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Atmospheric Environment 46 (2012) 430e440

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Atmospheric Environment

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

Retrieval of the single scattering albedo in the El Paso-Juarez Airshedusing the TUV model and a UV-MFRSR radiometer

Richard Medina a, Rosa M. Fitzgerald b,*, Qilong Min c

aComputational Science Program, Department of Mathematics, University of Texas at El Paso, Bell Hall #322 Wiggins St., El Paso, TX 79912, USAbDepartment of Physics, University of Texas at El Paso, Physical Science Building, 500 W. University Avenue, El Paso, TX 79968, USAcAtmospheric Sciences Research Center, State University of New York at Albany, 251 Fuller Road, Albany, NY 12203, USA

a r t i c l e i n f o

Article history:Received 8 February 2011Received in revised form13 September 2011Accepted 14 September 2011

Keywords:Aerosol optical propertiesUV-MFRSRDirect-to-diffuse irradiance ratioSingle scattering albedoAsymmetry parameterTropospheric ultraviolet and visible model

* Corresponding author. Tel.: þ1 915 747 7530; faxE-mail address: rfitzgerald@utep.edu (R.M. Fitzger

1352-2310/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.atmosenv.2011.09.028

a b s t r a c t

A methodology to retrieve Single Scattering Albedo (SSA) values employing the ratio of Direct to DiffuseIrradiances (DDR) is used and applied to the El Paso-Juarez Airshed, a challenging region where airmasses interact. The TUV model was used to obtain the calculated DDR irradiances, and the experimentalirradiances were obtained from a UV-MFRSR instrument located in the city of El Paso, Texas. Thewavelengths used were 332 nm and 368 nm. The retrieved SSA values at both 332 nm and 368 nm werehigher in a lightly polluted day (0.66e0.81 at 332 nm, and 0.61 to 0.80 at 368 nm) than in a heavierpolluted day (0.56e0.70 at 332 nm and 0.53e0.66 at 368 nm).

A sensitivity study of the ground albedo and the asymmetry parameter was performed, which indi-cated that the variation of the asymmetry parameter is a secondary effect in the retrievals of SSA. Inaddition, the variation of SSA values during the day was also analyzed for the El Paso-Juarez Airshed andlinked to the flow of air masses into the region using HYSPLIT trajectories. A presence of absorptiveaerosols was observed during the late morning and the middle of the day. This methodology can beapplied in any area, and is particularly useful for cities that experience episodes of high PMconcentrations.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

In recent years the study of atmospheric aerosols has becomevery relevant because of the impacts they have on air quality, healthand on Earth’s radiative balance. While solar radiation is essentialto life on earth, it can also be harmful at high energy levels or shortwavelengths. The range of UV-wavelengths where this radiationbecomes dangerous for Earth’s biosphere is 100 nme400 nm.Because of this concern many studies have been performed on thisresearch topic using both mathematical simulations and variousexperimental methods, e.g., remote sensing techniques usingground and satellite measurements.

Atmospheric aerosols play an important role in the attenuationof UV energy, but in large concentrations they can be harmful tohuman health. The ground albedo (ga) and some important aerosoloptical properties, such as the aerosol optical depth (AOD), thesingle scattering albedo (SSA), and the asymmetry parameter (g)provide us with a better understanding of the solar radiationpresent and its interaction with aerosols.

: þ1 915 747 5447.ald).

All rights reserved.

Retrieval of the SSA parameter is essential in characterizingaerosols present in the atmosphere. The single scattering albedoprovides information about the scattering and absorptive proper-ties of the aerosols. It is defined as

SSA ¼ bsbe

¼ bsbs þ ba

(1)

Where bs is the aerosol scattering coefficient and ba is the aerosolabsorption coefficient. Values of SSA range from zero in a purelyabsorbing medium to one in a purely scattering medium (Petty,2004). Dust and soot aerosols tend to have lower values of SSA(0.5e0.7), while sulfate aerosols have values closer to 1.0 (Reuderand Schwander, 1999).

The asymmetry parameter, g, represents the phase function. Itranges from 1 for scattered irradiance that is scattered in the forwarddirection to �1 when it is scattered in the backward direction. Forawavelength of 400 nm the lowest values of g are approximately 0.3for the soot-type aerosols, and the highest are approximately 0.9 fordust like components (Weihs and Webb, 1997).

