CICEET Progress Report for the period 08/01/01 through 01/31/02

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

Accomplishments
Scheduled Tasks:

1. Fabricate and test a new microporous hollow-fiber membrane module (MHFMM) based on experimental results of commercial system that was previously tested.
2. Integrate the new MHFMM with the aerosol collection system (ACS).
3. Present paper on instrument development at the American Association of Aerosol Research (AAAR) Annual Convention
4. Deploy instrument during a field intercomparison study for atmospheric ammonia/um measurements alongside several other collection and analysis methods for ammonia/um (e.g., filter pack methods, denuders, and impingers).

Progress on Tasks

1. A custom MHFMM was fabricated at Clarkson University, attempting to address the shortcomings of the previously utilized commercial MHFMM's. The parameters of the custom MHFMM were determined by utilizing the previously developed mathematical model for mass transfer optimization. Preliminary testing of the custom module was performed at Clarkson University using ion-selective electrode analysis.

   The custom MHFMM was brought to CBL in September 2001 for ammonia mass transfer testing using the long pathlength absorbance spectroscopy (LPAS) analytical system for ammonia/um. Gas-phase ammonia was introduced into the system at a constant rate through a bottled gas containing a known mixture of ammonia and clean dry air. This primary standard could then be diluted further by dry air to vary the gas-phase ammonia concentrations. Mass flow controllers were used to control the flow rates of the gases. The mass transfer coefficient was calculated by measuring the ammonium concentration within the aqueous phase of the system over time. Experimental data gathered throughout the testing failed to result in a definitive range of mass transfer coefficients for the MHFMM. Nitric acid mass transfer testing was not performed with the custom MHFMM due to the lack of definitive results with the ammonia testing.

2. The custom MHFMM was not integrated with the ACS due to the inability to adequately quantify the mass transfer coefficients (see above). Although we are not currently using the MHFMM to measure gas-phase ammonia concentrations, we have modified our design to allow for the measurement of ammonia (see below).

3. A paper was presented during a platform session at the American Association for Aerosol Research.
   "Development and Application of a Rapid and Robust Sensor to Measure Nitrogen Species in the Coastal Atmosphere", R. K. Larsen III, R. L. Siefert, J. E. Baker, K. Graves, and A. Zander, American Association of Aerosol Research (AAAR) Conference, Portland, Oregon, October 2001.

4. We have been involved in two field studies around the Chesapeake Bay airshed in the last 6 months. The first was a pilot study at a poultry house on the Eastern Shore of the Chesapeake Bay. This was the first time we deployed our instrument at a site characterized by highly variable concentrations of ammonia/um. The instrument collected data for less than one day but helped us prepare for another study at a dairy facility. The second study was an inter-comparison study at the USDA dairy facilities in Beltsville, Maryland. The focus of this inter-comparison study was to co-deploy several collection systems for measuring atmospheric gas-phase ammonia and particulate-phase ammonium. This provided us an excellent opportunity to test our instrument side by side with current state of the art techniques. The other measurement techniques co-deployed at the site included 1) a URG denuder - filter pack systems, 2) a honeycomb denuder - filter pack systems. 3) several different types of passive collectors and 4) a bubbler technique (i.e. impinger). Groups from the USDA, University of Delaware, and NOAA were the other study participants. This study occurred for about 2 weeks during late November and early December 2001.

Difficulties Encountered
At this time, we do not believe that the proposed MHFMM system can provide the sensor with a known constant removal rate of gas-phase ammonia and nitric acid within a continuous atmospheric sampling environment. The data that has been gathered through mass transfer experiments with the custom MHFMM does not result in a definitive range of mass transfer coefficients. It appears that the system used to analyze the mass transfer within the MHFMM cannot measure the results with consistency and reproducibility. This inconsistency has not been pinpointed to one factor ¨ it is believed that propagation of error within the measurement procedure and analysis is causing the primary difficulty.

Anticipated Success in Meeting Project Objectives in Scheduled Project Period
From our proposal:

"The overall objective of this proposal is to integrate several novel technologies into a robust field instrument for the near real-time, high temporal resolution (i.e., minutes) analysis of gas-phase and particulate-phase atmospheric nitrogen species relevant to the nutrient budgets in coastal waters (i.e.., NH₃(gas)/NH₄⁺(particulate), HNO₃(gas)/NO₃^-(particulate))."

We have met the overall objective of our proposal although we have modified the design due to difficulties that we encountered. We have built a system that can simultaneously measure gas-phase and particulate-phase atmospheric ammonia/um concentrations. The system can measure these two channels in near real-time with a temporal resolution of less than 10 minutes. Field experience with the instrument has shown that an ammonia/um preliminary concentration can be calculated within about minutes. The data can then be further refined to provide more precise and accurate values by incorporating instrument drift into the calculation of ammonia/um concentrations.

We had five specific objectives listed in the original proposal. These five objectives were steps in constructing a robust instrument that can measure ambient gas-phase and particulate-phase nitrogen species (i.e., NH₃/NH₄⁺ and HNO₃/NO₃^-) with a high temporal resolution. The main changes we have made from the original proposed design were 1) the use of a fluorescence method for analyzing ammonia instead of the proposed LPAS method for ammonia, 2) the use of a continuous mist chamber instead of a vapor condensation aerosol collection system, 3) and the modification of the instrument to a two channel system with a gas-phase ammonia scrubber on one inlet instead of the proposed MHFMM to transfer the gas-phase nitrogen species from ambient flow to an aqueous stream.

The modified system has been successfully deployed at numerous urban, rural and agricultural field sites in the Chesapeake Bay airshed. Figure 1 is a picture of the system setup in the rear of a minivan parked in a field just downwind of a poultry house on the Delmarva Peninsula. These field studies have all measured ammonia/um. We have successfully setup a nitrate/nitrite analytical technique based on long pathlength absorbance spectroscopy (LPAS) but have not yet field-tested this analytical system. However we are confident that switching the ammonia/um analytical technique with the nitrate/nitrite analytical technique will not be difficult.

Preliminary Results
During this past progress report period we participated in an inter-comparison field study at the USDA Dairy Facility in Beltsville, Maryland. The system was setup in a trailer located on a field a couple hundred meters from a dairy facility (see Figure 2). We anticipated highly variable ammonia/um concentrations dependent on meteorological conditions (wind direction, wind speed, temperature, atmospheric stability). The system was run continuously for 9 days at this field site. Daily calibrations and normal maintenance (e.g. tubing replacement) required a period of about 2 hours when the instrument did not collect ambient data. Figure 3 is a graph of total ammonia/um concentration versus time during this study. Concentrations ranged from less than 1 µg m^-3 to over 150 µg m^-3, a dynamic range of over two orders of magnitude. The system occupies about 8 cubic feet and has been deployed as a stand-alone instrument exposed to the elements. However, we have found that the instrument is easier to calibrate and maintain if placed in an environmentally controlled building (e.g., a field lab trailer).

Tasks and activities for next reporting period

Tasks for the next reporting period
Our task for the next reporting period is to finish and submit a manuscript describing the instrument.

Work plan to accomplish tasks

• Analyze honeycomb denuder and filter samples collecting concurrently with the continuous ammonia/um measurements.
• Investigate ammonia/um measurements between different sampling platforms
• Write the manuscript.

Concerns or difficulties
None.