CICEET Progress Report for the period 8/01/00 through 1/31/01

Project Title:

Development and Application of a Rapid and Robust Sensor to Determine Nitrogen Species in the Coastal Atmosphere

Principal Investigator(s):

Joel E. Baker
Ronald L. Siefert
Amy K. Zander

I. Accomplishments

A. Scheduled Tasks

  1. The ammonium and nitrate analytical methods will be tested for possible interferences. To test the methods we will use filter packs to collect ambient gas-phase and particulate-phase atmospheric samples for ammonia/um and nitric acid/nitrate at CBL. The filters will then be analyzed for ammonium and nitrate after extraction of the soluble ions in aqueous solution. Aliquots of these extracts will be analyzed using ion chromatography (IC) and the analytical techniques discussed

  2. The vapor condensation aerosol collection system (VCACS) will be tested. Initially, gas-phase ammonia will be used to determine the VCACS response time and dispersion through the system. The dispersion through the VCACS will limit the temporal resolution. We will also test the system using model aerosols (e.g., ammonium sulfate, ammonium nitrate). We will generate the aerosol using an atomizer or nebulizer with a diffusion dryer, and control the concentrations by dilution. We will use filter pack methods to calibrate the aerosol concentrations.

  3. Particle loss through the MHFMM will be investigated using an aerosol generation system.

  4. The analytical system developed by CBL will be integrated into the dual-phase mass transfer system at Clarkson University to enhance the MHFMM optimization process and to ensure equipment compatibility. The new analytical system will allow for more accurate measurement of low nitrogen species concentrations, typical of atmospheric conditions.

  5. Optimization of the MHFMM will be performed, where a target gas-phase removal efficiency of 98 percent will be reached. Optimal removal rates will be determined by changing sensitive parameters in the system (flow rates, pH, fiber geometry) and analyzing their effect on mass transfer. Other MHFMMs will be tested as necessary.

  6. Meet at Clarkson in early Nov 2000 to integrate MHFMM with VCACS. We have scheduled a meeting in early November to connect the hollow fiber module to the vapor condensation aerosol collection system (VCACS).

  7. Troubleshooting of the combined system will be performed. Concerns and difficulties arising from the integration of the MHFMM and VCACS with the analytical system will be addressed and solved.

  8. Preliminary samples will be taken with the combined system to verify equipment compatibility and to complete the troubleshooting process.

 

B. Progress on Tasks

  1. Testing for ammonia/um and nitric acid/nitrate interferences has not been done yet, although we have been testing the methods in the field. We recently acquired a new ion chromatography (IC) system and spent part of our time installing and learning how to use the system. We expect to start these interference tests shortly. Although we have not completely tested the systems for possible interferences, we have built several ammonia fluorometric systems and have taken these systems into the field to measure ammonia in both estuarine waters and open ocean waters.

    The ammonia fluorometric method uses a transversally illuminated liquid core waveguide (LCW) and has proven successful at meeting the objectives of the ammonia detector. Calibration curves for dissolved ammonium have been made down to 5 nM, although typical detection levels are on the order of 20 to 50 nM. The reagents for the ammonia fluorometric method are simple to work with and are stable for weeks. The LCW fluorometer is also relatively simple and inexpensive to construct and maintain. The LCW fluorometer also occupies less than 2' x 2' x2' of space making it highly portable and field deployable. The LCW fluorometer has been used to measure dissolved ammonium concentrations in a Chesapeake Bay subestuary aboard the University of Maryland’s 22' Pisces. Additionally, the LCW fluorometer was used to measure dissolved ammonium concentrations in the Atlantic Ocean during a NSF sponsored research cruise aboard the RV Seward Johnson. A LCW fluorometer was provide to Dr. Zander to assist in the development of the microporous hollow-fiber membrane unit.

  2. A vapor condensation aerosol collection system (VCACS) was assembled and tested. This system was borrowed from Prof. Dasgupta at Texas Tech University. Initial results found the residency time of the VCACS were on the order of tens of minutes that did not satisfy the design goals of an ammonium analyzer with response times on the order of minutes. The overall purpose of the VCACS is to remove particles from a large volume of air into a minimum volume of liquid eluent. An alternative method to reach the same goal is to pass the ambient air stream through a nebulizer and collect the resulting mist solution. Currently a commercially available atomizer is being modified to allow liquid eluent to flow through the mixing reservoir. A Teflon filter was placed across the atomizer outlet that allows the air stream to pass through, while causing the mist to collect and recycle back to the mixing reservoir. This modification will allow water to flow at ~ 1 ml/min. through the reservoir (~ 3 ml) to the LCW fluorometer. It is believed that this will reduce the response time to less than 10 minutes. Materials have been ordered for the construction of an aerosol generator to test the collection efficiency and residency time of this instrument. The response time and efficiency of the aerosol collection system (ACS) has not been done yet. We are in the progress of building an aerosol generation system needed to test the ACS. The aerosol generation system consist of a nebulizer to produce aerosols, a charge neutralizer (using alpha radiation from a Po210 source) and a diffusion dryer. The system is being designed to generate monodisperse aerosol populations of fluorescent tagged polystyrene latex microspheres (Duke Scientific) from an aqueous suspension of these microspheres. The system should also be able to produce polydisperse populations of model aerosols (e.g., ammonia sulfate, ammonia nitrate). Filter packs will be used to collect the aerosol to quantify concentrations of the aerosol generated. The fluorescent tagged polystyrene latex microspheres will be extracted from the filters and the extraction solution can be analyzed on a fluorometer to quantify aerosol concentrations.

