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.