Deposition of atmospheric mineral particles in the North Pacific Ocean

Total deposition of atmospheric mineral particles (wet plus dry) has been measured during consecutive two-week sampling intervals from January, 1981 to March, 1982 at four island stations (Midway, Oahu, Enewetak, and Fanning) of the SEAREX Asian Dust Study Network in the North Pacific. The total deposition of mineral aerosol during the period from February to June is higher than that during the period from July to January at most of the stations. A systematic geographical trend is apparent in the dust flux, with greater fluxes at higher latitudes. The deposition values are correlated with the atmospheric mineral particle concentrations at these stations. The mineral particles are transported from arid regions in Asia to the North Pacific, and the annual dust deposition to the ocean appears to be dominated by sporadic dust events of short duration. Wet deposition dominates the removal of dust particles from the atmosphere over the North Pacific. The total deposition of atmospheric mineral material to the central North Pacific is estimated to be 20×10¹² g yr^-1.     

Analysis of rainfall characteristics of the Madden–Julian oscillation using TRMM satellite data

Rainfall characteristics of the Madden–Julian oscillation (MJO) are analyzed primarily using tropical rainfall measuring mission (TRMM) precipitation radar (PR), TRMM microwave imager (TMI) and lighting imaging sensor (LIS) data. Latent heating structure is also examined using latent heating data estimated with the spectral latent heating (SLH) algorithm.The zonal structure, time evolution, and characteristic stages of the MJO precipitation system are described. Stratiform rain fraction increases with the cloud activity, and the amplitude of stratiform rain variation associated with the MJO is larger than that of convective rain by a factor of 1.7. Maximum peaks of both convective rain and stratiform rain precede the minimum peak of the outgoing longwave radiation (OLR) anomaly which is often used as a proxy for the MJO convection. Stratiform rain remains longer than convective rain until 4000 km behind the peak of the mature phase. The stratiform rain contribution results in the top-heavy heating profile of the MJO.Associated with the MJO, there are tri-pole convective rain top heights (RTH) at 10–11, 7 and 3 km, corresponding to the dominance of afternoon showers, organized systems, and shallow convections, respectively. The stratiform rain is basically organized with convective rain, having similar but slightly lower RTH and slightly lags the convective rain maximum. It is notable that relatively moderate (7 km) RTH is dominant in the mature phase of the MJO, while very tall rainfall with RTH over 10 km and lightning frequency increase in the suppressed phase. The rain-yield-per flash (RPF) varies about 20–100% of the mean value of 2–10 × 10⁹ kg fl⁻¹ over the tropical warm ocean and that of 2–5 × 10⁹ kg fl⁻¹ over the equatorial Islands, between the convectively suppressed phase and the active phase of MJO, in the manner that RPF is smaller in the suppressed phase and larger in the active phase.     

Stable Isotope Ratios: Hurricane Olivia

The oxygen and hydrogen isotopic compositions of rains from HurricaneOlivia (1994) in the eastern Pacific were measured. The rains werecollected on 24 and 25 September during airplane flights conducted at anelevation of 3 km. Hurricane Olivia peaked in intensity to a category-4storm between the two dates. Isotope ratios of rains from HurricaneOlivia were markedly lower ( ¹⁸O = –13.9to –28.8) than that of rain collected from a thunderstormat an elevation of 2.3 km outside the influence of Olivia (¹⁸O = –3.8). A distinct decrease in isotoperatios from the first day to the next ( ¹⁸O =–18.4 to –21.9) in Hurricane Olivia wasattributed to decreased updraft velocities and outflow aloft. Thisshifted the isotopic water mass balance so that fewer hydrometeors werelifted and more ice descended to flight level. A decrease in the averagedeuterium excess from the first day to the next (d = 15.5 to 7.1)was attributed to an increase in the relative humidity of the watervapor `source' area. We hypothesize that the `source' region for therain was in the boundary layer near the storm center and that becausethe hurricane was at peak intensity prior to the second day the relative humidity was higher.     

