Atmospheric Aerosols and Climate Change: Process and Closure Studies

A Component of the NOAA OGP Aerosol-Climate Program

Timothy S. Bates, NOAA/PMEL
September 5, 1995

Summary:

This project summary describes a NOAA Office of Global Programs (OGP) core research effort to quantify the combined chemical and physical processes controlling the evolution and properties of the atmospheric aerosol relevant to radiative forcing and climate. This effort is the short-term-intensive field component of the NOAA OGP Aerosol-Climate Program and complements the long-term continuous observations (CMDL & PMEL) and global climate modeling (GFDL) components. This effort is also the major NOAA contribution to the International Global Atmospheric Chemistry Program's (IGAC) Aerosol Characterization Experiments (ACE).

The Aerosol Process and Closure Studies component of the NOAA OGP Aerosol Climate Program began in FY 1990 with an emphasis on the role of the marine biogeochemical sulfur cycle in the generation of atmospheric aerosol particles, cloud microphysics, cloud albedo, and global climate. More recently the emphasis has shifted to characterizing the chemical, physical, and radiative properties of marine aerosols, the relationships between these properties and the anthropogenic perturbations to the background aerosol. OGP funding to this project during the past six years has resulted in 37 peer-reviewed publications.

1. Program Goal, Scientific Questions, and History:

The goal to the Aerosol Process and Closure Studies component of the NOAA OGP Aerosol-Climate Program is to quantify the combined chemical and physical processes controlling the evolution and properties of the atmospheric aerosol relevant to radiative forcing and climate. These process and closure studies are designed to provide the necessary data to incorporate aerosols into global climate models and to reduce the overall uncertainty in the calculation of climate forcing by aerosols.

The focus of our program has been on the marine atmosphere. This environment provides an opportunity to establish the chemical, physical and radiative properties of the natural aerosol and to compare and quantify the anthropogenic perturbations to this background aerosol. The initial experiments have been designed to address the following questions:

• What are the biological, chemical, and physical processes controlling the concentration of dimethylsulfide (DMS) in surface seawater and its flux to the atmosphere?

 

• What are the rates and efficiencies of the processes controlling the nucleation, growth, distribution, and removal of particles in the marine atmosphere?

 

• Can the measured physical and chemical properties of the aerosol be used to predict the radiative and cloud nucleating properties of that same aerosol?

This program began in FY 1990 ($150K), providing funds for NOAA and the University of Washington to participate in the Pacific Sulfur-Stratus Investigation (PSI-2). Funding in FY 1991 ($400K) allowed expanded participation in PSI-3 and additional measurements of sulfur gases and aerosols on cruises in the both the Atlantic and Pacific Oceans. In FY 1992 ($350K) we participated in two major international research programs, both under the auspices of the International Global Atmospheric Chemistry Program's (IGAC) Marine Aerosol and Gas Exchange, Atmospheric Chemistry and Climate (MAGE) activity. The first cruise, in the Equatorial Pacific in February and March 1992 (PMEL and UW), was part of an IGAC-JGOFS (Joint Global Ocean Flux Study) field experiment to look at the natural cycling of carbon, sulfur, and nitrogen in the surface ocean and the flux of these elements both to the deep ocean and the atmosphere. Our primary effort in this program was to observe the atmospheric diurnal cycling of natural sulfur gases and aerosol particles and the processes controlling the formation and growth of both the total particle population (CN) and those particles that serve as cloud condensation nuclei (CCN). The second cruise, in the Eastern Atlantic Ocean in June 1992 (AOML), was part of ASTEX (Atlantic Stratocumulus Transition Experiment) and was designed to compare the impacts of natural and anthropogenic sulfur on marine aerosol chemistry and the formation and dissipation of marine clouds. In FY 1993 ($350K) and FY 1994 ($300K) we conducted two major field experiments in the mid-Pacific (67°S to 57°N along 140°W) to study the latitudinal and seasonal variations in the chemical, physical and radiative properties of marine aerosol particles. In FY 1995 ($300K) we completed the analyses of these data and prepared for the IGAC Aerosol Characterization Experiment (ACE-1) scheduled for October-December 1995.

