolar radiation). Air monitoring at SEL includes a full suite ofmeasurements including CO, NO, NO2, O3, and PM10 and PM2.5.Meteorology and air quality is typically measured on the ScienceBuilding 2 rooftop at PSU. However, this capability has beentemporarily interrupted as a result of a building remodel. Formeteorology at PSU, nearby data from the downtown Portland DEQsite is used, located w1 km from PSU (45.52�N, 122.67�W, 13 m).

3. Results and discussion

3.1. Measurements of CO2 concentrations

Continuous measurements of CO2 were recorded from our threestation network July 22 through December 31, 2009 at 1 minintervals (Fig. 2). Measurements at PSU were initiated earliest(July 13), followed by SEL (July 18) and SIS (July 22). Short breaks inthe data result from instrument calibration and longer periods frominstrument and data storage failures. One minute CO2 concentra-tions were passed through a 6s filter to remove irregular spikes inthe data. This analysis preserved >99.9% of all raw data. Hourmeans were calculated from filtered data on the half-hour mark.Considerable variability is observed in hourly means at each site,which ranges 360e610 ppm. Over the measurement period, thesites averaged 408 � 24 ppm, 407 � 30 ppm, and 403 � 25 ppm atPSU, SEL, and SIS, respectively (�1s).

Several features are notable in the resulting dataset. First, thetwo urban Portland sites (PSU and SEL) are typically and on averagehigher in CO2 concentration than the rural SIS site, commonly byseveral ppm or more. This is due to sources of CO2 in the Portlandmetropolitan region, primarily the combustion of petroleum inmotor vehicles and natural gas and petroleum for heating andindustrial processes. To quantify the enhancement of CO2 concen-trations in urban Portland, we use the upgorge feature of air flow inthe region. Over the measurement period, wind speed at SISaveraged 2.3 � 1.5 m s�1 and was dominated by a northwest-erlyenortherly direction (Fig. 3). Selecting hourly CO2 data witha 270e360� wind component at SIS we calculate hourly differencesin coincident CO2 concentrations between in-city sites (PSU andSEL) and the upwind SIS site (DCO2). The distributions of theseanalyses are shown as histograms in Fig. 4; both results showa significant enhancement of in-city concentrations over theupwind site with 79% of hourly averages higher in-city duringupgorge wind conditions. The mean difference between PSU andSIS was 5.1�1.4 ppm (95% confidence interval, p value<0.001), the

at PSU (blue), SEL (green), and SIS (red) between July 13 and December 31, 2009.o the web version of this article.)

Fig. 3. Windrose at SIS showing meteorological features of the Columbia River Gorge.

A. Rice, G. Bostrom / Atmospheric Environment 45 (2011) 1138e1144 1141

difference between SEL and SIS was 5.5 � 1.5 ppm (95% CI, p value<0.001). The difference between in-city and rural area CO2concentrations for Portland is on the low end compared to studiesin other urban centers, which typically are 30e100 ppm aboveoutlying areas (George et al., 2007; Idso et al., 2001; Newman et al.,2008). This may result from the relatively high rate of ventilationfor the Portland metropolitan region.

a

b

Fig. 4. Histogram showing the difference between hourly in-city (PSU (a) and SEL (b))and upwind (SIS) CO2 concentrations (DCO2) selected for a westerly to northerly windvector (270e360�). Coefficients of skewness and kurtosis indicate distributions haveslight left-skews and are heavy-tailed from normal.

Differences betweenweek and weekend concentrations are alsofound at all three sites; weekday mean concentrations are4.0� 0.2 ppm, 2.8� 0.3 and 1.3� 0.3 (95% CI, p value<0.001) higherthan weekend mean concentrations at PSU, SEL, and SIS respec-tively. This finding for PSU and SEL is consistent with increases inweekday traffic volume in the region and consistent with previousobservations in cities (Gratani and Varone, 2005). The existence ofa smaller but significant difference betweenweek andweekend CO2concentrations at SISmaybe due to increases in traffic volume in thelarger metropolitan region or to periods when SIS is downwind ofthe Portland region. Similar differences have been observed inremote and semi-remote locations (Thoning et al., 1989).

