Joe F. Boatman

Heterogeneous sulfur conversion in sea-salt aerosol particles: the role of aerosol water content and size distribution

Abstract Meteorological and chemical conditions during the July 1988 Bermuda-area sampling appear to have been favorable for conversion of sulfur gases to particulate excess sulfate (XSO4). Observed average XSO4 and SO4 concentrations of 11 and 2.1 nmol m−3, respectively, at 15 m a.s.l. in the marine boundary layer (MBL) upwind of Bermuda, indicate that conversion of SO2 to XSO4, over and above homogeneous conversion, may be necessary to explain the > 5.0 average molar ratio of XSO4 to SO2. Given an observed cloud cover of Aerosol water content, estimated as a function of particle size distribution plus consideration of SO2 mass transfer for the observed particle size distribution, shows that SO2 was rapidly transferred to the sea-salt aerosol particles. Assuming that aqueous-phase SO2 reaction kinetics within the high pH sea-salt aerosol water are controlled by O3 oxidation, and considering mass-transfer limitations, SO2 conversion to XSO4 in the sea-salt aerosol water occurred at rates of approximately 5% h−1 under the low SO2 concentration, Bermuda-area sampling conditions. All of the 2 nmol XSO4 m−3 associated with sea-salt aerosol particles during low-wind-speed, Bermuda-area sampling can be explained by this conversion mechanism. Higher wind speed, greater aerosol water content and higher SO2 concentration conditions over the North Atlantic are estimated to generate more than 4 nmol XSO4 m−3 by heterogeneous conversion of SO2 in sea-salt aerosol particles.

Ozone oxidation of sulfur in sea‐salt aerosol particles during the Azores Marine Aerosol and Gas Exchange experiment

Sea-salt aerosol particles in the lowest tens of meters abovc the ocean are, typically, more than three-fourths water on a volume basis. Calculations herein indicate that aqueous-phase conversion of sulfur dioxide dissolved in the water associated with sea-salt particles (sea-salt aerosol water) supported the production of 2-8 nmol m −3 of non-sea-salt sulfate (nssSO 4 = ) during the Marine Acrosol and Gas Exchange (MAGE) experimcnt intensives. This production is based on ozone oxidation of dissolved SO 2 in sea-salt aerosol water and accounts for sulfur gas and ozonc mass transfer limitations as a function of sea-salt particle size. Measurements showed that 1-15 nmol m −3 of nssSO 4 = was actually present in the sea-salt particle mode except for four enhanced concentration cases due to continental sulfur input. The range in predicted, as well as observed, nssSO 4 = was primarily due to variability in sea-salt aerosol water volume. The nssSO 4 = produced by ozone oxidation of sulfur dioxide, being in the sea-salt particle mode with observed volume geometric median diameter of 3.5-5 μm, is dry deposited at a fairly rapid rate. The remainder, being large-particle cloud condensation nuclei, may contribute little to cloud albedo over the global oceans. The two papers following this one, Kim et al. (this issue) on aerosol size distribution and water content and Parfai et al. (this issue) on compositional variations of sea-salt-mode aerosol particles observed by electron microscopy, complement and support results presented here. All three present results of the MAGE experiment and precede a forthcoming special issue of the Journal of Geophysical Research-Atmospheres on MAGE (B. Huebert, guest editor).

Trends in global marine cloudiness and anthropogenic sulfur

Abstract A statistical analysis using published data on the global distribution of total cloud cover and cloud type amounts over the ocean, reduced from the Comprehensive Ocean–Atmosphere Data Set (COADS), shows a significant positive trend (4.2% increase from the 1930 baseline) in total oceanic cloud amount in the period between 1930 and 1981. The increase of total cloud amount for the Northern Hemisphere (5.8% ) was twice that for the Southern Hemisphere (2.9% ), The more consistent 30-yr ( 1952–1981 ) data show that the change in cloud amount ( 1952 base) was 1.5% for the globe, 2.3% for the Northern Hemisphere, and 1.2% for the Southern Hemisphere. The analysis also shows that the greatest cloud amount increase was for altocumulus and altostratus clouds and that this increase was most pronounced at midlatitudes (30°–50°N). The trend and the pattern of cloud amount variations appear to be in accord with the temporal trend and geographic distribution of S02 emissions. It is hypothesized that sulfate par...

