Developing and Testing Prototype Compact Denuders for Ambient Air Sampling

Applications

Misha Schurman (1), Jeffrey L. Collett, Jr. (1), Susanne V. Hering (2), Derek E. Day (3), William C. Malm (3), Brian Lee (4): (1)

Department of Atmospheric Science, Colorado State University, Fort Collins, CO; (2) Aerosol Dynamics Inc., Berkeley, CA; (3) Cooperative

Institute for Research in the Atmosphere (CIRA)/National Park Service, Colorado State University; (4) USEPA, Washington, DC

Outline

• Motivation• CASTNET Overview• Sampling Setups of the Major Networks• Proposed Sampling Trains for CASTNET and the

Chemical Speciation Network• Features of SASS Denuder Prototype 1• SASS Testing: Experimental Design• Results: Blanks, Collection Efficiency, Total Load Capacity• Conclusions: Prototype 1• Features of SASS Denuder Prototype 2• Results: Blanks and Collection Efficiency• Conclusions: Prototype 2

• Concentrations and speciation of atmospheric constituents such as sulfate, nitrate, nitric acid, and ammonia/ammonium are relevant to research areas such as acid deposition, aquatic chemistry, aerosol and cloud chemistry and formation.

• Currently, speciation and gas-phase quantification are poor in national networks .

• This leads to miscalculation of dry deposition because the deposition velocities of gas and particulate phases can be very different.

• Technologies exist to measure and speciate these constituents, but they are generally expensive, time-consuming, and fragile.

• The goal is to make denuders that allow us to collect more sophisticated data from existing national networks such as CASTNET and the Chemical Speciation Network.

Motivation

• About 25% of nitrogen deposition occurs via dry processes in both spring and summer.

• Dry deposition rates (average estimates, for spring/summer):– HNO 3(g) ~ 1.75 cm/sec NO 3(particulate) ~ 0.25 cm/sec

– NH 3(g) ~ 1.2 cm/sec NH 4(particulate) ~ 0.25 cm/sec

Spring

NO3 wet23.76%

NH313.75%NH4 dry

3.15%

ON wet16.65% NO3 dry

1.08%

HNO37.49%

NH4 wet34.11%

Motivation: Quantifying Dry Deposition

Summer

NO3 wet27.83%

NH316.48%

NH4 dry1.41%

NH4 wet34.24%

ON wet12.33%

HNO37.50% NO3 dry

0.20%

Beem 2009

CASTNET Overview

• The Clean Air Status and Trends Network (CASTNET) has 86 sites in rural and/or sensitive ecosystems.

• 27 of these sites are in national parks and other Class-I areas.

• CASTNET:– Aims to monitor ambient concentrations [C] and help to quantify

dry acidic deposition (D = [C]Vd).

– Measures sulfate, nitrate, ammonium, sulfur dioxide, and nitric acid, plus other pollutants such as ozone.

– Involves weekly samples for gaseous and particulate species on a three-filter cartridge.

Sampling Train: CASTNET

Particles

Teflon

NH4+,

SO42-,

NO3-,

Ca2+, Cl-

Mg2+, Na+, K+

Gases + Volatilized Particles

Nylon

HNO3, NO3-, SO4

2-

Gases

Cellulose: SO2

Pro: Simple, easier to ship and extract than denuders.

Con: Cannot distinguish between gas and volatilized particle for species such as ammonia/ammonium, nitrate, and sulfate.

• The IMPROVE network utilizes a four-channel sampling system on various single-filter substrates.

• Species relevant to dry deposition, including nitrate and sulfate, are collected through Channel B on Nylasorb filters.

Sampling Train: IMPROVE

Pro: Simple, speciates nitrate particles.

Con: Incomplete speciation of sulfur species; no ammonia species; no collection of volatilized particulates.

IMPROVE focuses on monitoring visibility/aerosol effects in sensitive areas such as Denali National Park

Figure and photo: http://vista.cira.colostate.edu/improve/Default.htm

Denuder removes HNO3

• Three channel system (one channel currently empty).

• Non-extractable magnesium oxide denuder removes HNO3.

Sampling Train: Chem. Speciation Network

MgO

HNO3 removed

Particles

Nylon

NH4+,

SO42-,

NO3-, K+

Pro: Simple, speciates nitrate particulate.

Con: Does not speciate ammonia/ammonium or measure HNO3, SO2 or volatilization.