Fluxes of gases and particles above a deciduous forest in wintertime

Eddy-correlation measurements of the vertical fluxes of ozone, carbon dioxide, fine particles with diameter near 0.1 m, and particulate sulfur, as well as of momentum, heat and water vapor, have been taken above a tall leafless deciduous forest in wintertime. During the experimental period of one week, ozone deposition velocities varied from about 0.1 cm s^–1 at night to more than 0.4 cm s^-1 during the daytime, with the largest variations associated primarily with changes in solar irradiation. Most of the ozone removal took place in the upper canopy. Carbon dioxide fluxes were directed upward due to respiration and exhibited a strong dependence on air temperature and solar heating. The fluxes were approximately zero at air temperatures less than 5 °C and approached 0.8 mg m^–2 s^–1 when temperatures exceeded 15 °C during the daytime. Fine-particle deposition rates were large at times, with deposition velocities near 0.8 cm s^–1 when turbulence levels were high, but fluxes directed upward were found above the canopy when the surface beneath was covered with snow. Diffusional processes seemed to dominate fine-particle transfer across quasilaminar layers and subsequent deposition to the upper canopy. Deposition velocities for particulate sulfur were highly variable and averaged to a value small in magnitude as compared to similar measurements taken previously over a pine forest in summer.     

Atmospheric inorganic nitrogen in marine aerosol and precipitation and its deposition to the North and South Pacific Oceans

Aerosol and rain samples were collected between 48°N and 55°S during the KH-08-2 and MR08-06 cruises conducted over the North and South Pacific Ocean in 2008 and 2009, to estimate dry and wet deposition fluxes of atmospheric inorganic nitrogen (N). Inorganic N in aerosols was composed of ~68% NH₄⁺ and ~32% NO₃^– (median values for all data), with ~81% and ~45% of each species being present on fine mode aerosol, respectively. Concentrations of NH₄⁺ and NO₃⁻ in rainwater ranged from 1.7–55 μmol L⁻¹ and 0.16–18 μmol L⁻¹, respectively, accounting for ~87% by NH₄⁺ and ~13% by NO₃⁻ of total inorganic N (median values for all data). A significant correlation (r = 0.74, p < 0.05, n = 10) between NH₄⁺ and methanesulfonic acid (MSA) was found in rainwater samples collected over the South Pacific, whereas no significant correlations were found between NH₄⁺ and MSA in rainwater collected over the subarctic (r = 0.42, p > 0.1, n = 6) and subtropical (r = 0.33, p > 0.5, n = 6) western North Pacific, suggesting that emissions of ammonia (NH₃) by marine biological activity from the ocean could become a significant source of NH₄⁺ over the South Pacific. While NO₃⁻ was the dominant inorganic N species in dry deposition, inorganic N supplied to surface waters by wet deposition was predominantly by NH₄⁺ (42–99% of the wet deposition fluxes for total inorganic N). We estimated mean total (dry + wet) deposition fluxes of atmospheric total inorganic N in the Pacific Ocean to be 32–64 μmol m⁻² d⁻¹, with 66–99% of this by wet deposition, indicating that wet deposition plays a more important role in the supply of atmospheric inorganic N than dry deposition.     

Fluxes,sizes, morphology and compositions of particles in the Mt. Erebus volcanic plume,December 1983

Use of an airborne quartz crystal microbalance cascade impactor instrument together with a correlation spectrometer has allowed the flux of particles and their size distribution to be determined at Mount Erebus. The plume contributes 21±3 metric tomnes/day of aerosol particles to the Antarctic upper troposphere. The aerosol particles consist of larger (5–25 m) particles of elemental sulfur and silica, a middle sized group of iron oxides and smaller particles (less than 1 m) of complex liquids. Unlike many volcanic plumes, the Erebus plume has only a small amount of sulfate particles. The concentrations of particles in the Erebus plumes was 70–370 m/m³. Limited sampling of the Antarctic atmosphere at 8 km altitude but hundreds of km away from Erebus obtained a few large particles of sulfur and silicates, suggesting a similarity with the Erebus plume. The fallout of these particles occurs slowly over a broad area of the Antarctic continent.     