Size Calibration Corrections for the Active Scattering Aerosol Spectrometer Probe (ASASP-100X)

The response of the active scattering aerosol spectrometer probe (ASASP-100X) is affected by the optical properties of measured particles. Response functions of the ASASP-100X probe were calculated for different complex refractive indices corresponding to different types of atmospheric aerosol particles under various relative humidity conditions. Based on these response functions, corrected calibration bin diameters were determined for 15 size channels at six relative humidity values (0%, 50%, 70%, 80%, 90%, and 99%) and three typical aerosol types (rural, urban, and maritime). Sample calculations with these corrected calibration data show that a significant underestimation of the aerosol volume distribution can result if uncorrected manufacturers size calibration data are used.

The relationship between dimethyl sulfide and particulate sulfate in the mid-atlantic ocean atmosphere

Abstract Dimethyl sulfide (DMS) and atmospheric aerosols were sampled simultaneously over the Atlantic Ocean in the vicinity of Bermuda using the NOAA King Air research aircraft. Total and fine (50% cutoff at 2 μm diameter) aerosol fractions were sampled using two independent systems. The average nonsea-salt (nss)SO 4 2− concentrations were 1.9 and 1.0 μg m −3 (as SO 4 2− ) for the total and the fine fractions in the boundary layer (BL) and 0.53 and 0.27 μg m −3 in the free troposphere (FT). Non-sea-salt SO 4 2− in the two aerosol fractions were highly correlated ( r = 0.90), however a smaller percentage (55%) was found in the fine aerosol near Bermuda relative to that (90%) near the North American continent. The BL SO 4 2− concentrations measured in this study were higher than those measured by others at remote marine locations despite the fact that the 7-day air mass back trajectories indicated little or no continental contact at altitudes of 700 mb and below; the trajectories were over subtropical oceanic areas that are expected to be rich in DMS. DMS concentrations were higher near the ocean surface and decreased with increasing altitude within the BL; the average DMS concentration was 0.13 μg m −3 . Trace levels of DMS were also measured in the FT (0.01 μg m −3 ). Computer simultation of the oxidation and removal of DMS in the marine atmosphere suggests that 4 2− observed could be related to the natural S cycle.

Microanalysis of the aerosol collected over south-central New Mexico during the alive field experiment, May–December 1989

Abstract Thirty-eight size-segregated aerosol samples were collected in the lower troposphere over the high desert of south-central New Mexico, using cascade impactors mounted onboard two research aircraft. Four of these samples were collected in early May, sixteen in mid-July, and the remaining ones in December 1989, during three segments of the ALIVE field initiative. Analytical electron microscope analyses of aerosol deposits and individual particles from these samples were performed to physically and chemically characterize the major particulate species present in the aerosol. Air-mass trajectories arriving at the sampling area in the May program were quite different from those calculated for the July period. In general, the May trajectories showed strong westerly winds, while the July winds were weaker and southerly, consistently passing over or very near the border cities of El Paso, Texas, and Ciudad Juarez, Mexico. Aerosol samples collected during the May period were predominantly fine (0.1–0.5 μm dia.), liquid H2SO4 droplets. Samples from the July experiment were comprised mostly of fine, solid (NH4)2SO4 or mostly neutralized sulfate particles. In both sampling periods, numerous other particle classes were observed, including many types with probable terrestrial or anthropogenic sources. The numbers of these particles, however, were small when compared with the sulfates. Composite particle types, including sulfate/crustal and sulfate/carbonaceous, were also found to be present. The major differences in aerosol composition between the May and July samples (i.e. the extensive neutralization of sulfates in the July samples) can be explained by considering the different aerosol transport pathways and the proximity of the July aerosol to the El Paso/Juarez urban plume. Winds during the December experiment were quite variable, and may have contributed to the widely varying aerosol compositions observed in these samples. When the aircraft sampled the El Paso/Juarez urban plume, high concentrations of carbonaceous particles were collected. These C-rich particles were of three distinct types, two of which showed combustion morphologies and the third an irregular morphology. Concurrent aethalometer measurements of aerosol black carbon concentration were well correlated (r = 0.83) with the total carbonaceous particle fraction in the aerosol samples. Carbonaceous particles were not observed in abundance in any of the May or July samples (even when the winds passed over El Paso), and we attribute the high concentrations in December to increased wintertime burning of wood, fossil fuels and other combustibles in the urban area.