There is a 20e30 ppm diurnal cycle in CO2 observed at all threesites in the hourly data (Fig. 5a). Highest concentrations occur in theearly morning hours (5e8am, local time) and lowest concentrationsoccur in the afternoon (3e5pm). This feature is driven by a combi-nation of biological activity (photosynthesis and respiration) andvariations in the planetary boundary layer height (Thoning et al.,1989). The two urban sites have 5e10 ppm higher concentrationsthan SIS during the daytime when anthropogenic sources are mostsignificant. PSU, located in downtown Portland, experiences higherdaytime concentrations than either SEL or SIS on the average. Thedifference between PSU and SIS is greatest in late afternoon,w15 ppm. In contrast, early morning concentrations are comparableat SIS and SEL and lower at PSU by 5e10 ppm. The causes of lowerearly morning concentrations in downtown Portland are not knownbut could relate to a smaller local plant respiration source ora weakening of stratification and thickening of the nocturnalboundary layer due to the urban heat island effect (Fan and Sailor,2005; Martilli, 2002). As a result of higher afternoon concentrations

a

b

Fig. 5. Mean diurnal cycle of CO2 concentration (a) and d13C of CO2 (b) at PSU (blue), SEL(green), and SIS (red) over the measurement periods (JulyeDecember, 2009). Data are1 h means centered around the half hour; error bars are the standard error of hourlymeans. Time is Pacific Standard Time (UTC-8). (For interpretation of the references tocolour in this figure legend, the reader is referred to the web version of this article.)

Fig. 6. The relationship between d13C of CO2 and inverse concentration (1/[CO2]) for5 min mean values at PSU. The y-intercept is �30.5 � 0.2& versus VPDB (95% confi-dence interval) and correlation coefficient (r2) is 0.85.

A. Rice, G. Bostrom / Atmospheric Environment 45 (2011) 1138e11441142

and lower evening concentrations, PSU experiences the smallestamplitude diurnal cycle of the three sites (23 ppm). The largestamplitude diurnal cycle is at SIS (32 ppm). Notably, though thediurnal cycle in CO2 at all sites are in-phase, the SIS and SEL sites aremore alike in sinusoidal behavior. In particular, the diurnal cycle atPSU has a strong first harmonic in the signal which is apparent asa later rise in CO2 in the earlymorning hours and an afternoon rise inCO2 to approximately steady state levels between w6pm and 3am.Spectral analysis of both 1 h and 1 min data, reveals a uniquefrequency at PSUof two cycles per dayof near equalmagnitude to thefundamental (one cycle per day). We speculate that this occurrencemay be largely due the importance of traffic sources in and neardowntown Portland which have peak volumes typically centeredaround7e8amand5pmfromrushhour (data fromPortlandRegionalTransportation Archive Listing (PORTAL): http://portal.its.pdx.edu/portal2.1/index.php/highways). When differentiated by week andweekend, the two cycle per day spectral feature at PSU is unchangedin week data (large compared with fundamental) whereas the twocycle per day feature inweekend data is small (i.e., comparable withharmonic contribution at other sites).

To our knowledge, there is only one previous studywithwhich tocompare our results in the Portlandmetropolitan region (Khalil andRasmussen, 2004). In that work, authors compared carbon dioxideconcentrations in discrete flask samples collected in the Portlandregion with those collected at a coastal site (Cape Meares, Oregon,45.48�N, 123.97�W, 30 m) and found an enhancement of w3%.When we compare mean values observed in our work withboundary layer CO2 observations made at Trinidad Head, California(41.05�N, 124.15�W), a coastal site operated by the NOAA EarthSystem Research Laboratory Global Monitoring Division, we findour in-city values higher by w6% on average (C. Sweeney, PersonalCommunication).