Previous research work using the TUV model (Madronich andFlocke, 1998), and irradiance measurements to retrieve pollutantconcentrations and optical properties has proven successful in

R. Medina et al. / Atmospheric Environment 46 (2012) 430e440 431

other regions in the U.S., (Goering et al., 2005, and Slusser et al.,2001). Furthermore, studies using the direct-to-diffuse irradianceratio method to obtain values of SSA have been performed usingother radiative transfer models such as UVSPEC (Mayer et al., 1997).

This research work uses the radiative transfer model TUV andan Ultraviolet Multi-filter Rotating Shadowband Radiometer(UV-MFRSR) installed in the city of El Paso to retrieve SSA values.The direct and diffuse irradiances were calculated for the El Paso-Juarez Airshed using the Tropospheric Ultraviolet and Visiblemodel, TUV. Subsequently, the calculated direct-to-diffuse irradi-ance ratios (DDR) using the TUV model were compared againstthose obtained with the UV-MFRSR instrument to retrieve thecorresponding single scattering albedo (SSA). A sensitivity study ofthe relevant optical and physical parameters was also performed toassess the accuracy of this method. Finally, the variationthroughout the day of the retrieved SSA values was performed forthe El Paso-Juarez Airshed and linked to the flow of air masses,proving this method can be useful in studying aerosol loadingchanges in the atmosphere.

2. Methodology and description

2.1. Site description

The city of El Paso is situated at the extreme western tip of thestate of Texas at 1.202 km above sea level. It is the sixth-largest cityin Texas and the 19th-largest city in the United States. Itsgeographic coordinates are: latitude 31.86 (31�450 North), andlongitude �106.44 (106� 290 West). With a population of over600,000, the city is contiguous with the industrial city of Juarez(population over 1 million), Mexico and some adjacent suburbs inthe state of New Mexico. The combined urban area, known as ElPaso-Juarez Airshed, is one of the largest bi-national metropolitanareas in the world, and one of the fifty largest metropolitan areas inthe Western Hemisphere. The El Paso-Juarez metro area is isolated,being more than 500 km away from the nearest urban area ofcomparable size, thus making it an ideal location for air qualitystudies of an isolated large urban environment. The metro area isalso in the heart of the Chihuahuan Desert (area of 362,000 km2),which is otherwise sparsely populated. The climate is characterizedby well-defined seasons, with hot summers and cold winters, andwith an average of approximately 22 cm of annual precipitation.The metropolitan area contains the river valley of the Rio Grande(Rio Bravo del Norte), as well as the Franklin Mountains whichbisect the city of El Paso and the Sierra de Juarez in the city ofJuarez. This complex topography and location of the mountains andriver valley result in a constrained air basin where, on calm days,especially in the winter, anthropogenic air pollution is trapped overthe metropolitan area (Noble et al., 2003; Pearson and Fitzgerald,2001). On windy days, especially during the winter and earlyspring, mineral dust and sand blowing out of the surroundingdesert causes high particulate matter concentrations (Rivera et al.,2009). Both low-wind inversions and high-wind dust events lead toreduced visibility, occasional high exceedances of air quality regu-lations, and potential concerns for the health of the El Paso-Juarezresidents. The combination of a topographically-restricted urban-ized air basin, surrounded by a dust-producing desert, also makesthe El Paso-Juarez Airshed an ideal location for the study of aerosolsfrom different sources. Furthermore, the Air Quality in this region isamong the worst along the U.SeMexico border.

Since 1990 the El Paso-Juarez Airshed has been non-compliantwith U.S. Standards for particulate matter, with frequent days ofsevere air pollution. This is an important reason for pursuing airquality studies in this region. Optical studies, in addition to theirtechnical capability, have the advantage of not requiring special

permission from Mexican authorities to sample pollution in theadjacent city of Juarez.

2.2. Instrument description

A UV Multi-filter Rotating Shadow Band Radiometer(UV-MFRSR) was used for this study. The UV-MFRSR instrumentmeasures global, direct and diffuse solar irradiance, at 3-minintervals and up to 7 UV wavelengths: 300-, 305-, 311-, 317-,325-, 332-, and 368- nm with a full-width at half maximum ofabout 2.0 nm. It consists of a broadband channel together witha rotating shadow-band. The UV-MFRSR provides irradiancemeasurements which can be used to retrieve optical depth andcolumn ozone information in the UV-A and UV-B regions. The useof this type of instrument is well established in the scientificliterature. It is also known that an estimated uncertainty inaccuracy of the Langley calibrations ranged from �3.8% at 300 nmto �2.1% at 368 nm (Slusser et al., 2000). The total optical depth,st, is obtained using the UV-MFRSR for cloud free sample days. Theaerosol optical depth (AOD), sa, is extracted from the followingequation (Slusser et al., 2000):

sa ¼ st � s0 � sR � sw (2)

where s0 refers to the ozone optical depth, sR to the Rayleigh opticaldepth and sw to the water vapor optical depth.