Analysis of the Saharan dust regional transport

Summary The Balkan Peninsula is situated in the impact zone of Saharan dust storms. The case of Saharan dust transport to Belgrade in the period of 14–17 April 1994 is analyzed using the Eta model for synoptic and meso scale processes. Air back trajectories are calculated at six model levels from 434 up to 5129m with horizontal grid resolutions of 1°×1° and 10×10. Following cyclonic circulation the dust was picked up from North Africa, and transported over Mediterranean. Simultaneously, according to the analysis of the three lowest trajectories, transport of trace metals from Macedonia and southern Serbia by the Koshava wind might be dominant in the observed episode. Turbulent flow enhanced the coagulation process of initially clean dust particles with particles containing Pb and Cd. The coagulation and scavenging processes below and in clouds increased deposition rates of Pb and Cd in Belgrade in the course of wet removal, and consequently trough resuspension processes. Dry deposition samples contained characteristic particles up to 30µm in diameter with Fe content of 11 to 15 atomic% and significant ratio Si/Fe of 3 to 5, determined for selected single particles by the SEM/EDX method. Following dry and wet deposition of Cd and Pb, a residual effect of dustfall is noticed throughout the vegetational period.     

Characteristics of the ozone decline in the northern polar and middle latitudes during the winter-spring

Summary The total ozone decline during the past twenty years, especially strong during the winter-spring season poleward from 50° N, is well established with known average trends of 5–7% per decade. This study presents a number of additional characteristics such as ozone-mass deficiency (O₃MD) from the pre- 1976 base average, and areal extent with negative deviations greater than2 and3. Gridded satellite data combined with ground-based total ozone maps, permit calculations of daily and regional ozone deficiencies from the anthropogenically undisturbed average ozone levels of the 1960s and early 1970s. Then the quantity of the O₃MD and the changes in surface area, with deficiencies larger than-10 and-15% are integrated for the 1 January to 15 April period for each of the last 20 years, and compared. In addition, the polar vortex extent during the last 10 years is determined using the PV at 475°K. The quantity of the O₃MD within the sunlit part of the vortex is shown to contribute from15 to 35% of the overall ozone deficiency within the-10% contours over the area 35–90°N. The ozone deficiency, integrated for the first 105 days of each year, has increased dramatically from 2,800Mt in the early 1980s to7,800Mt in the 1990s, exceeded 12,000Mt in the winter-springs of 1993 and 1995. The latter quantity is comparable with the average O₃MD over the same Southern latitudes in the last ten austral springs. During the 1990s over the 35–90° latitudes the average ozone deficiency in the Southern hemisphere belt is less than over the Northern hemisphere belt by40%. It is known that the main ozone decline is observed in the lower stratosphere and the ozone loss over the Arctic is very sensitive to decreasing stratospheric temperatures; negative 50hPa monthly anomalies greater than 4°C have occurred during 7 of the springs in the last decade, thus possibly facilitating doubling the area with negative ozone deviations greater than-10% in the 1990s to5,000.10⁶km² and nearly tripling the O₃MD as stated above. The changes in total eddy heat fluxes as a proxy indicator of the long wave perturbations are positively correlated with the ozone deficiency in the 45–75°N. The strong anticorrelation between the ozone deficiency in the region>55° N. versus the 35–50° N belt is discussed in relation to possible transport of air masses with low ozone from the sub-tropics, which in some years are the dominant reason for the observed ozone deficiency.With 11 Figures     

Numerical study of the cloud seeding effects

The microphysical model with the bulk water parameterization is applied to simulated both contact and deposition nucleation as well as the imersion freezing for unseeded cases and the cases immediately after seeding performed for the cold continental Cb clouds with small cloud droplets. The injection of agent AgI is performed in temperature region between –8°C and –12°C. The four groups of sensitivity experiments are executed.

Rainwater Chemistry and Wet Deposition over the Equatorial Forested Ecosystem of Zoétélé (Cameroon)