3.2. The isotopic composition of CO2

Measurements of the 13C/12C isotope ratio of CO2 began October1, 2009 at PSU. Fiveminute averages from the Picarro instrument areshown in Fig. 2d alongside CO2 concentration time series (as d13C).These data are further degraded to hourly averages of d13C of CO2which vary between �7 and �15&with an average of �9.2 � 1.0&(1s, n¼ 2184). Themean value is significantly lower than the d13C ofCO2 of clean coastal air at this latitude and time of year whichranges �8.0 to �9.0& relative to VPDB (White and Vaughn, 2009).The depletion of 13C relative to 12C in CO2 is the result of regional CO2sources. Two fossil sources, in particular, have isotopic signatureswhich can cause this effect; petroleum has a d13C w�28& andnatural gas has a d13Cw�40& versus VPDB (Pataki et al., 2006). Therelationship between d13C and inverse CO2 concentration (“Keelingplot”) can provide some constraint on the partitioning betweenthese sources. Fig. 6 shows the high degree of correlation between5 min averaged values of d13C and 1/CO2 (r2 ¼ 0.85); least squaresregression of these data, considering errors in both variables, resultsin a y-intercept value of �30.5 � 0.2& versus VPDB (95% CI).Considering anthropogenic sources alone, petroleum and naturalgas sources account for 80% and 20% of the CO2 source, respectively.An important caveat in this analysis is that it neglects biogenicsources of respiration, which have a d13C w�25& for C3 plantswhich are dominant respiration sources in the region. To addressthis bias,we selected data for daytimehours,whenplant respirationis lowest and anthropogenic sources are at the greatest. This anal-ysis resulted in a y-intercept value of �31.0 � 0.2& versus VPDB(95% CI), which corresponds to a 75/25 split betweenpetroleumandnatural gas. Excluding electricity which is generated outside themetropolitan region, petroleum sources (including oil-based heat-ing fuel) in Multnomah County are thought to be responsible for

w67% of CO2 emissions based on emissions inventories (Adamset al., 2009). Petroleum sources may be lower in emissions inven-tories compared with our estimates due to traffic into MultnomahCounty fueled from gasoline stations located outside the countybecause emissions inventories are based on fuel sales. In contrast,natural gas sources are non-mobile and better defined in space.

The diurnal cycle in d13C of CO2 at PSU is also apparent in hourlyaveraged data (Fig. 5b). It is in anti-phase with CO2 concentrationswith lowest values (most 13C depleted) occurring in early morning(w7e9am) and highest values (most 13C enriched) occurring inafternoon (12e4pm). As with CO2 concentration, this effect isdriven by both biological processes and boundary layer meteo-rology. The first harmonic observed in the CO2 diurnal cycle is alsoapparent in the d13C results. These observations of d13C of CO2 showpotential to identify temporal trends in CO2 concentration. Forexample, the dramatic drop in d13C value at w5pm (Fig. 5b) coin-cides with a corresponding rise in CO2 concentration (Fig. 5a) atPSU and is further evidence of a petroleum source (depleted in 13C)intensifying at this time of day.

3.3. Ancillary measurements

Co-location of our Li-Cor CO2 instruments at Oregon DEQregional air quality stations allows the examination of relationshipsbetween CO2 concentrations and both meteorological variables andair pollutants. These can help to identify primary drivers of vari-ability in CO2 in the Portlandmetropolitan region. Variations in CO2

concentrations with hourly averaged wind speed and with severalcriteria air pollutants observed at SEL are shown in Fig. 7.

At low wind speeds (0e2 m s�1) CO2 concentrations displayconsiderable variability up to 610 ppm due to variations in sources,sinks, and dynamical processes (Fig. 7a). The relationship betweenCO2 and wind speed approaches an asymptotic value near 400 ppmat wind speeds above 2 m s�1 as ventilation is sufficient that theenhancement above CO2 background concentrations due to urbansources is not significant and concentrations are <410 ppm. Rela-tionships of CO2 with additional meteorological variables were nothighly correlated (r2 < 0.6). Primary pollutants carbon monoxide

b

c d

e f

a

Fig. 7. Variability observed in hourly mean CO2 concentrations plotted against wind speed (a) and criteria air pollutant (bef) concentrations measured at SEL. Primary pollutants CO(b, r2 ¼ 0.80) and NO (c, r2 ¼ 0.77) are highly correlated with CO2; secondary pollutants NO2 (d, r2 ¼ 0.29) and O3 (e, r2 ¼ 0.36) and aerosols (f, r2 ¼ 0.52) have lower degrees ofcorrelation.

A. Rice, G. Bostrom / Atmospheric Environment 45 (2011) 1138e1144 1143

(CO) and nitric oxide (NO) show strong correlations with CO2variability at SEL (Fig. 7b and c, r2 ¼ 0.80 and 0.77, respectively).Both CO and NO are produced by the internal combustion engineand the strength of these correlations shows the importance ofautomotive sources on CO2 concentrations in the Portland region.This finding is consistent with both d13C observations and withemissions inventories which place transportation as the leadingregional source of CO2. Correlations with secondary air pollutantstend to beweaker. Though NO2 and O3 are linked tomobile sources,they are dependent on photochemical processes which are unre-lated to CO2 sources. As a result, the correlation between variabilityof CO2 and NO2 in ambient air is weaker (Fig. 7d, r2 ¼ 0.29) and CO2

and O3 are bifurcated due to dayenight differences (Fig. 7e,r2 ¼ 0.36). Finally, aerosols show only a medium degree of corre-lation with CO2 (Fig. 7f, r2 ¼ 0.52). Though particulates are gener-ated during combustion of automotive fuels, there appear to beother significant controlling variables for aerosols in the Portlandmetropolitan region, which include industrial processes, non-roadequipment, wood burning, and secondary organic aerosol.