• Pro: Speciates ammonia/ium, sulfate, nitrate, nitric acid, and can measure volatilized particulates.

• Con: More expensive and time consuming than filter-only sampling.

Proposed Sampling Train for CASTNET

Cationic Gaseous Species

Anionic Gaseous Species

Particles

Volatilized

Particles

• Focuses on differentiating ammonia and ammonium.• Reduced speciation ability for anionic species, but also

reduced time and cost.

Proposed Sampling Train for C.S.N.

Cationic Gaseous Species

Particles

Volatilized

Particles

Comparative CASTNET and SASS denuder analytical capabilities:

Species Current CASTNet SASS Prototype

Particulate

NH4+

SO42-

NO3-

Ca2+,Cl-,Mg2+

Na+,K+

Quantify volatilized particulates

---

Gaseous

NH3 ---

NO3- not speciated speciated

SO42-

SO2

The denuder must be:• Small – fit into existing SASS sampling canister.• Robust – must ship well.• Easily extractable – must coat and extract cleanly and

easily.

• Efficient – must collect above ~90% HNO3 and ~95% NH3.

• Sufficient in capacity for up to a week of sampling.• Cheap(ish) – to outfit a national network, the cost must

be low.

Requirements for the prototype denuder:

Prototype 1 has two removable denuders that fit into a test tube for extraction, washing, and coating.

SASS Denuder Prototype 1

FlowFlow

Denuder A

Denuder B

Filter 2

Filter 1Spacing Ring

Assembled SASS Canister

SASS Sampling SetupDenuder cartridge detail

Detachable 2.5 μm cyclone

SASS Testing: Experimental Design• Ammonia gas generated via Dynacalibrator permeation tube.• Sample flow diluted and split between URG and SASS systems [6.7 LPM]. All denuders were coated with a 1% phosphorous acid solution and dried under nitrogen.• Two denuders of each type were run in series. • Denuders A and B were extracted separately and analyzed via ion chromatography.• Because URG denuders have ~99% efficiency for ammonia and a large load capacity, the total load on the URG system (A+B) is assumed to represent the total amount of ammonia available to each system for a given sample.

Dynacalibrator

NH3 Generator

Dilution Flow Scrubber: HEPA, H2O, NH3, SO4

A B

6.7 LPM

6.7 LPMA B

Results: Blanks

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

SASSunco

ated

A

SASSunco

ated

B

URGunco

ated

A

URGunco

ated

B

SASScoate

dA

SASScoate

dB

URGcoat

edA

URGcoat

edB

mic

rog

ram

s

calcium

magnesium

potassium

sodium

Figure 1: Blank values (micrograms) from various species comparing coated and uncoated SASS and URG denuders.

31 13 blanks - ammonia

0

0.2

0.4

0.6

0.8

1

1.2

SASS0521

08SActB

LK

SASS0521

08SBctB

LK

SASS0521

08U2c

tBLK

SASS0521

08U6c

tBLK

mic

rog

ram

s

uncoated

uncoated

Figure 2: Coated ammonia blanks (red). Uncoated blanks showed no ammonia.

•SASS blanks contain more Ca2+, K+, and Mg2+ than URG blanks.•SASS extracted surfaces are physically handled, while URG surfaces are not.• Most likely, the difference in blank values is either contamination from handling or entrainment of room air into the drying apparatus.•Subsequent prototypes are sealed to the drying rack with Parafilm to prevent entrainment.

Results: Collection Efficiency

• Collection Efficiency = 1- (B/A)

Figure 3: Total load of ammonia captured by URG (left axis) plotted with SASS denuder efficiency (right axis).

0

200

400

600

800

1000

1200

0 1 2 3 4 5 6 7

Sample Run Number

Mic

rog

ram

s A

mm

on

ia

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

SA

SS

Co

lle

cti

on

Eff

icie

nc

y

Total Load fromURG

SASS efficiency

• URG calculated efficiency measured 99.3 +/- 0.76 %.

• SASS efficiency measured 89.8 +/- 14.5 %.

• SASS efficiency is decreased in the presence of high loads, indicating that SASS capacity may be low.

• Prototype 1 denuders have NOT achieved target (>95%) efficiency levels.

• SASS load capacity must be determined to evaluate the denuder’s applicability to outdoor sampling.