Within the framework of IDAF (IGAC DEBITS AFRICA: International GlobalAtmospheric Chemistry/DEposition of Biogeochemically Important TraceSpecies/Africa) network, data analysis is realised on precipitation chemical composition collected in Zoétélé, in Southern Cameroon. This station, located atabout 200 km from the Atlantic Ocean, is representative of a so-called `Evergreen Equatorial Forest' ecosystem. An automatic wet-only precipitation collector was operated at the station from 1996 to 2000. The rainfall regime, associated with eastward advection of moist and cool monsoon air masses, amounts to an average of 1700 mm/year. Inorganic and organic content of the precipitation were determined by IC in 234 rainfall events, representing a total 4,583 mm of rainfall from an overall of 7,100 mm.The mean annual precipitation chemistry and wet deposition fluxes characteristic of an African equatorial forest are quantified. Typical atmospheric gases and particles sources influence the precipitation chemical content and the associated deposition of chemical species. Indeed, hydrogen concentration is the highest (12.0 eq.L^–1) of the IDAF measurements, leading to acid rains with a low mean pH 4.92. The mineral species are dominated by nitrogenous compounds (NH₄ ⁺:10.5 and NO₃ ^–: 6.9 eq.L^–1), Ca²⁺ (8.9 eq.L^–1) and SO₄ ^{2 –} 5.1 eq.L^–1. Relationship between Ca^{2 +} and SO₄ ^{2 –} indicated aterrigeneous particulate source and an additional SO₄ ^{2 –} contributionprobably due to swamps and volcano emissions. Na⁺ and Cl^–concentrations, around 4.0 eq.L^–1, seem very low for this site,accounting for the marine source. Besides, strong correlations between NH₄ ⁺/K⁺/Cl^– indicate the biomass burning originof these species. Accordingly, precipitation chemistry in Zoétéléis influenced by three major sources: biogenic emissions from soil and forest ecosystems, biomass burning from savannah, and terrigenous signature from particles emissions of arid zones; and three minor sources: marine, volcano and anthropogenic. In spite of the relatively low concentration of all these elements, the wet deposition is quite significant due to the high precipitation levels, with for example a nitrogenous compounds deposition of 34 mmol.m^–2.yr^–1.     

Vertical distribution of dimethylsulfide,sulfur dioxide,aerosol ions,and radon over the Northeast Pacific Ocean

Dimethylsulfide (DMS), sulfur dioxide (SO₂), methanesulfonate (MSA), nonsea-salt sulfate (nss-SO₄ ^2–), sodium (Na⁺), ammonium (NH₄ ⁺), and nitrate (NO₃ ^–) were determined in samples collected by aircraft over the open ocean in postfrontal maritime air masses off the northwest coast of the United States (3–12 May 1985). Measurements of radon daughter concentrations and isentropic trajectory calculations suggested that these air masses had been over the Pacific for 4–8 days since leaving the Asian continent. The DMS and MSA profiles showed very similar structures, with typical concentrations of 0.3–1.2 and 0.25–0.31 nmol m^–3 (STP) respectively in the mixed layer, decreasing to 0.01–0.12 and 0.03–0.13 nmol m^–3 (STP) at 3.6 km. These low atmospheric DMS concentrations are consistent with low levels of DMS measured in the surface waters of the northeastern Pacific during the study period.The atmospheric SO₂ concentrations always increased with altitude from <0.16–0.25 to 0.44–1.31 nmol m^–3 (STP). The nonsea-salt sulfate (ns-SO₄ ^2–) concentrations decreased with altitude in the boundary layer and increased again in the free troposphere. These data suggest that, at least under the conditions prevailing during our flights, the production of SO₂ and nss-SO₄ ^2– from DMS oxidation was significant only within the boundary layer and that transport from Asia dominated the sulfur cycle in the free troposphere. The existence of a sea-salt inversion layer was reflected in the profiles of those aerosol components, e.g., Na⁺ and NO₃ ^–, which were predominantly present as coarse particles. Our results show that long-range transport at mid-tropospheric levels plays an important role in determining the chemical composition of the atmosphere even in apparently remote northern hemispheric regions.     

Coastal rainwater hydrogen peroxide: Concentration and deposition

Correlation analysis between rainwater component concentrations (hydrogen peroxide, hydrogen ion, nitrate, nonseasalt sulfate and chloride ion) was used to investigate patterns of variation in hydrogen peroxide concentrations in rain collected in Wilmington, North Carolina, a coastal southeastern United States location, between October 1992, and October 1994. Rainwater hydrogen peroxide concentrations in general correlated positively with the pollutant components (hydrogen ion, nitrate and non-seasalt sulfate). This pattern suggests that destruction of hydrogen peroxide by sulfur dioxide is not the dominant factor controlling the concentration of hydrogen peroxide in this rainwater, with the possible exception of winter rain from coastal storms where an inverse correlation between hydrogen peroxide and nonseasalt sulfate was observed. Sequential sampling indicates rapid production of hydrogen peroxide and incorporation into rain within time periods of hours during summer daytime rains.Rain is an important transport mechanism for removal of atmospheric hydrogen peroxide, which may affect the oxidizing capacity of surface waters that receive the rain. During this study time, the annual deposition of hydrogen peroxide by rain was 12 mmole m^-2 yr^-1. An average rain event added approximately half of the resident amount of hydrogen peroxide to the shallow lakes typical of eastern North Carolina; extreme rain events can triple the amount normally present. The episodic nature of rain contributes to the variability in hydrogen peroxide concentration in surface waters. Higher hydrogen peroxide concentrations and greater rainfall amounts cause wet deposition of hydrogen peroxide to be approximately seven times greater during the warm season than the cold season.     