4. Conclusions

Analysis of continuous ambient CO2 concentrations at threelocations in the Portland, Oregon metropolitan region during theperiod July 13eDecember 31, 2009 provides a first accounting formean behavior and variability of CO2 levels in the region due toanthropogenic and natural sources. On average, we observea 20e30 ppm diurnal cycle in CO2 concentration with highestconcentrations in early morning and lowest concentrations in lateafternoon as a result of biological activity and planetary boundarylayer dynamics. Urban CO2 sources result in enhanced concentra-tions of CO2 at PSU and SEL over SIS during the daytime. At nightinto early morning, CO2 concentrations at SEL and SIS are closelymatched whereas PSU displays lower concentrations. PSU alsodisplays somewhat unusual behavior consistent with a strongcontribution from the first harmonic. The diurnal cycle in d13C ofCO2 measured at PSU is in anti-phase with CO2 concentrations withlowest values in the early morning and highest values in theafternoon.

A. Rice, G. Bostrom / Atmospheric Environment 45 (2011) 1138e11441144

Based on coincident analyses at in-city and SIS locations, we findthat CO2 concentrations are enhanced in the city of Portland by anaverage of 5e6 ppm over incoming air due to regional sources. Thesources of CO2 in the region appear to be dominated by mobileautomotive sources based on the carbon isotopic composition (d13C)of CO2 which shows a source value near �31& relative to VPDB,providing a split of 75e80% petroleum and 20e25% natural gassources, when ignoring biogenic sources. Strong correlations withprimary automotive pollutants NO and CO support the assertion thatthis sector drives much of the variability in CO2.

Data resulting from this ongoing work are useful to betterunderstand regional sources of CO2 and their changes in time. Thestraightforward approach presented here using continuous moni-toring of upwind and in-city CO2 concentrations is effective forquantifying the enhancement in CO2 levels due to regional sourcesin mid-sized cities. Additional monitoring stations or mobile fieldcampaigns would be useful to better characterize spatial dimen-sions of this urban CO2 enhancement. Finally, long-standing timeseries will be particularly useful for measuring changes in regionalCO2 sources in time responding to regional approaches to reduceGHG emissions. Accompanying these datasets, fine resolutiongridded inventories in time and space and chemical transportmodels will be useful for quantitatively testing carbon emissionsinventories at regional scales.

Acknowledgements

We thank Oregon Department of Environmental Quality forproving field location and auxiliary support for instrumentationand data, and in particular Ron Cunningham, Keith Iding, and EliMurphy. Additional thanks to Colm Sweeney of NOAA ESRL GMDfor CO2 data from Trinidad Head. We also are grateful to ToddRosenstiel, David Sailor, Ben Burnett, Chris Butenhoff, Linda George,Dean Atkinson, and Doaa Teama for their thoughtful discussions inanalysis and Mariela Brooks, Erica Hanson, J’reyesha Brannon, andPatrick Reynolds with help in manuscript preparation. Trans-portation data was provided by the Portland Regional Trans-portation Archive Listing (PORTAL) and Oregon Department ofTransportation (ODOT). Research support for this work wasprovided by a grant from the James F. and Marion L. Miller Foun-dation and PSU Center for Sustainable Processes and Practices.

References

Adams, S., Anderson, S., Cogen, J., 2009. City of Portland and Multnomah Countyclimate action plan 2009. City of Portland Bureau of Planning and Sustainabilityand Multnomah County Sustainability Program.

Andres, R.J., Boden, T.A., Marland, G., 2010. Annual Fossil-fuel CO2 Emissions: Massof Emissions Gridded by One Degree Latitude by One Degree Longitude. CarbonDioxide Information Analysis Center, Oak Ridge National Laboratory, U.S.Department of Energy, Oak Ridge, Tenn.

