Tropospheric Chemistry
The Photochemical Oxidation/Products Group (Fred Eisele, Bruce Henry, Edward Kosciuch, R. Lee Mauldin, James Smith and Mark Zondlo) made progress in a number of areas this past year. Tropospheric Ozone Production about the Spring Equinox (TOPSE) data was analyzed and written up highlighting several interesting observations, including the response of OH to O3 depletion events, anthropogenic MSA plumes, and particle nucleation. Much effort was put into improving the aircraft nitric acid instrument. Lessons learned on TOPSE allowed a redesign of the ion source, suggested the need for modified ion chemistry, and led to improved calibration geometry. Laboratory testing of the modified instrument took place throughout the fall, and test flights were carried out in February 2001. The new instrument was then used successfully to make fast, sensitive measurements of nitric acid in the NASA (Transport and Chemistry Evolution over the Pacific (TRACE-P) campaign. Two major field studies were also completed this past year: Investigation of Sulfur Chemistry in the Antarctic Troposphere (ISCAT 2000) and TRACE-P. The first of these was a study of sulfur chemistry, photochemistry, and aerosol composition at the South Pole. Measurements performed by our group included those of OH, sulfuric acid, methane sulfonic acid, CO, and H2O. A new HO2 measurement was also added in the latter part of this study. The study was highly successful, and the new HO2 measurements, when combined with those of OH, provided a more complete picture of the South Pole photochemistry.
Hydroxyl radical concentrations were again found to be quite high. Experience gained from the past field study at the Pole and the earlier arrival of a few investigators greatly reduced set-up time and provided a significantly longer measurement period. Measurements early and late in the study yielded particularly large variabilities in OH and H2SO4 concentration. These were typically correlated with changes in NO and hazy conditions respectively, and led to a large dynamic range over which the models could be tested. Gas phase methane sulfonic acid (MSA) measurements again yielded very low concentrations, but this is consistent with a small, local production of MSA due to low DMS concentrations. Most of the MSA and probably the bulk of the sulfuric acid that reaches the polar snow are probably transported for large distances on particles. The combination of OH concentration and NO flux measurements has added greatly to our understanding of the unique photochemical environment just above the South Pole. Good correlation between NO and OH were observed, and HO2 measurements provided an independent confirmation of the high and variable measured NO concentrations. The combination of OH, HO2, CO, and NO measurements also provided a particularly detailed and consistent picture of surface photochemistry at the South Pole. Results of the earlier ISCAT study have been submitted for publication (Mauldin et al., Geophys. Res. Lett., in press).
TRACE-P was a NASA-sponsored study, which took place off the coast of Asia. The group’s participation involved the use of a unique multi-channel mass spectrometer to measure OH, sulfuric acid and methane sulfonic acids in ppq (parts per quadrillion) and nitric acid in the ppt (parts per trillion) concentration range in support of TRACE-P's goals to understand the composition and chemical evolution of outflow from Asia. The measurements were made using selected ion chemical ionization mass spectrometric techniques, and calibrations were performed in flight. Measurements were made in a 30-second or less time period (depending on the compound) on a near continuous basis throughout the campaign. Preliminary results for all four of these compounds have been submitted to the NASA archive. Some of the preliminary highlights of the study include: observation of a previously unknown nighttime oxidation mechanism for SO2 and DMS and sulfuric acid driven nucleation at the edges of plumes.
Much progress has also been made in developing a field instrument that can chemically analyze ultrafine particles at ambient concentrations. The prototype instrument that initially demonstrated that the Electrostatic Precipitation/Volatilization/Selected Ion Chemical Ionization Mass Spectrometry technique could be used to analyze ambient ultrafine particles has been significantly modified to allow automated field operation. The large, heavy diffusion pumped vacuum system has been replaced with a much lighter weight, less power-consuming turbo pumped vacuum system. The electrostatic precipitator apparatus has been redesigned to improve repeatability and to reduce contamination during the analysis of collected aerosol. The flow control system is automated and under computer control. The manual collection filament positioning system has been replaced with a stepper motor-based linear actuator, also under computer control.
The components that were used for initial testing, on short-term loan from the University of Minnesota, have been replaced with a commercial electrospray aerosol generator (TSI, Inc.), a commercial nano-DMA (TSI, Inc.), and a home built electrometer. This allows continuous operation/testing and optimization of the instrument. The nano-DMA is now also computer controlled and the output of the electrometer is fed into the computer so that the signal can be normalized to the total number of particles collected. Work has recently begun on measuring ambient aerosols and such measurements should be possible in the next few weeks. Several laboratory measurements have also been completed in this same time frame showing very sensitive and linear response to aerosols produced in the laboratory.
Work has also begun on a new ion trap mass spectrometer. Ion traps offer the ability to enhance sensitivity far beyond a scanning quadrupole instrument, while permitting the acquisition of tandem mass spectra to improve specificity. These qualities are essential for characterizing gas and particle phase organic constituents. The ion trap under development is unique because of the measures that will be taken to improve ion throughput above that provided by commercial instruments. These will include conventional techniques such as high efficiency ion transfer optics and large sampling orifices as well as more experimental state-of-the-art techniques such as broadband nondestructive ion detection and the use of asymmetric trapping fields for ion ejection. The initial phase of instrument development has concluded with the acquisition an assembly of the ion trap and vacuum chamber. Testing will begin shortly.