Aerosols in Alaskan air masses

Results from measurements of the composition and size distribution of aerosol particles advected into central Alaska are reported. It is argued that the aerosol predominant in number, but not necessarily in mass, consists of submicron droplets of sulfuric acid. The major aerosol by mass in arctic air is a removal-resistant accumulation mode (radius 0.3 m) probably to large extent originating from pollution sources 10³ km upstream (mostly in central Eurasia) from the site in Alaska. The accumulation mode aerosol disappears when arctic air masses are replaced with relatively warmer air masses flowing in from the northern Pacific. The latter air mass systems have been strongly scavenged by clouds and precipitation associated with the Aleutian low pressure system and with forced orographic uplifting over the Alaska Mountain Range; nevertheless the Pacific air masses contain substantial (i.e., 500–1000 cm^-3) quantities of small (several hundredths of a micron in radius) particles. Arctic-derived air masses are enriched in large (i.e, 0.3 ) particles compared to Pacific Marine air masses, whereas the opposite trend is found for smaller, Aitken, particles. The smaller particles are found in greatest abundance in warmer air mass systems, presumably because of the relatively brief time since such air masses were last exposed to sunlight with attendant production of small particles from the gas phase.     

An Examination of the Atmospheric Chemistry of Mercury Using 210Pb and 7Be

Measurements of Hg (total gas-phase, precipitation-phase andparticulate-phase), aerosol mass, particulate ²¹⁰Pb and⁷Be and precipitation ²¹⁰Pb were made at an atmosphericcollection station located in a near remote area of northcentral Wisconsin,U.S.A. (46°10N, 89°50W) during the summers of 1993, 1994and 1995. Total Hg and ²¹⁰Pb were observed to correlate strongly(slope = 0.06 ± 0.03 ng mBq^-1; r ² =0.72) in rainwater. Mercury to ²¹⁰Pb ratios in particulate matter(0.03 ± 0.02 ng mBq^-1; r ² = 0.06) wereconsistent with the ratio in rain. Enrichment of the Hg/mass ratio (approx.5–50×) relative to soil and primary pollutant aerosols indicatedthat gas-to-particle conversion had taken place during transport. Comparisonof these results with models for the incorporation of Hg into precipitationindicates that atmospheric particles deliver more Hg to precipitation than canbe explained by the presence of soot. A lack of correlation between totalgas-phase Hg (TGM) and a ⁷Be/²¹⁰Pb function suggests novertical concentration gradient within the troposphere, and allows an estimateof TGM residence time of 1.5 ± 0.6 yr be made based on surface airsamples.     

Sensitivity of simulated Asian and African summer monsoons to orbitally induced variations in insolation 126, 115 and 6 kBP

We have conducted four numerical experiments with an atmospheric general circulation model (AGCM) to investigate the sensitivity of Asian and African monsoons to small changes (–5 to +12%), with respect to present-day, in incoming solar radiation at the top of the atmosphere. We show that, during the mid-Holocene (6 kBP where kBP means thousands of years before present-day) and the last interglacial (126 kBP), the Northern Hemisphere seasonal contrast was increased, with warmer summers and colder winters. At the time of glacial inception (115 kBP) however, summers were cooler and winters milder. As a consequence, Asia and tropical North Africa experienced stronger (weaker) summer monsoons 6 and 126 kBP (115 kBP), in agreement with previous numerical studies. This present study shows that summer warming/cooling of Eurasia and North Africa induced a shift of the main low-level convergence cell along a northwest/southeast transect. When land was warmer (during the summer months 6 and 126 kBP), the monsoon winds converged further inland bringing more moisture into northern India, western China and the southern Sahara. The southern tips of India, Indochina and southeastern China, as well as equatorial North Africa became drier. When land was cooler (during the summer 115 kBP), the main convergence zone was located over the west Pacific and the wet (dry) areas were those that were dry (wet) 6 and 126 kBP. The location and intensity of the simulated precipitation maxima were therefore very sensitive to changes in insolation. However the total amount of monsoon rain in Asia as well as in Africa remained remarkably stable through the time periods studied. These simulated migrations of convective activities were accompanied by changes in the nature of precipitation events: increased monsoon rains in these experiments were always associated with more high precipitation events (> 5 mm day ^–1), and fewer light showers (1 mm day^–). Rainy days with rates between 1 and 5 mm day^–1 were almost unchanged.     

