1. 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

2. 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

3. Motivation
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.

4. Motivation: Quantifying Dry Deposition
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
Beem 2009

5. 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.

6. Sampling Train: CASTNET
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.
Particles
Teflon
NH4+, SO42-, NO3-, Ca2+, Cl-Mg2+, Na+, K+
Gases + Volatilized Particles
Nylon
HNO3, NO3-, SO42-

7. Sampling Train: IMPROVE
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.
Denuder removes HNO3
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:

8. Sampling Train: Chem. Speciation Network
Three channel system (one channel currently empty).
Non-extractable magnesium oxide denuder removes HNO3.
Pro: Simple, speciates nitrate particulate.
Con: Does not speciate ammonia/ammonium or measure HNO3, SO2 or volatilization.
MgO
HNO3 removed
Particles
Nylon
NH4+, SO42-, NO3-, K+

9. Proposed Sampling Train for CASTNET
Cationic Gaseous Species
Anionic Gaseous Species
Particles
Volatilized Particles
Pro: Speciates ammonia/ium, sulfate, nitrate, nitric acid, and can measure volatilized particulates.
Con: More expensive and time consuming than filter-only sampling.

10. Proposed Sampling Train for C.S.N.
Cationic Gaseous Species
Particles
Volatilized Particles
Focuses on differentiating ammonia and ammonium.
Reduced speciation ability for anionic species, but also reduced time and cost.

11. 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
not speciated
speciated
SO2

12. Requirements for the prototype denuder:
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.

13. SASS Denuder Prototype 1
Prototype 1 has two removable denuders that fit into a test tube for extraction, washing, and coating.
Detachable 2.5 μm cyclone
Flow
Denuder A
Denuder B
Filter 1
Spacing Ring
Filter 2
Denuder cartridge detail
SASS Sampling Setup
Assembled SASS Canister

14. 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. A B
Dynacalibrator
NH3 Generator
6.7 LPM
Dilution Flow Scrubber: HEPA, H2O, NH3, SO4
6.7 LPM A B

15. Results: Blanks 31 13
Figure 1: Blank values (micrograms) from various species comparing coated and uncoated SASS and URG denuders.
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.

16. Results: Collection Efficiency
Collection Efficiency = 1- (B/A)
URG calculated efficiency measured / %.
SASS efficiency measured / %.
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 3: Total load of ammonia captured by URG (left axis) plotted with SASS denuder efficiency (right axis).

17. Results: Total Load Capacity
Figure 4: Total ammonia load (A+B) on SASS and URG denuder systems run in parallel.
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:
μg/m3 over one day
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.

18. 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.

19. SASS Denuder Prototype 2
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
Denuder A
Denuder B
Filter 1
Filter 2
SASS denuder prototype 2
Keck, L. and Wittmaack, K. Aerosol Science 37 (2006) 1165 – 1173

20. Results: Blanks

21. Results: Blanks
1.5 LPM
1.75 LPM
2 LPM
2.5 LPM
Flow rates refer to the amount of nitrogen under which the samples were dried.
Average: / μg ammonia

22. Results: Collection Efficiency
Blank-corrected SASS ammonia collection efficiency: /- 5.07%

23. Results: Comparison to URG
Collection disparity unexplained.
Field comparison is more relevant to performance evaluation.

24. Conclusions: Prototype 2
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.

25. Ongoing Work 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.

26. Keck, L. and Wittmaack, K. Aerosol Science 37 (2006) 1165 – 1173
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
References
Keck, L. and Wittmaack, K. Aerosol Science 37 (2006) 1165 – 1173
Beem, K., Atmospheric Nitrogen and Sulfur Deposition in Rocky Mountain National Park. M.S. thesis. Atmospheric Science Department, Colorado State University, Fort Collins, CO
Ralph Oberg, “Rocky Mountain Way”