2. Rationale:

Atmospheric aerosol particles affect the Earth's radiative balance both directly through the upward scatter of solar radiation and indirectly as cloud condensation nuclei (CCN). The natural aerosol derived primarily from biogenic sulfur emissions has been substantially perturbed by anthropogenic aerosols, particularly sulfates from SO₂ emissions and organic condensates and soot from biomass and fossil fuel combustion. The global mean radiative forcing due to the direct effect of anthropogenic sulfate aerosol particles is calculated to be of comparable magnitude (approximately -0.3 to -1.1 watt/m^-2) but opposite in sign to the forcing due to anthropogenic CO₂ and the other greenhouse gases (Charlson et al., 1991, 1992; Kiehl and Briegleb, 1993; Penner et al., 1994). More uncertain is the radiative forcing due to the indirect cloud-mediated effects of aerosol particles. Although aerosol particles have a potential climatic importance over and down wind of industrial regions that is equal to that of anthropogenic greenhouse gases, they are still poorly characterized in global climate models. This is a result of a lack of both globally distributed data and a clear understanding of the processes linking gaseous precursor emissions, atmospheric aerosol properties, and the spectra of aerosol optical depth and cloud reflectivity. At this time, tropospheric aerosols pose one of the largest uncertainties in model calculations of the climate forcing due to anthropogenic changes in the composition of the atmosphere. Clearly, considerable attention must be focused on quantifying the processes controlling the natural and anthropogenic aerosol and on defining and minimizing the uncertainties in the calculated climate forcings.

3. Research Strategy and Progress Summary:

The three scientific questions posed under section 1 fall under two categories of experiments: process studies and closure studies. Process studies are needed to evaluate the factors controlling the sources, formation and fate of aerosols and the way these processes affect the number size distribution, chemical composition and radiative and cloud nucleating properties of the particles. The details of these processes are needed to make the critical links in climate models between gas emissions, aerosol particles and the associated impact on the Earth's radiation budget. Closure studies are designed to document the chemical, physical and radiative characteristics of aerosols and to investigate the relationships between these aerosol properties. The objective of the closure studies is to over-determine aerosol properties using a variety of measurement and modelling techniques in order to examine the internal consistency of these different measurement and modelling strategies.

Our strategy to address the three research questions posed under section 1 and a summary of our progress are discussed below.

3.1. What are the biological, chemical, and physical processes controlling the concentration of DMS in surface seawater and its flux to the atmosphere?

DMS is emitted from the oceans and has been shown to be the largest natural source of sulfur to the atmosphere and thus the major source of the nss sulfate mass in aerosol particles over the more remote regions of the Earth (Charlson et al., 1987; Bates et al., 1992a; Calhoun et al., 1992). Our first process study has focused on quantifying the factors controlling the sources and sinks of DMS in surface ocean waters and its flux to the atmosphere. This has required a coordinated plankton biology, microbiology, and photochemistry study of DMS, its precursors, and its seawater oxidation products. The production of dimethylsulfoniopropionate (DMSP), the precursor of DMS, is extremely species specific, tending to be higher for coccolithophores and dinoflagellates and lower for diatoms. It has been shown that DMSP contributes significantly to the osmotic pressure within the algal cell and serves to maintain the osmotic balance required for cell growth. DMSP is released from phytoplankton cells both during senescence and during zooplankton grazing and is cleaved enzymatically to yield DMS and acrylic acid. Once in the water column, DMS is degraded both microbially and photochemically and is lost to the atmosphere via air-sea exchange. Although the sources and sinks of DMS have been identified, their magnitudes are poorly constrained and the factors controlling them are still unknown. Initial results suggest that the microbial sink of DMS controls its cycling in the water column and that air-sea exchange is a relatively minor sink (Kiene and Bates, 1990). Without further quantitative data, we have no way to predict how seawater DMS concentrations may respond to a changing climate. The seawater DMS data are extremely important since these data are the starting point for calculations of the sea to air flux of DMS (Bates et al., 1987a) and the entire marine atmospheric biogeochemical sulfur cycle. It is therefore imperative that the environmental factors which regulate DMS release and cycling within the surface waters be quantified.

Our first major study of this objective was in FY 1991 during PSI-3 (Pacific Sulfur/Stratus Investigation). A similar coordinated seawater study was conducted in FY 1992 in the equatorial Pacific. The principal scientific conclusions from this work to date include:

• Air-sea exchange of DMS was shown to be only a minor sink in the seawater sulfur cycle of the temperate latitudes of the North Pacific and thus there is the potential for much higher DMS emissions under different climatic conditions (Bates et al., 1994).

 

• The photolysis of DMS in the equatorial Pacific Ocean proceeds at significant rates at ambient DMS concentrations and light levels and the photochemical turnover rate constants for DMS are comparable to turnover rate constants for other known removal mechanisms including atmospheric ventilation and biological consumption (Kieber et al., 1995).