Coplen, T.B., 1995. Reporting of stable carbon, hydrogen, and oxygen isotopicabundances. In: Reference and Inter-comparison Materials for Light Elements.IAEA, Vienna, Austria, pp. 31e34.

Decker, E.H., Elliott, S., Smith, F.A., Blake, D.R., Rowland, F.S., 2000. Energy andmaterial flow through the urban ecosystem. Annual Review of Energy and theEnvironment 25, 685e740.

Duncan, A., et al., 2009. Report to the Legislature. Oregon Global Warming Com-ission, State of Oregon, Salem, Oregon.

EPA, 2010. Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008. U.S.Environmental Protection Agency, Washington, DC.

Fan, H., Sailor, D.J., 2005. Modeling the impacts of anthropogenic heating on theurban climate of Philadelphia: a comparison of implementations in two PBLschemes. Atmospheric Environment 39, 73e84.

George, K., Ziska, L.H., Bunce, J.A., Quebedeaux, B., 2007. Elevated atmospheric CO2concentrations and temperature across an urban-rural transect. AtmosphericEnvironment 41, 7654e7665.

Gratani, L., Varone, L., 2005. Daily and seasonal variation of CO2 in the city of Romein relationship with traffic volume. Atmospheric Environment 39, 2619e2624.

Grimmond, C.S.B., King, T.S., Cropley, F.D., Nowak, D.J., Souch, C., 2002. Local-scalefluxes of carbon dioxide in urban environments: methodological challenges andresults from Chicago. Environmental Pollution 116, S243eS254.

Idso, C.D., Idso, S., Balling, R.C., 2001. An intensive two-week study of an urban CO2dome in Phoenix, Arizona, USA. Atmospheric Environment 35, 995e1000.

Khalil, M.A.K., Rasmussen, R.A., 2004. Changes in the regional emissions of green-house gases and ozone-depleting compounds. Environmental Science & Tech-nology 38, 364e366.

Martilli, A., 2002. Numerical study of urban impact on boundary layer structure:sensitivity to wind speed, urban morphology, and rural soil moisture. Journal ofApplied Meteorology 41, 1247e1266.

Newman, S., Xu, X., Affek, H., Stolper, E., Epstein, S., 2008. Changes in mixing ratioand isotopic composition of CO2 in urban air from the Los Angeles basin, Cal-ifornia, between 1972 and 2003. Journal of Geophysical Research 113.doi:10.1029/2008JD009999.

Pataki, D.E., Bowling, D.R., Ehleringer, J.R., Zobitz, J.M., 2006. High resolutionatmospheric monitoring of urban carbon dioxide sources. Geophysical ResearchLetters 33. doi:10.1029/2005GL024822.

Pataki, D.E., Emmi, P.C., Forster, C.B., Mills, J.I., Pardyjak, E.R., Peterson, T.R.,Thompson, J.D., Dudley-Murphy, E., 2009. An integrated approach to improvingfossil fuel emissions scenarios with urban ecosystem studies. EcologicalComplexity 6, 1e14.

Proehl, R.S., 2008. 2007 Oregon Population Report. Population Research Center,Portland State University, Portland, Oregon.

Schauer, A.J., Lott, M.J., Cook, C.S., Ehleringer, J.R., 2005. An automated system forstable isotope and concentration analyses of CO2 from small atmosphericsamples. Rapid Communications in Mass Spectrometry 19, 359e362.

Sharp, J., Mass, C.F., 2004. Columbia gorge gap winds: their climatological influenceand synoptic evolution. Weather and Forecasting 19, 970e992.

Thoning, K.W., Tans, P.P., Komhyr, W., 1989. Atmospheric carbon dioxide at MaunaLoa Observatory 2. Analysis of the NOAA GMCC data, 1974-1985. Journal ofGeophysical Research 94, 8549e8565.

Velasco, E., Pressley, S., Allwine, E., Westberg, H., Lamb, B., 2005. Measurements ofCO2 fluxes from the Mexico City urban landscape. Atmospheric Environment39, 7433e7446.

White, J.W.C., Vaughn, B.H., 2009. Stable Isotopic Composition of AtmosphericCarbon Dioxide (13C and 18O) from the NOAA ESRL Carbon Cycle CooperativeGlobal Air Sampling Network, 1990-2007. University of Colorado, Institute ofArctic and Alpine Research (INSTAAR).