• Figure 4 shows under-measurement of ammonia by the SASS denuders.

• From Figure 3, we can estimate the maximum load on the SASS denuders that will allow the efficiency to remain above 90%:

• Conservative total load estimate = 200 μg; liberal total load estimate = 400 μg.

• This produces the ability to sample maximum average concentrations of:– 20.7-41.5 μg/m3 over one day– 2.9-5.9 μg/m3 over one week

• Since CASTNET samples for a period of one week, and the maximum sampling concentrations listed above are less than usual ambient concentrations, the SASS Denuder Prototype 1 has insufficient capacity for outdoor use.

Results: Total Load CapacityFigure 4: Total ammonia load (A+B) on SASS and URG denuder systems run in parallel.

0

200

400

600

800

1000

1200

1 2 3 4 5 6

Sample Run Number

Mic

rog

ram

s A

mm

on

ia

URG

SASS

Conclusions: Prototype 1

• Collection efficiency and load capacity do not meet sampling requirements.

• Blanks for K+, Mg2+, and Ca2+ are higher than desired.

• Delrin® (polyoxymethylene) denuder slide holders are incompatible with the phosphorous acid coating solution.

• Increased coating solution concentration to 5% phosphorous acid per Keck and Wittmaack 2006.

• Fixed slides in holder to increase capacity and reduce handling.• Capped denuder ends for extraction and coating to reduce handling

and contamination.• Switched materials to low density polyethylene to resolve

incompatibility with coating solutions

Keck, L. and Wittmaack, K. Aerosol Science 37 (2006) 1165 – 1173

SASS Denuder Prototype 2

SASS denuder prototype 2

Denuder A

Denuder B

Filter 2Filter 1

Results: Blanks

potassium: m = 1.9769

R2 = 0.81

sodium: m = -1.1163

R2 = 0.0041

magnesium: m = 4.4567

R2 = 0.1925

0

5

10

15

20

25

0 2 4 6 8 10

URG microNormal

SASS

mic

roN

orm

al

sodium

potassium

magnesium

y = 4.494x + 59.702

R2 = 0.0015

0

20

40

60

80

100

120

140

160

180

200

0 1 2 3 4 5 6

URG microNormalS

AS

S m

icro

Nor

mal

calcium

• Flow rates refer to the amount of nitrogen under which the samples were dried.

• Average: 1.73 +/- 0.78 μg ammonia

Results: BlanksSASS coated blanks: ammonia

0

0.5

1

1.5

2

2.5

3

3.5

1 2 3 4 5 6 7 8 9 10

coated blank number

mic

rog

ram

s am

mo

nia

ammonia

1.5 LPM 1.75 LPM 2 LPM 2.5 LPM

SASS and URG Ammonia Collection Efficiency

0.75

0.8

0.85

0.9

0.95

1

0 2 4 6 8 10 12 14 16 18

Sample Number

Am

mo

nia

Co

llec

tio

n E

ffic

ien

cy

URG

blank-correctedSASS

Blank-corrected SASS ammonia collection efficiency: 95.06 +/- 5.07%

Results: Collection Efficiency

Results: Comparison to URG

• Collection disparity unexplained.

• Field comparison is more relevant to performance evaluation.

• Efficiency is acceptable if we can get it to be more consistent.

• Coated blanks are too high, esp. for ammonia.– Reduces apparent efficiency– Increases limit of detection

• Materials seem compatible with sol’ns used.

Conclusions: Prototype 2

• Field URG/SASS comparison testing.• Developing protocols for field use and to reduce blanks.• Investigate the cause of efficiency inconsistency.

• Evaluation for HNO3 collection efficiency.

Ongoing Work

Acknowledgements

• Gregory S. Lewis at Aerosol Dynamics Inc. (denuder engineering)

• Met One Instruments, Inc. (denuder manufacture)

• Collett Group, especially Amy Sullivan, Florian Schwandner, Taehyoung Lee, & Leigh Patterson

Funding

53-4135 53-4107

Ralph Oberg, “Rocky Mountain Way”

References

Keck, L. and Wittmaack, K. Aerosol Science 37 (2006) 1165 – 1173

Beem, K., 2009. Atmospheric Nitrogen and Sulfur Deposition in Rocky Mountain National Park. M.S. thesis. Atmospheric Science Department,

Colorado State University, Fort Collins, CO 80523.