  3. We have not investigated the loss of particles through the MHFMM because we are still in the process of building an aerosol generation system (see above)

  4. The ammonia fluorometry analytical system was integrated into the dual-phase mass transfer system at Clarkson University and calibrated to ensure equipment compatibility. This system replaced the previous analytical tool, an ion-selective electrode.

  5. Experiments involving mass transfer of gas-phase ammonia via the MHFMM were conducted to determine the removal efficiency of the module. Ammonia was introduced into the system at a constant rate through a bottled gas containing a known mixture of ammonia and clean air. Analysis of the mass transfer was conducted using the ammonia fluorometry analytical system developed by CBL that measured ammonium concentration within the aqueous-phase of the system over time. Optimization of the system was performed by varying aqueous flow rates at a constant gas flow rate of 10 L/min (see Figure_01.gif) as well as varying gas flow rates at a constant aqueous flow rate of 10 mL/min (see Figure_02.gif). In addition, the module parameters were integrated into the previously developed mathematical model to determine the accuracy of the model. By plotting both the measured and predicted gas-phase ammonia mass transfer coefficient versus the flowrate varied, it was seen that the trends are the same as expected from the prediction of the mathematical model. Diffusion through the bulk gas phase is the controlling resistance within the system, therefore gas flowrate will be the controlling variable for this module.

  6. We have delayed integrating the MHFMM with the aerosol collection system until later this spring to allow for the testing of the individual systems. The labs are in contact with one another on a regular basis and when CBL has finished constructing the aerosol generation system we will begin testing the MHFMM for particle loss and the ACS for collection efficiency.

7 & 8. We have not integrated the MHFMM and ACS yet.

C. Difficulties Encountered

Overall we are encouraged with our progress although we have not met all of our scheduled tasks for this progress period because of our ambitious schedule outlined in the previous progress report. We have not had any major difficulties with the design and construction of the aerosol collection system except that it is progressing slower than we had hoped.

The experimental data that has been measured using the mass transfer system for the Fiberflo MHFMM is consistently lower than predicted by the mathematical model developed during the first progress period. This is not an entirely unexpected outcome. It is in fact common for mass transfer experiments not to reach predicted optimum mass transfer. This could be due to a number of factors including variability in the estimation of the model input parameters, the packing geometry of the fibers within the module, and/or the local depletion of H+ ions within the aqueous solution at the sites of ammonia diffusion. We are planning to run experiments with constant air and water flow rates and varying pH values to determine if the H+ ion depletion is the main cause for the discrepancy between experimental data and model predictions.

The measured gas-phase removal efficiency provided by the FiberFlo MHFMM is currently less than the targeted removal rate of 98 percent. It appears that the target removal rate cannot be achieved using this particular Fiberflo MHFMM, so a larger Fiberflo module has been ordered and will be tested within the dual-phase mass transfer system. Initial calculations using the mathematical model indicate that the target removal rate should be met by the larger MHFMM.

D. Anticipated Success in Meeting Project Objectives in Scheduled Project Period

We still expect to meet the original project objectives in the scheduled project period.

E. Preliminary data

Figure 1 is a plot of the measured gas-phase ammonia mass transfer coefficient versus aqueous flow rate at a constant gas flow rate of 10 L/min.


Figure 2 is a plot of the measured gas-phase ammonia mass transfer coefficient versus gas flow rate at a constant aqueous flow rate of 10 mL/min.

 

II. Tasks and activities for next reporting period

A. Tasks for the next reporting period

  1. Complete construction of aerosol generation system.
  2. Test aerosol collection system (ACS) for efficiency and response time.
  3. Investigate particle loss through the MHFMM.
  4. Optimize the MHFMM for a target gas-phase removal efficiency of 98 percent.
  5. Replication studies will be performed with the optimized MHFMM for removal of nitric acid.
  6. Integrate MHFMM with aerosol collection system (ACS).

B. Work plan to accomplish tasks

  1. Complete construction of aerosol generation system. This system will be used to generate monodisperse aerosol populations of polystyrene latex microspheres and polydisperse model aerososol populations (e.g., ammonium nitrate, ammonium sulfate).
  2. The aerosol collection system (ACS) will be tested. Initially, gas-phase ammonia will be used to determine the ACS response time and dispersion through the system. We will also test the system using model aerosols (e.g., ammonium sulfate, ammonium nitrate). We will generate the aerosol using a nebulizer with a diffusion dryer, and control the concentrations by dilution. We will use filter pack methods to calibrate the aerosol concentrations.
  3. Particle loss through the MHFMM will be investigated using a monodisperse aerosol generation system with fluorescent polystyrene latex (PSL). Comparison analysis will take place using a control filter pack and a filter pack attached to the MHFMM.
  4. Optimization of the MHFMM will be completed, where a target gas-phase removal efficiency of 98 percent will be reached. Optimal removal rates will be achieved by changing sensitive parameters in the system (flow rates, pH, fiber geometry) and analyzing their effect on mass transfer. A larger version of the current Fiberfo MHFMM will be analyzed to achieve adequate ammonia removal.
  5. Replication studies will be performed with the optimized MHFMM for removal of nitric acid. The LPAS system will be used to analyze mass transfer utilizing the nitrate technique developed by CBL.
  6. Meet at CBL in late spring 2001 to integrate MHFMM with aerosol collection system (ACS).

C. Concerns or difficulties

None

 

III. Expenditures

Expenditures were in the range anticipated for the work accomplished to date.