 

• During the equatorial Pacific IGAC/MAGE 1992 experiment, the sea-to-air flux of DMS was calculated by two independent methods: seawater DMS concentrations coupled with air-sea exchange models and the atmospheric diurnal cycle of DMS concentrations coupled with atmospheric photochemical models. The data suggest that the sea-to-air DMS flux may be higher than previously thought and that the rate of oxidation of DMS by OH is underestimated using the currently accepted rate constant for this reaction and the modeled OH field (Yvon et al., 1995).

3.2. What are the rates and efficiencies of the processes controlling the nucleation, growth, distribution, and removal of particles in the remote marine atmosphere?

The partitioning of DMS oxidation products between new particle production and particle growth will affect the sub-micron aerosol size distribution and, in turn, the effect of these particles on climate. Parameters that determine whether gaseous species condense to form new particles or condense onto existing particles include the saturation vapor pressure of the condensing species (H₂SO₄, MSA, and NH₄), RH, temperature, and the existing particle number concentration and surface area. Our second process study has focused on quantifying the factors controlling the nucleation, growth, distribution and removal of particles in the remote marine atmosphere. Observations of ultra-fine particle (3nm < D_p < 20nm) number concentrations have been used to identify conditions and regions of new particle formation. Simultaneous measurements DMS, the chemical aerosol mass and aerosol number size distributions, and the CCN number concentration have been used to determine the role of specific chemical species in aerosol growth, the effect of cloud cycling on the aerosol size distribution, and the effect of precipitation on aerosol removal. The principal scientific conclusions from this work to date include:

 

• DMS is oxidized in the atmosphere to NSS sulfate and MSA. The ratio of these oxidation products has been shown to vary with temperature and/or latitude (Bates et al., 1992b). In addition, size resolved aerosol chemical measurements (Pszenny, 1992; Huebert et al., 1993; Quinn et al., 1993) have shown that over the remote ocean, appreciable MSA is found in the super-micron size fraction. These results help to indicate if DMS sulfur results in the formation or growth of particles and what atmospheric conditions promote one process over the other.
• New particle production in the marine boundary layer occurs in bursts and is a function of the existing particle size distribution, vapor concentration of aerosol precursors, relative humidity and temperature (Covert et al., 1992; Hegg et al., 1992).
• A doubling of the non-seasalt sulfate mass was shown to correspond to a 40 to 50% increase in the accumulation mode size range and CCN number concentration (Quinn et al., 1993; Berresheim et al., 1993).
• The relative number concentration of the modes within the MBL aerosol size distribution was found to depend on regional and mesoscale meteorology. Subsidence of air from the free troposphere to the MBL in the high and mid-latitudes resulted in the injection of ultra-fine particles to the MBL. In the tropics, where aerosol residence times in the MBL were longer, the aerosol size distribution reflected the growth of particles from the Aitkin to the accumulation mode size range. In this extensive data set from 1992-1994 there was no indication of new particle formation in the MBL, rather the source of new particles seemed to be exclusively from the free troposphere (Covert et al., 1995).
• The fraction of the aerosol mass that was non-sea-salt (nss) sulfate aerosol was highest in regions having the longest MBL residence times or the largest sulfur sources (naturally produced DMS or anthropogenically produced sulfur dioxide). The latitudinal distribution of nss sulfate light scattering was similar to that of the nss sulfate mass fraction, but amplified due to the concentration of nss sulfate in the accumulation mode where the scattering efficiency per unit aerosol volume is the highest (Quinn et al., 1995b).
• Ammonia may be a key species in tertiary nucleation reactions which produce new particles in the atmosphere. Measured seawater and atmospheric ammonia concentrations for the North Atlantic Ocean, where the input of anthropogenic nitrogen to the ocean surface could be significant, were reviewed (Quinn et al., 1995a). Flux estimates based on these measurements were compared to those derived from modeled seawater and atmospheric ammonia concentration. The lack of measured data prevented a thorough comparison, however, and emphasized the need for additional simultaneous measurements of ammonia in the aerosol, gaseous and seawater phases. An analytical system is currently under development at PMEL to accurately measure ammonia in the gaseous and seawater phases.

3.3. Can the measured physical and chemical properties of the aerosol be used to predict the radiative and cloud nucleating properties of that same aerosol?