In this research work two wavelengths, 332 nm and 368 nm(which are less affected by O3, NO2 and SO2 atmospheric gases), andtwo days with markedly different characteristics in the winter of2009 (January 28, 2009 and February 05, 2009) were selected forthe retrievals, as described below.

2.3. The tropospheric ultraviolet and visible model- parameterinputs

The tropospheric ultraviolet and visible radiative transfer modelTUV4.4, was used to solve the radiative transfer equation using thediscrete ordinate method in the 4-streammode, and assuming onlya downward radiation.

For the calculations, the atmosphere was considered in thealtitude range from 0 to 50 km and the number of layers along thevertical direction used was also 50.

The surface pressures used were 885.4 mbar (88,540 Pa) forJanuary 28, 2009 and 887.2 mbar (88,720 Pa) for February 05, 2009,provided by Dave Novlan, from NWS/NOAA.

The reflection of radiation by the surface is characterized by theground albedo. For many surfaces in the UV, except for snow, theground albedo lies between 0.0 and 0.2. Some authors have foundvalues for the ground albedo to be between 0.01 and 0.03(Blumthaler and Ambach, 1988; Diffey et al., 1995; Feister andGrewe, 1995; McKenzie et al., 1996). Kylling et al. (2000) madestudies in Tromsø, Norway and found maximum values of 0.08 and0.16 for 320 nm and 450 nm respectively in completely snow-freeconditions. Finally, Corr et al. (2009) have used a fixed groundalbedo of 0.06, in addition to spectrally varying ground albedosduring the MILAGRO field campaign in Mexico.

Forour regionweusedvalues for thegroundalbedoof0.05, for thelightpolluteddayand0.06 for theheavierpollutedday. Theseaveragevalues were obtained using the Nimbus7-TOMS satellite data.

The value used for the asymmetry parameter, g, was 0.70, for theselected wavelengths of 332 nm and 368 nm through all the alti-tudes in accordance with the value used in some previous studies(Petters et al., 2003).

Finally, the vertical aerosol profile used for scaling was Elter-man’s, (Elterman, 1968) and it is included in the model.

R. Medina et al. / Atmospheric Environment 46 (2012) 430e440432

2.4. Retrieval of SSA

It is well known that irradiance values are more sensitive to AODvalues (Corr, 2008) than to any other parameters. Sensitivity studiesof relevant aerosol optical parameters, performed in this work, alsoreveal that changes of AOD have a greater impact on irradiancevalues than other atmospheric parameters. Consequently, in ourcalculations instantaneous experimental AOD values were usedrather than average values. These instantaneous AOD values wereobtained directly from the UV-MFRSR at three-minute intervalsusing the Langley calibration as described in Slusser et al. (2000).

In order to select the values of SSA, the calculated DDR irradi-ances from the TUV model were compared with those obtainedfrom the UV-MFRSR instrument. The criterion we used was toretain only those differences between the calculated and theexperimental DDR values corresponding to less than 1% (see Fig. 1)to retrieve the corresponding SSA values.

2.5. Criteria for day selection

The retrieval of the single scattering albedo for the El Paso-Juarez Airshed was performed under two different day scenarioswith different pollution concentration. The criteria for the dayselection was based on the concentrations of PM2.5 (monthlysummaries of hourly average data) provided by the TexasCommission for Environmental Quality (TCEQ) for the city of ElPaso. In addition, the days selected for this research include cloud-free days and having complete instantaneous optical depth data.

The concentrations of PM2.5 are presented in Table 1, and Figs. 2and 3. Table 1 shows PM2.5 data along with the correspondingstatistics for the monitoring station CAMS 12 (Continuous AmbientMonitoring Station), located at the University of Texas at El Paso(UTEP). This site was selected because of its proximity to theUV-MFRSR shadow-band radiometer.