Rainfall over oceans: Remote sensing from satellite microwave radiometers

Summary Microwave radiometer brightness temperature (T _b) measurements obtained from satellites over the oceans in dual polarization, at frequencies ranging from 6.6 to 85 GHz, reveal information about the rain and precipitation sized ice. These multifrequency measurements are composited from observations made by the Scanning Multichannel Microwave Radiometer (SMMR) and the Special Sensor Microwave/Imager (SSM/I). TheT _b measurements at 37 GHz, having a field of view (fov) of about 30 km, show relatively strong emissions due to rain, reaching values as large as 260 K over the tropical and mid-latitude rainbelts. Only marginal effects due to scattering by ice above the rain clouds are revealed. At frequencies below 37 GHz, where the fov is much larger than 30 km and the extinction is weaker,T _b is significantly smaller than 260 K. Additional information content about rain, at these low frequencies, is not appreciable. On the other hand, at 85 GHz (fov 15 km), where the extinction is very strong, the sea surface below the clouds is often masked and scattering due to ice above the rain clouds is vividly noticed. However, these high frequency measurements do not yield direct information about rain below the clouds.Recognizing the above merits inherent in the 37 GHz observations the SMMR and SSM/I data at this frequency are utilized to develop and empirical method to retrieve rain rate over oceans. In this method it is assumed that over an oceanic area, the statistics of the observedT _b must be derivable from the statistics of the corresponding rain rates. Furthermore, the underestimation of rain rate, arising from the inability of the radiometer to respond sensitively to rain above a given threshold is empirically rectified with the help of two parameters that depend on the total water vapor content in the atmosphere. Rain rates deduced over the oceans around Japan using the SSM/I data, when compared with those measured by radars that are calibrated against rain gauges, show a good correlation; there is, however, a systematic overestimation. Seasonal mean maps of the rainfall over the global oceans based on SMMR data compare favorably with climatological rain maps over the Atlantic and Pacific Oceans developed by Dorman and Bourke (1979, 1981).With 16 Figures     

In-cloud and below-cloud scavenging of aerosol ionic species over a tropical rural atmosphere in India

The temporal variation in concentrations of major water soluble ionic species has been studied from several rain events occurred over Gadanki (13.5 °N, 79.2 °E), located in tropical semi arid region in southern India. The contribution from rain-out (in cloud) and wash-out (below cloud) processes to the total removal of ionic species by rain events is also estimated using the pattern of variations of ionic species within an individual event. A number of rain samples were collected from each rain event during June–November in 2006, 2007 and 2008. On average, nearly 20% of the total NH ₄ ⁺ and non-sea SO ₄ ^2? is removed by in-cloud scavenging, suggesting that their removal by “below cloud” washout is relatively dominant. In contrast Na⁺, Ca²⁺, Mg²⁺, NO ₃ ^? and sea-SO ₄ ^2? are mainly removed by below-cloud scavenging or wash-out process. A significant variation in the acidity was observed within rain events with successive precipitation showing higher acidity at the final stage of the precipitation due to partial neutralization of non-sea SO ₄ ^2? . Overall, greater influence of both terrestrial and anthropogenic sources is recorded in the rain events compared to that from marine sources.     

Rainwater composition over a rural area with special emphasis on the size distribution of insoluble particulate matter

The rainwater composition in the vicinity of Mainz, FRG, has been investigated with special emphasis on insoluble constituents. The number size distribution was determined in the range from 0.1 m up to 100 m radius. For particles with r>0.5 m radius the shape of the size distribution of insoluble particles in rain follows the shape of the average urban and rural aerosol. In this particular size range no major size selective removal processes could be seen. For r<0.5 m the number size distribution tends to flatten compared to the average aerosol. This might be the indication of a size selective removal process (Greenfield Gap).     