The accuracy of the estimated radiative forcing of tropospheric sulfate aerosol depends on the quality and spatial coverage of the aerosol chemical, physical, and radiative data that serve as input to global climate models. Closure experiments provide data for testing climate models and in addition allow us to:

• estimate measurement uncertainties from the internal consistencies of the data set,

 

• test the validity of models to predict:
• a) aerosol radiative properties based on Mie theory and measurements of chemical and physical aerosol properties and

 

• b) cloud nucleating properties based on Köhler theory and measurements of chemical and physical aerosol and cloud properties,
• test and calibrate remotely sensed and indirectly determined aerosol properties for use in climate models,

 

• identify areas where improvements in instrumentation or modelling are needed.

Our initial closure studies have focused on mass and scattering closure. Key measurements for all the closure studies include the aerosol number concentration and chemical composition as a function of particle size. It is important, therefore, to validate these measurements and to estimate their uncertainty. Our initial efforts have been to compare the chemically analyzed mass and the mass derived from the number size distribution. We have then extended this comparison to aerosol scattering by comparing the direct measurements of light scattering properties with independent predictions from the aerosol number and chemical mass size distributions, some simplifying assumptions (e.g. spherical particle shape, an externally mixed aerosol, particle refractive index, and density), and Mie theory. The principal scientific conclusions from this work to date include:

• Measured aerosol scattering and backscattering coefficients were compared with calculated values derived from Mie theory applied to measured number and chemical mass size distributions. The calculated scattering values were within 3% of the measured values indicating that the aerosol chemistry and scattering properties were parameterized adequately in the model. The calculated backscattering values were about 40% lower than the measured values, however, indicating a need for an improved characterization of the nephelometer measurement or model assumptions (Quinn et al., 1995c).

 

• For continentally impacted air masses, mass concentrations determined from ion chromatography (IC) analysis were consistently lower than those calculated from the number size distribution indicating that a more complete aerosol chemical analysis is needed for closure to be achieved. This analysis would include determining the mass concentration of organic species, elemental carbon, trace metals, and mineral dust in addition to a gravimetric measure of the total aerosol mass size distribution (Quinn et al., 1995c; McInnes et al., 1995).

4. Research Products:

Results from all field programs and laboratory studies are being reported at national meetings and in refereed journal publications. Results from the Aerosol Process and Closure Studies component of the NOAA OGP Aerosol-Climate Program have been reported in special sessions of the 1989, 1991, and 1992 fall AGU meetings. The following refereed journal articles have resulted from this program (1990-1995):

5. Research Direction:

FY 1996 funding will be used to participate in ACE-1. The field experiment is scheduled to begin in October 1995 with a transit cruise from Seattle to Hobart. ACE-1 is the first of a series of international aerosol experiments and is aimed at the minimally polluted marine troposphere south of Australia. ACE-2 will focus on the polluted marine atmosphere of the North Atlantic during the summer of 1997. The Aerosol Process and Closure Studies component of the NOAA OGP Aerosol Program will be the major NOAA contribution to these international aerosol experiments.

6. Ties to Other Programs:

NOAA programs- PMEL is currently involved in NOAA's Radiatively Important Trace Species (RITS) and CO₂ programs studying the biogeochemical cycling of key elements such as carbon, oxygen, and nitrogen in the remote marine troposphere and the potential roles these elements may play in global climate change. The aerosol process and closure studies described here add a necessary and valuable complement to the RITS and CO₂ work.

National programs

The Aerosol Process and Closure Studies component of the NOAA OGP Aerosol-Climate Program was the NOAA contribution to the interdis-ciplinary/interagency Pacific Sulfur/Stratus Investigation (PSI). This study began in 1989 to coordinate NOAA, NASA, NSF, DOE, and DOD funded research on the climatic role of the marine biogenic sulfur cycle. Three field experiments were conducted between 1989-1991. This national program evolved into the international IGAC-Multiphase Atmospheric Chemistry (MAC) activity. MAC has recently been divided into 3 separate aerosol activities as part of the merger between the International Global Aerosol Program (IGAP) and IGAC.

International programs

The Aerosol Process and Closure Studies component of the NOAA OGP Aerosol-Climate Program has been designed as a concise, clearly focused research effort which has been an integral part of the "Marine Aerosol and Gas Exchange, Atmospheric Chemistry and Climate" activity (MAGE) and the "Aerosol Closure and Process Studies" activity (ACAPS) within the International Global Atmospheric Chemistry (IGAC) program. T. Bates is the co-convenor of the IGAC-ACAPS activity. NOAA was a major contributor to both IGAC/MAGE field activities in 1992 and is taking a leading role in organizing ACE-1. Under the auspices of the Commission on Atmospheric Chemistry and Global Pollution (CACGP) of the International Association of Meteorology and Atmospheric Physics (IAMAP), IGAC has been created in response to the growing international concern about rapid atmospheric chemical changes and their potential impact on mankind.