January 28 was one of the lowest polluted days of the season; ithad an average PM2.5 concentration of 4.97 mg m�3. This day inparticular was almost cloud-free and all the data was high quality.In contrast, February 5 was one of the most polluted days of the

Fig. 1. Flow chart for retrieval of SSA. The figure shows the procedure for retrie

season (18.8 mgm�3) and with the highest 1-h PM2.5 concentrationfor the entire season (89.69 mg m�3).

The following Figs 2 and 3, show an increase-decrease trend ofmonthly concentration levels. In Fig. 2, January 28 has one of thesmallest average concentrations of PM2.5 for that month. In Fig. 3,the highest average concentration of PM2.5 for February is for the5th day. Consequently, January 28th and February 5th of 2009 werechosen as representative days for low and high polluted dayscenarios.

The values of the AOD at 340 nm (i.e. a wavelength in betweenthe selectedwavelengths) in Elterman’s profile, (Elterman,1968), at1 and 2 km high are 0.106 and 0.0456. The TUV model cuts thisprofile at the surface altitude (1.2 km) and uses an average value forthe whole layer. The estimated value of the AOD at the surface isaround 0.07. Assuming the value of 0.07 could correspond toa relatively clean atmosphere, the selected days are polluted if thevalues of their AOD are greater than 0.07. Using the PM2.5concentrations and the above criteria, the days of January 28, 2009(low polluted) and the day of February 05, 2009 (higher polluted),were selected for the retrieval of SSA values. In addition to theabove, only those days having a complete data set of instantaneousoptical depth were selected for the retrieval.

The days selected were also virtually free of clouds, which is anissue that must be taken into account because of the high value ofoptical depth observed in clouds (Liao et al., 1999, and Luccini et al.,2003).

2.6. Sensitivity and uncertainty analysis

The irradiances and AODs produced by the UV-MFRSR are mostaccurate at small Solar Zenith Angles, SZA, (less than 70�), due toincreases in cosine correction errors with increases in SZA (Krotkovet al., 2005a). The SSA retrievals were made at times with SZA lessor equal to 65�.

Values of total Ozone column (TOC) were selected from themeasurements of the UV-MFRSR radiometer. It is known thatthe Ozone absorption is more relevant in the UVB region(280 nme315 nm) than other UV regions, and therefore will not

ving SSA using irradiance values from UV-MFRSR and from the TUV model.

Table 1Selected day statistics.

Jan, 09 Statistics Feb, 09 Statistics

Day Max SH Min Avg STD Day Max SH Min Avg STD

1 23.79 23.53 2.03 10.87 7.3 1 18.76 16.83 3.41 8.40 3.82 18.49 17.09 1.38 5.84 5.4 2 28.17 14.18 2.64 8.79 4.83 54.56 42.39 1.93 11.05 12.6 3 24.13 17.66 2.18 8.99 4.84 41.01 31.52 1.30 9.60 9.9 4 17.00 12.20 3.15 6.45 3.35 17.50 12.66 1.70 6.05 4.0 5 89.69 67.77 5.15 18.80 20.06 6.31 5.86 0.62 3.39 1.5 6 40.48 20.38 2.39 10.17 8.27 8.54 5.35 0.10 3.10 1.7 7 19.36 16.53 2.61 9.31 4.08 12.36 11.37 0.17 3.76 2.9 8 28.90 15.51 1.09 8.66 6.29 10.84 6.85 1.58 3.97 2.0 9 8.57 6.61 �0.02 2.55 2.010 58.68 55.54 0.75 12.05 16.4 10 29.98 27.33 1.79 8.27 7.611 12.70 8.97 �0.03 4.23 3.1 11 32.87 27.27 0.66 6.61 8.612 8.97 8.92 0.21 3.97 2.3 12 18.29 18.17 3.35 9.02 4.713 35.26 31.30 2.74 8.13 8.0 13 37.79 23.02 0.41 8.57 9.014 50.58 47.63 1.69 15.28 14.5 14 21.28 14.50 0.26 7.85 4.815 69.64 25.64 2.39 12.48 13.3 15 14.40 12.95 0.77 5.57 3.516 55.73 36.88 3.98 12.82 12.2 16 21.59 19.26 2.18 8.52 4.817 18.55 15.37 3.32 10.36 4.0 17 14.12 13.87 1.97 8.37 2.918 40.54 32.94 0.50 9.96 9.5 18 21.66 17.66 0.09 5.21 5.619 27.91 21.42 0.73 8.97 6.8 19 46.23 14.61 0.09 6.77 9.120 24.92 23.29 7.28 12.68 4.3 20 19.87 15.43 1.98 7.30 4.621 56.39 33.43 2.32 19.10 11.5 21 15.09 12.09 4.19 7.47 2.822 20.47 19.70 5.76 12.24 4.0 22 34.81 28.22 3.00 10.14 8.323 31.35 19.45 2.50 8.51 5.9 23 31.48 29.44 5.68 16.59 6.824 11.41 11.03 1.04 6.80 3.0 24 22.52 21.28 1.91 8.16 6.125 35.25 29.09 �0.02 7.79 8.8 25 21.94 14.31 2.61 5.79 4.626 20.22 19.97 1.68 6.67 4.8 26 11.98 9.55 1.67 4.89 2.527 7.63 7.12 1.38 4.42 1.6 27 40.29 29.87 2.24 9.35 8.928 12.74 12.69 0.44 4.97 3.5 28 23.78 17.68 3.76 8.54 5.029 21.85 20.62 2.90 8.76 4.730 64.69 46.73 0.64 13.21 15.831 21.31 17.93 0.55 9.54 5.5

