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. 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).

1. Construction of aerosol generation system was completed. The system is able to disperse fluorescent polystyrene spheres (Duke Scientific, Inc.) from an aqueous solution into an aerosol. The system uses a nebulizer and drying chamber to disperse the solution and then dry the particles. These model aerosols were used to test the aerosol collection efficiency of the ACS for different aerosol size classes.
2. Duke Scientific Inc. fluorescent 0.5 micron spheres were dispersed in the aerosol generation system and then used to determine the efficiency of the aerosol collection system. Results greater than 95% were obtained using this size class. Two additional size cuts (1 and 2 microns) remain to be tested.
3. The MHFMM are used to collect gas-phase species and not particles. However, there is always some particle loss through the MHFMM and the modules were tested to make sure the loss was minimal. Particle loss experiments through the MHFMM were performed using 0.5 micron spheres as reported in section b. Three MHFMMs with different membrane materials were evaluated. Particle loss experiments showed that the commercially-made Fiberflo MHFMM retained a significant portion of the aerosol. Once it was determined that the Fiberflo MHFMM would not be adequate for the project, a custom MHFMM was fabricated at Clarkson University. Model calculations indicated that sufficient mass transfer would occur for a module that was 50 cm long with a diameter of 2.5 cm. A total of 80 fibers were contained within the module, and the active length of the fibers within the module was 35 cm. Similar particle loss experiments were performed with custom-made MHFMM's with much smaller fiber packing densities and each of these MHFMMs had a particle loss of less than 2% based on ratio of spheres present in the MHFMM divided by the spheres present in the MHFMM and a series of back up glass fiber filters. These experiments verified preliminary calculations showing no significant particle loss.
4. Optimize the MHFMM for a target gas-phase removal efficiency of 98 percent. (See Comments)
5. Replication studies will be performed with the optimized MHFMM for removal of nitric acid. (See Comments)
6. Integrate MHFMM with aerosol collection system (ACS). (See Comments)

Anticipated Success in Meeting Project Objectives in Scheduled Project Period There are currently two separate atmospheric collections systems that have been used in field tests around the Chesapeake Bay. These systems have collected high temporal (<10 min), near real-time gas-phase ammonia and particulate-phase ammonia data. However these systems are currently configured using denuders and we hope to re-configure the systems with MHFMMs in the near future.

Preliminary Results
The system as described has been deployed on three occasions. The initial deployment occurred on the pier of the Chesapeake Biological Laboratory in Solomons, MD (See Figure 2). This data shows a dramatic increase of ammonia in the late evening that coincided with a large thunderstorm. The second occurred as a co-deployment of other ammonia and atmospheric samplers at the Fort Meade, Maryland super site (See Figure 3). During this study period we can see several diurnal fluctuations, also we see that the difference between the NH3 only and NH3+ NH4+ signal is minimal indicating that the majority was particulate-phase ammonium. Further evaluations of these data are ongoing.

Tasks and activities for next reporting period

Tasks for the next reporting period

1. Fabricate a new MHFMM based on experimental results of initial system
2. Test and incorporate new MHFMM with ACS.
3. Present paper on development of instrument at the American Association of Aerosol Research's Annual Convention
4. Co-deploy instrument with conventional NHx samplers at an agricultural field site

Work plan to accomplish tasks
The new MHFMMs are currently under construction at Clarkson University. These units will be tested at CBL in October 2001. After the testing is complete and the units have the designed gas-phase collection efficiency, the ACS and MHFMMs will be integrated.

Concerns or difficulties

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. In addition, a significant portion of microspheres were retained by the Fiberflo during the particle loss experiments. This is likely due to the packing geometry of the fibers within the module. The packing density of the fibers in the module is too great, which restricts air from reaching a large portion of the fibers as well as leading to particle capture within the module. These results indicate that the Fiberflo module will not be an adequate MHFMM to use in the development of the nitrogen sensor.

The custom MHFMM that was tested also provided mass transfer data that was lower than predicted by the mathematical model. However, the experimental results are more favorable than those from the Fiberflo MHFMM and no significant particle capture is expected from the custom MHFMM. Potential factors leading to lower mass transfer results include contamination of the fibers during fabrication (oils and greases from fingers, workbenches, etc.), and chemical reaction of the nitrogen species (nitric acid in particular) with the epoxy used to hold the MHFMM together. These factors can be remedied, and a new MHFMM is currently in the process of being fabricated utilizing a different epoxy that has been used in previous MHFMM applications.