Aerosol size distribution and aerosol water content measurements during Atlantic Stratocumulus Transition Experiment/Marine Aerosol and Gas Exchange

Aerosol size distribution data measured during the June 1992 Marine Aerosol and Gas Exchange experiment are analyzed to investigate the characteristics of fine marine aerosol particles measured over the North Atlantic near the Azores Islands. Measured aerosol size distribution data were corrected using the corrected size calibration data based on the optical properties of particles being measured. The corrected size distribution data were then approximated with either one or two lognormal size distributions, depending on air mass conditions. Under clean air mass conditions <3 μm diameter aerosol size distributions typically exhibited two modes, consisting of an accumulation mode and the small end of the sea-salt particle mode. However, under the influence of continental polluted air masses, the aerosol size distribution was dominated by <1 μm diameter particles in a single mode with an increased aerosol concentration. Aerosol water content of accumulation mode marine aerosols was estimated from differences between several series of ambient and dried aerosol size distributions. The average aerosol water fraction was 0.31, which is in good agreement with an empirical aerosol growth model estimate. The average rate of SO4= production in the accumulation mode aerosol water by H2O2 oxidation was estimated to be <7×10−10 mol L−1 s−1, which is an insignificant contributor to the observed non-sea-salt SO4= in the accumulation mode.

Measured and calculated optical property profiles in the mixed layer and free troposphere

Nearly simultaneous measurements of the physical and optical properties of mixed layer and free tropospheric aerosols near Boulder, Colorado, were made on several occasions using aircraft, balloon, and ground-based sensors. This effort (Front Range Lidar, Aircraft, and Balloon experiment (FRLAB)) was conducted with the purpose of obtaining a diverse, self-consistent data set that could be used for testing optical model calculations based on measured physical characteristics such as apparent size distribution, composition, and shape. It was found that even with the uncertainties involved, the model predictions are in good agreement with the measurements in the visible and near infrared wavelength regions. At CO2 lidar wavelengths there is considerably more uncertainty in both the calculated and measured values; however, within the estimated errors there appears to be satisfactory agreement except for the highest free tropospheric layer studied. The results also indicate that during FRLAB the aerosol in the boundary layer and free troposphere behaved as spherical particles for optical modeling purposes. The utility of the observations for determining the extinction-to-backscatter ratio relevant to aerosols in the boundary layer and free troposphere is described with typical measured values being in the 20 to 30 sr range.

Atmospheric sulfur dioxide at Mauna Loa, Hawaii

Measurements of sulfur dioxide (S02) were made at the National Oceanic and Atmospheric Administrations Mauna Loa Observatory in Hawaii, during a 12-month period beginning in December 1988. SO2 concentrations varied from background levels of less than 0.05 ppbv to a maximum of 50 ppbv, during episodes that lasted from 2 to 24 hours. Emissions from the Kilauea crater, approximately 35 km southeast of the observatory at an elevation of about 1000 m above sea level (asl), and the current eruption at Puu O′o 50 km east-southeast, are the most likely sources for the higher concentrations. These episodes occurred 10–25 times each month, mostly during the day; peak concentrations were usually recorded at mid-day. The SO2 concentrations can be grouped into three periods; low (June–September), high (October–January) and intermediate (February–May). A clear diurnal cycle of SO2 concentration exists throughout the year, although day-night changes were greatest during October–January and were barely detectable during the June–September period. The highest SO2 concentrations were recorded when the predominant wind direction was northerly to northwesterly, even though the apparent sources are in the southeastern sector. Nighttime concentrations were usually at background levels; however, many exceptions were observed. A few cases of higher than background SO2 were observed when free tropospheric (FT) conditions were identified. The possibility that long-range transport was the cause for elevated SO2 concentrations under FT conditions was examined using air mass back trajectories analyses. The highest nighttime SO2 concentrations, under FT conditions, were observed during periods with slow easterly trajectories, and the lowest concentrations were found during westerly flows. Twenty-four nighttime free tropospheric events were recorded when the SO2 concentration exceeded 0.2 ppbv. During 18 of these episodes, unusually high CO2 concentrations were observed.

Sulfur dioxide flux measurements over the western atlantic ocean

Aircraft measurements of SO2 were made along the U.S. East Coast and in the vicinity of the Bermuda Islands during the period 2 March–11 April, 1985. SO2 was detected in all samples taken inside the boundary layer 100 km off shore. The maximum 1.0-min average concentration observed was 9.9 ppb (on the East Coast), and the average for the duration of the study was 2.1 ppb. The SO2 concentration in the free troposphere at the same location ranged from < 0.1 ppb to a maximum of 4.2 ppb. The air parcel sampled during the maximum event was back-tracked across the Ohio Valley region. Sulfur dioxide concentrations in the vicinity of the Bermuda Islands, inside and above the boundary layer, were less than the detection limit (0.1 ppb) during most of the time. On one event elevated SO2 levels were recorded, however they could not be traced to a source in N America. On the basis of the concentration and wind speed data, an altitude profile of SO2 flux was constructed for a portion of the U.S. East Coast. Integration of the analytical function describing the profile provided an estimate of SO2 flux eastbound of ~ 1 Tg (S)a−1.