Bold values are the maximum average values of PM2.5 concentrations for each month.

R. Medina et al. / Atmospheric Environment 46 (2012) 430e440 433

significantly affect the DDR irradiances at the wavelengthsselected for this study. Barnard et al. (2003), established thatvariations of tropospheric ozone do not affect significantly the UVradiation when compared to stratospheric ozone. However,tropospheric ozone may still impact to some degree the absorp-tion process, which can introduce uncertainties to the retrievalmethodologies used. Using the TUV model for testing purposeswe observed that there were not significant changes in the valuesof DDR irradiances, or in the SSA retrievals, when modifying thevalues of TOC for the wavelengths of 332 nm and 368 nm. Fig. 4and Fig. 5 depict the columnar ozone for our region during the

Fig. 2. Average concentrations of

months of January and February of 2009 respectively. Thesegraphs help us understand the variations of columnar ozone forour region. Our calculations indicate that changes of 20 DU (thecalculated standard deviation) do not affect the SSA retrievals forthe wavelengths used.

The column values of NO2 were also varied by a factor of 0.1 DUin the input values of the TUVmodel, and observed that this gas didnot significantly change the DDR irradiances or the SSA retrievals. Itwas observed that variations in SSA retrievals were at most lessthan 0.1% when changes of 10% DU were made in the NO2 columnvalues.

PM2.5 for January 28, 2009.

February 2009

Ave

rage

PM

2.5

( g/

m3 )

El Paso, Texas

Minimum = 2.55

Maximum = 18.8

01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 280

5

10

15

20

25

Fig. 3. Average concentrations of PM2.5 for February 05, 2009.

January 2009

O3 C

olum

n (D

U)

El Paso, Texas

22 23 24 25 26 27 28 29 30 310

50

100

150

200

250

300

350

O3 average = 258.5

STD = 20.86

Fig. 4. Ozone column for January 28, 2009.

R. Medina et al. / Atmospheric Environment 46 (2012) 430e440434

Similarly, changes in sulfur dioxide (SO2) did not affect the SSAretrievals. There was no noticeable change in the values of DDR orin the SSA retrievals when the input values of SO2 were varied bya factor of 10%.

Feb

O3 C

olum

n (D

U)

El P

02 03 04 05 06 07 08 09 10 11 12 13 140

50

100

150

200

250

300

350

400

Fig. 5. Ozone column for

The value of the asymmetry parameter, g, equal to 0.7 was usedfor all the retrieved SSA values based on previous studies (Petterset al., 2003). Furthermore, a sensitivity study for g was performedto assess the effects of this input parameter. It was concluded that

ruary 2009

aso, Texas

O3 Average = 272.4

O3 STD = 18.05

15 16 17 18 19 20 21 22 23 24 25 26 27 28

February 05, 2009.

10:00 10:30 11:00 11:30 12:00 12:30 13:00 13:30 14:00 14:30 15:00 15:300.50

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

Local time

Sing

le S

catte

ring

Alb

edo

El Paso, TXFebruary 05, 2009

g = 0.65g = 0.70g = 0.75

332 nm

Fig. 6. Sensitivity study of g at 332 nm.

R. Medina et al. / Atmospheric Environment 46 (2012) 430e440 435

a change in value of g did not affect significantly the retrieved SSAvalues. It was observed that changes of 5% in g, around the fixedvalue of 0.7, produced a variation of 1% in SSA retrievals. A repre-sentative sample of this study is shown in Fig. 6 for the morepolluted day scenario of February 05, 2009.

