Detailed elemental characterization of Saharan

dust to quantify its contributions to PM2.5 and

PM10 during episodic intrusions in Houston

Shankar Chellam Department of Civil and Environmental Engineering

Department of Chemical and Biomolecular Engineering University of Houston

Texas Air Research Center, August 2014

Clinton Drive

Channelview

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Daily PM10

Clinton Drive

Daily PM10

Time series of ambient PM2.5 and PM10 mass conc.

2008

High PM levels on July 25-27, 2008 traced to N. Africa • Increases in PM concentrations were concomitant with satellite

images of Saharan dust intrusion to the greater Houston area

• 800 Tg of soil dust emitted annually from North Africa (estimated)

• Trade winds carry this crustal material across the Atlantic Ocean

reaching Texas few times a year largely during the summer months

• We want to quantitatively distinguish between locally-emitted

versus long-range transported particulate matter

• Enables us to rigorously exclude exceptional events that impact

local air quality but are beyond regulatory control

Aeolian resuspension from Northwest Africa on July 16, 2008

Dust reaches Houston on July 25, 2008

Dust laden Saharan Air Layer. Cumulus clouds can be seen.

(NASA SALEX mission 20060916n)

• Quantify episodic intrusions of Aeolian

dust from the Sahara-Sahel region to

Houston

• PM samples collected at two receptor

locations (Clinton Drive and Channelview)

were analyzed for 43 elements

• [PM10] remained high at one of the sites

(Clinton Drive) but not the other even

after the episode (more on this later)

Track and vertical profile of Saharan dust (CALIPSO)

Aerosol type from CALIPSO vertical feature mask: yellow is “dust” and brown is “polluted dust”

Total attenuated backscatter (532nm) along the path

July 27

Clinton Drive (29.73; -95.26)

Clinton Drive (29.73; -95.26)

Enrichment factors for Houston ambient PM

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NickelO

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PM2.5

Clinton Drive PM10

Clinton Drive

PM2.5

Channelview PM10

Channelview

Antimony

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Chromium

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t fa

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t fa

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(Ti in UCC)

Li

Be

Na

Mg

Si K

Ca

Sc

Ti

Mn

Cu

Zn

Ga

As

Se

Rb Sr

Zr

Mo

Cd

Sn

Sb

Cs

Ba

WP

b

Al V

Cr

Fe

Co

Ni Y

La

Ce

Pr

Nd

Sm

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Gd

Tb

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Er

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Th U

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Enrichm

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Se

Cd

Titanium used as crustal reference

Saharan dust collected in Barbados (Joe Prospero)

Enrichment factors for all elements w.r.t. upper continental crust (except Se and Cd) → 1

• Se and Cd emitted by marine biosphere

• Cd coatings used to reduce Al corrosion in marine environments

Ragged point

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Gasoline

monazite

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Cerium

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Lanthanum x 2.03

UCC

Refining catalyst

PM2.5

,PM10

Refining episode

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Cerium

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Lanthanum x 2.03

UCC

Refining catalyst

PM2.5

,PM10

Refining episode

Saharan dust

(Barbados)

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UCCSaharan dust

PM10 before/after event

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Lanthanum x 2.03

UCCSaharan dust

PM2.5 before/after event

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Cerium

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Lanthanum x 2.03

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Saharan dust

Event PM 10

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Saharan dust

Event PM 2.5

Lanthanoid ternary diagram: Long range transport

Chemical mass balancing of primary PM sources

• On average 50% and 72% of measured PM2.5 and PM10 were apportioned to

primary emissions. This is consistent with significant secondary aerosol

formation during the summertime in southeast Texas (especially PM2.5)

• Mineral dust sources (cement manufacturing and soil and road dust) were

dominant on non-Saharan days

• Of particular interest was that small amounts of Saharan dust was present even

on “routine” days

• On average Saharan aerosols accounted for 54% of PM2.5 mass and 63% of

PM10 mass during the 3-day episode

• Concentrations of Al, Si, Ca, Fe, and Ti were converted to their respective

mineral oxides to calculate the “reconstructed measured mineral mass”

• As seen next, chemical mass balancing and reconstructed measured mineral

mass showed significant contributions from cement manufacturing and local

soil for PM10 at Clinton Drive after the episode

Chemical mass balancing of primary PM sources

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Saharan dust

Soil and road dust

Cement manufacturing

Reconstructed measured mineral

material

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Local soil

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Ti x 21.25

Local soil Barbados Saharan dust Cement mfg

PM2.5

Saharan intrusion PM2.5

Routine

PM10

Saharan intrusion PM10

Routine

Al

Ca x 3.18

Mineral PM released from cement manufacturing

Concluding remarks

• Various lines of evidence demonstrated that dust-laden African air masses

reached Houston and overwhelmed local emissions in late July 2008.

• For the first time, we isolated, differentiated, and quantified relative

contributions of long-range transported dust from local and global mineral

dust sources through detailed measurements of a wide suite of elements in

ambient PM.

• Importantly, we were able to determine that local emissions of crustal

minerals dominated the period immediately following the Saharan dust

episode. Simple quantification of bulk crustal materials may have

misappropriated this elevated PM to trans-Atlantic transport of Saharan

dust.

• Although the effect of African dust stands out clearly during the peak event

on 25-27 July, there is evidence from chemical mass balancing that

advected African dust is also present as a “background” aerosol component

in Houston even during non-event days.

On-going work

• By advancing chemical analysis of PM, we better understand contributions

from different sources. Regulatory agencies can rigorously isolate and

quantify long-range transported dust to obtain exceptions when local PM

levels exceed federal standards under the “exceptional circumstances

category.”

• We have published our results from the first episode. Bozlaker et al.

Environmental Science and Technology 47 10179-10187.

• A second Saharan dust episode was identified in the Southern Texas region

(including Houston) between June 28 and July 2, 2008 using satellite

imagery and meteorology.

• We also have archived PM2.5 and PM10 filters from this period and five days

immediately preceding and succeeding this event. Similar laboratory

analysis and receptor modeling will be performed using these samples to

characterize and quantify a second Saharan dust episode.