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Bates, T.S., J.A. Calhoun, and P.K. Quinn. Variations in the concentration ratio of methane-sulfonate to sulfate in marine aerosol particles over the South Pacific Ocean. J. Geophys. Res., 97, 9859-9865, 1992.

Bates, T.S., J.E. Johnson, P.K. Quinn, P.D. Goldan, W.C. Kuster, D.S. Covert, and C.J. Hahn. The biogeochemical sulfur cycle in the marine boundary layer over the northeast Pacific Ocean. J. Atmos. Chem., 10, 59-81, 1990.

Bates, T.S., K.C. Kelly, and J.E. Johnson. Concentrations and fluxes of dissolved biogenic gases (DMS, CH4, CO, CO2) in the equatorial Pacific during the SAGA-3 experiment. J. Geophys. Res., 98, 16,969-16,978, 1993.

Bates, T.S., R.P. Kiene, G.V. Wolfe, P.A. Matrai, F.P. Chavez, K.R. Buck, B.W. Blomquist, and R.L. Cuhel. The cycling of sulfur in surface seawater of the northeast Pacific during PSI-3. J. Geophys. Res., 99, 7,835-7,843, 1994.

Bates, T.S., B. K. Lamb, A.B. Guenther, J. Dignon, and R.E. Stoiber. Sulfur emissions to the atmosphere from natural sources. J. Atmos. Chem., 14, 315-337, 1992.

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Calhoun, J.A., T.S. Bates, and R.J. Charlson. Sulfur isotope measurements of submicro-meter aerosol particles over the Pacific Ocean. Geophys. Res. Let., 18, 1877-1880, 1991.

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McInnes, L.M., P.K. Quinn, D.S. Covert, and T.L. Anderson. Gravimetric analysis, ionic composition, and associated water mass of the marine aerosol, Atmos. Environ., submitted, 1995.

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Pszenny, A.A.P. Particle size distributions of methanesulfonate in the tropical Pacific marine boundary layer. J. Atmos. Chem., 14, 273-284, 1992.

Pszenny, A.A.P., C. Fischer, A. Mendez, and M. Zetwo. Direct comparison of cellulose and quartz fiber filters for sampling submicrometer aerosols in the marine boundary layer. Atmos. Environ., 27A, 281-284, 1993.

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Quinn, P.K., K.J. Barrett, F.J. Dentener, F. Lipschultz, and K.D. Kurz. Estimation of the air/sea exchange of ammonia for the North Atlantic basin, Biogeochemistry, submitted, 1995a.

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Quinn, P.K., S. Marshall, T.S. Bates, D.S. Covert, and V.N. Kapustin. Comparison of measured and calculated aerosol properties relevant to the direct radiative forcing of tropospheric sulfate aerosol particles. J. Geophys. Res., 100, 8977-8992, 1995.

Saltzman, E.S., S.A. Yvon, and P.A. Matrai. Low-level atmospheric sulfur dioxide measurement using HPLC/fluorescence detection. J. Atmos. Chem., 17, 73-90, 1993.

Sievering, H., E. Gorman, T. Ley, A. Pszenny, J. Boatman, Y. Kim, C. Nagamoto, and D. Wellman. Ozone oxidation of sulfur in sea-salt aerosol particles during the Azores Marine Aerosol and Gas Exchange Experiment, J. Geophys. Res., in press, 1995.

Wolfe, G.V., T.S. Bates, and R.J. Charlson. Climatic and environmental implications of biogas exchange at the sea surface: modelling DMS and the marine biologic sulfur cycle. In: Ocean Margin Processes in Global Change. R.F.C. Mantoura, J.M. Martin and R. Wollast, eds., pp 383-400, 1991.

Wolfe, G.V. & R.P. Kiene. Effects of methylated, organic, and inorganic substrates on microbial consumption of dimethylsulfide in estuarine waters. Appl. Environ. Microbiol., 59, 2,723-2,726, 1993.

Yvon, S.A., E.S. Saltzman, D.J. Cooper, T.S. Bates, and A.M. Thompson. Atmospheric dimethylsulfide cycling at a tropical South Pacific station (12S, 135W): A comparison of field and model results. J. Geophys. Res., submitted, 1995.