Changes in AOD, particularly higher AOD values, have a greaterimpact on irradiance values than other atmospheric variables.Figs. 7 and 8 are representative samples of the effect on the direct todiffuse ratio for different AOD and SSAvalues for thewavelengths of332 nm and 368 nm respectively. Consequently, instantaneousexperimental AOD values were used rather than average AODvalues in all of our numerical calculations.

Alternative vertical profiles were also used in our calculations ofDDR Irradiance values, and these showed no discrepancy with thestandard Elterman’s vertical profile.

A value of ga equal to 0.05 was used for the clean day, anda valued of 0.06 was used for the heavier polluted day. These valueswere obtained using satellite data for our region. Fig. 9 shows howthis parameter affects the SSA retrieval. Changing ga in one percent

0.1 0.2 0.3 0.4 0.5

0.50

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

A

SS

A

Direct to Di

1.5

1.0

1.0

1.0

Fig. 7. Sensitivity study

affects the SSA retrievals in the same manner. However, the varia-tion of ga in the UV regime is known to be minimal.

3. Results and discussion

3.1. SSA retrieval results

Retrieved values of SSAwere acquired during the day. Change ofAOD values reflect the level of pollution during the days selected forthe retrievals. Our results are shown in Table 2.

The retrieval values of SSA at 332 nm ranges from 0.66 to 0.81,and from 0.61 to 0.80 at 368 nm, for January 28, 2009. The retrievalSSA values for February 5 at 332 nm ranges from 0.56 to 0.70 andfrom 0.53 to 0.66 at 368 nm, as illustrated in Table 2. Previouscalculations, at different locations, using inversion methods for thewavelengths, 332 nm (Petters et al., 2003), and 368 nm (Wetzelet al., 2003, and Krotkov et al., 2005b), and recently Corr et al.(2009) are shown in Table 3.Figs. 10 and 11 show the retrievals ofSSA for 332 nm and 368 nm respectively for the low polluted day.

0.6 0.7 0.8 0.9 1.0

OD

ffuse Ratio, 332nm

0.5

0.5

of AOD at 332 nm.

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

0.50

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.0

AOD

SS

A

Direct to Diffuse Ratio, 368nm

2.5

2.0

2.0

1.5

1.5 1.0

1.0

0.5

2.0 1.5 1.0

Fig. 8. Sensitivity study of AOD at 368 nm.

13:30 13:40 13:50 14:00 14:10 14:20 14:30 14:40 14:50 15:00 15:10 15:20 15:30

0.63

0.64

0.65

0.66

0.67

0.68

0.69

0.70

0.71

Local time

Sing

le S

catt

erin

g A

lbed

o

El Paso, TX, January 28, 2009 Values at 368nm

ga=0.04

ga=0.05

ga=0.06

Fig. 9. Sensitivity study of ga, at 368 nm.

Table 3Retrieved SSA values in former studies using DDR modeling and UV-MFRSRradiometer.

R. Medina et al. / Atmospheric Environment 46 (2012) 430e440436

3.1.1. Variation of SSA values during the dayFigs.12 and 13 are representative samples of the variation in SSA

values throughout the higher polluted day for wavelengths of332 nm and 368 nm respectively. A depression in the SSA curveswas observed from 10:30 a.m. until 1:30 p.m., which representedan increase in the presence of absorptive aerosols in the El Paso-Juarez Airshed. Similarly, a milder depression is shown for thelow polluted day on Figs. 14 and 15 at 332 nm and 368 nm. It is

Table 2Retrieved values of SSA-values of SSA for the selected days with their correspondingvalues of AOD.

Date l (nm) SSA range AOD range AOD average

January 28, 2009 332 0.66e0.81 0.072e0.308 0.097January 28, 2009 368 0.61e0.80 0.069e0.311 0.094February 05, 2009 332 0.56e0.70 0.100e0.264 0.150February 05, 2009 368 0.53e0.66 0.091e0.227 0.142

observed that although the higher and the lighter polluted daysexhibit some similarities in the graphs of the variation of SSAduring the day, yet some differences are seen as a consequence ofthe air masses coming from different directions.

Author l (nm) Location, Date SSA g ga

Corr et al., 2009 332 Mexico City,March, 2006

0.75e0.80 0.60 0.06368 0.76e0.82 0.75

Petters et al., 2003 332 Western NorthCarolina,JuleDec, 1999

0.77e0.97 0.7 0.04368 0.80e0.99

Wetzel et al., 2003 368 Poker Flat, Alaska,MareApr, 2001

0.63e0.95 0.7 Variable

Krotkov et al., 2005b 368 Greenbelt, MD,Summer 2003

0.88e0.95 0.7 0.028

06:00 09:00 12:00 15:00 18:00

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Local Standard Time

Direct to

D

iffu

se Irrad

ian

ce R

atio

January 28, 2009, El Paso, Texas

MFRSR

SSA=0.0

SSA=0.1

SSA=0.2

SSA=0.3

SSA=0.4

SSA=0.5

SSA=0.6

SSA=0.7

SSA=0.8

SSA=0.9

SSA=1.0

Fig. 10. Retrieval of SSA for low polluted day at 332 nm.

R. Medina et al. / Atmospheric Environment 46 (2012) 430e440 437

3.2. Analysis of air masses

Fig. 16 shows the HYSPLIT (Draxler and Rolph, 2003) backward-trajectory for the polluted day. It is observed that the air parcels ateach of the three predetermined heights came from the south of ElPaso, Texas, with a slow flow at the end of the trajectories directlyacross industrialized Juarez and into El Paso. It was inferred that thehigh particulate concentrationwas due primarily to influxes from thecity of Juarez urban region, which correspondedmostly to absorptiveaerosols, e.g., soot, in agreement with the observations in Figs.12 and13. One of the main contributors of soot in the region, is the use ofscrap tires as a source of heat for improvised furnaces during thewintermonths, primarily in the pauper zones of Ciudad Juarez and bytraditional brickmanufactureswhich frequently burn the tires at hightemperatures in kilns (Blackman and Palma, 2002). Anothercontributor is thewoodburningparticulatematter (Murr et al., 2006).

06:00 09:00 12

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Local S

Direct to

D

iffu

se Irrad

ian

ce R

atio

January 28,

Fig. 11. Retrieval of SSA for low

In addition, during both summer and winter, the PlanetaryBoundary Layer (PBL) height in our region is lowest at midnight orearly morning and highest during late afternoon. In general, in ElPaso PBL height increases gradually throughout the morning,approximately after 9 a.m., and reach its highest around 3e4p.m.local time. It is usually after 9 a.m. that the boundary layer startsexpanding and the inversion breaks up, wind speeds increase, andthere is increased concentration of dust and soot aerosols over ElPaso, including that from local urban sources and the desert (Cahillet al., 2009). There is also a significant amount of dust re-suspendedfrom unpaved roads, especially in Juarez, where the majority ofroads are unpaved. This may also be advected into the city of El Pasofrom Juarez after the local wind speeds increase and allow trans-port of pollutants after mid to late morning.

It should also be noted that the emission of soot occurring in thevicinity of highways and bridges builds up and peaks between 7

:00 15:00 18:00

tandard Time

2009, El Paso, Texas

MFRSR

SSA=0.0

SSA=0.1

SSA=0.2

SSA=0.3

SSA=0.4

SSA=0.5

SSA=0.6

SSA=0.7

SSA=0.8

SSA=0.9

SSA=1.0

polluted day at 368 nm.

09:30 10:00 10:30 11:00 11:30 12:00 12:30 13:00 13:30 14:00 14:30 15:00

0.58

0.60

0.62

0.64

0.66

0.68

0.70

Local time

Sing

le S

catte

ring

Alb

edo

El Paso, TXFebruary 05, 2009

332 nm

Fig. 12. Variation of SSA during the day for heavier polluted day at 332 nm.

09:30 10:00 10:30 11:00 11:30 12:00 12:30 13:00 13:30 14:00 14:30 15:00 15:300.52

0.54

0.56

0.58

0.6

0.62

0.64

0.66

Local time

Sing

le S

catte

ring

Alb

edo

El Paso, TXFebruary 05, 2009

368 nm

Fig. 13. Variation of SSA during the day for heavier polluted day at 368 nm.

08:00 08:30 09:00 09:30 10:00 10:30 11:00 11:30 12:00 12:30 13:00 13:30 14:00 14:30 15:00 15:30 16:00 16:300.66

0.68

0.70

0.72

0.74

0.76

0.78

0.80

Local time

Sing

le S

catte

ring

Alb

edo

El Paso, TexasJanuary 28, 2009

332 nm

Fig. 14. Variation of SSA during the day for lighter polluted day at 332 nm.

R. Medina et al. / Atmospheric Environment 46 (2012) 430e440438

07:00 07:30 08:00 08:30 09:00 09:30 10:00 10:30 11:00 11:30 12:00 12:30 13:00 13:30 14:000.60

0.62

0.64

0.66

0.68

0.70

0.72

0.74

0.76

0.78

0.80

Local time

Sing

le S

catte

ring

Alb

edo

El Paso, TexasJanuary 28, 2009

368 nm

Fig. 15. Variation of SSA during the day for lighter polluted day at 368 nm.

R. Medina et al. / Atmospheric Environment 46 (2012) 430e440 439

and 9 a.m. There is a delay while this is transported from down-town El Paso, and especially from Juarez, to the receptor monitoringsites in El Paso and to our instrument. Juarez, being a bigger city andwith less stringent pollution controls, has a greater amount ofabsorbing aerosols emitted than El Paso. It would be expected thatthese aerosols, originating in Juarez, would be detected at themonitoring sites in El Paso and to our instrument after 10 a.m., afterthe inversion over the El Paso-Juarez Airshed breaks down, theboundary layer expands, and mesoscale winds enhanced by theterrain cause local transport of aerosols. Cahill et al. (2009) ina study of aerosols in another part of the Paso del Norte (El Paso-Juarez) metropolitan area detected such a lag effect of

Fig. 16. HYSPLIT trajectory analysis for heavier polluted day.

topographically forced winds causing a slow drainage of highlyabsorptive aerosols from the direction of Juarez to monitoring sitesin El Paso County.

The HYSPLIT backward-trajectory is shown on Fig. 17 for the lowpolluted day, January 28, 2009. It is observed that the air parcels forthis case scenario came from the northwestern part of the U.S.Aerosols being transported from the northwest on an otherwiseclean (relatively light winds) day would still tend to experiencesome topographic funneling down the valley of the Rio Grande andpass across the western El Paso metropolitan area immediatelybefore arriving at the receptor site. The valley they would traversehas a population of few hundred thousand people as well as

Fig. 17. HYSPLIT trajectory analysis for lighter polluted day.

R. Medina et al. / Atmospheric Environment 46 (2012) 430e440440

agricultural, rural activities, some industry, and a major trans-portation corridor, immediately upwind of the receptor site. Thus,some absorbing aerosols (but a lesser amount than when the aircomes from Juarez and the rest of El Paso) would be expected to befound in this situation, as observed with the milder depression ofthe SSA values in Figs. 14 and 15.

4. Conclusions

The retrieved values of SSA for the low polluted day (0.66e0.81at 332 nm and 0.61e0.8 at 368 nm) and for the polluted day(0.56e0.7 at 332 nm and 0.53e0.66 at 368 nm) were successfullyobtained for the El Paso-Juarez Airshed using the DDR irradiancemethod and are in agreement with previous studies. In addition,the presence of both moderately absorbing and absorptive particleswas observed in the Airshed, which is symptomatic of a complexinterface region such as the El Paso-Juarez Airshed, locatedbetween an urban and a rural site and surrounded by the Chihua-huan desert. An increase in the concentration of absorptive aerosolsduring the late morning and middle of the day is observed in theatmosphere, especially for the polluted day case, as shown in thegraphs of the variation of the SSA values throughout the day, whichcorrelated well with the analysis of the air masses flow in the sameday. It was also observed in the uncertainty analysis that changes of5% in g produced a variation of only 1% in the retrieved SSA values,denoting that g is a secondary effect in the retrieval of SSA. It wasfound that the TUV model can be used as a diagnostic model tointerpret UV-MFRSR irradiance data and successfully to retrieve thesingle scattering albedo in this region. The studies performed inthis work will have an impact on improving the air quality for thisand similar high PM polluted regions.

Acknowledgments

The authors wish to express their immense gratitude to Dr. JohnDavis and Dr. James Slusser, Chelsea Corr, and to Dr. Dave Dubois forvery helpful advice in this work, to Angel Esparza for providing uswith some pollution data and to Roger Tree for help with theinstallation of our instrument. This work was supported by NOAAthrough the Educational Partnership Program for Minority ServingInstitutions (EPP/MSI) Cooperative Agreement NA17AE1623 andpartially by a grant for bi-national air studies from the NewMexicoHealth agency.

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