Airborne measurements of dimethylsulfide,sulfur dioxide,and aerosol ions over the Southern Ocean South of Australia

Vertical distributions of dimethylsulfide (DMS), sulfur dioxide (SO₂), aerosol methane-sulfonate (MSA), non-sea-salt sulfate (nss-SO₄ ^2-), and other aerosol ions were measured in maritime air west of Tasmania (Australia) during December 1986. A few cloudwater and rainwater samples were also collected and analyzed for major anions and cations. DMS concentrations in the mixed layer (ML) were typically between 15–60 ppt (parts per trillion, 10^–12; 24 ppt=1 nmol m^–3 (20°C, 1013 hPa)) and decreased in the free troposphere (FT) to about <1–2.4 ppt at 3 km. One profile study showed elevated DMS concentrations at cloud level consistent with turbulent transport (cloud pumping) of air below convective cloud cells. In another case, a diel variation of DMS was observed in the ML. Our data suggest that meteorological rather than photochemical processes were responsible for this behavior. Based on model calculations we estimate a DMS lifetime in the ML of 0.9 days and a DMS sea-to-air flux of 2–3 mol m^–2 d^–1. These estimates pertain to early austral summer conditions and southern mid-ocean latitudes. Typical MSA concentrations were 11 ppt in the ML and 4.7–6.8 ppt in the FT. Sulfur-dioxide values were almost constant in the ML and the lower FT within a range of 4–22 ppt between individual flight days. A strong increase of the SO₂ concentration in the middle FT (5.3 km) was observed. We estimate the residence time of SO₂ in the ML to be about 1 day. Aqueous-phase oxidation in clouds is probably the major removal process for SO₂. The corresponding removal rate is estimated to be a factor of 3 larger than the rate of homogeneous oxidation of SO₂ by OH. Model calculations suggest that roughly two-thirds of DMS in the ML are converted to SO₂ and one-third to MSA. On the other hand, MSA/nss-SO₄ ^2- mole ratios were significantly higher compared to values previously reported for other ocean areas suggesting a relatively higher production of MSA from DMS oxidation over the Southern Ocean. Nss-SO₄ ^2- profiles were mostly parallel to those of MSA, except when air was advected partially from continental areas (Africa, Australia). In contrast to SO₂, nss-SO₄ ^2- values decreased significantly in the middle FT. NH₄ ⁺/nss-SO₄ ^2- mole ratios indicate that most non-sea-salt sulfate particles in the ML were neutralized by ammonium.     

Nitrate in the atmospheric boundary layer of the tropical South Pacific: Implications regarding sources and transport

Weekly bulk aerosol samples collected at Funafuti, Tuvalu (8°30S, 179°12E), American Samoa (14°15S, 170°35W), and Rarotonga (21°15S, 159°45W), from 1983 through most of 1987 have been analyzed for nitrate and other constituents. The mean nitrate concentration is about 0.11 g m^–3 at each of these stations: 0.107±0.011 g m^–3 at Funafuti; 0.116±0.008 at American Samoa; and 0.117±0.010 at Rarotonga. Previous measurements of mineral aerosol and trace metal concentrations at American Samoa are among the lowest ever recorded for the near-surface troposphere and indicate that this region is minimally affected by transport of soil material and pollutants from the continents. Consequently, the nitrate concentration of 0.11 g m^–3 can be regarded as the natural level for the remote marine boundary layer of the tropical South Pacific Ocean. In contrast, over the tropical North Pacific which is significantly impacted by the transport of material from Asia and North America, the mean nitrate concentrations are about three times higher, 0.29 and 0.36 g m^–3 at Midway and Oahu, respectively. The major sources of the nitrate over the tropical South Pacific are still very uncertain. A very significant correlation between the nitrate concentrations at American Samoa and the concentrations of ²¹⁰Pb suggests that transport from continental sources might be important. This continental source could be lightning, which occurs most frequently over the tropical continents. A near-zero correlation with ⁷Be indicates that the stratosphere and upper troposphere are probably not the major sources. A significant biogenic source would be consistent with the higher mean nitrate concentrations, 0.16 to 0.17 g m^–3, found over the equatorial Pacific at Fanning Island (3°55N, 159°20W) and Nauru (0°32S, 166°57E). The lack of correlation between nitrate and nss sulfate at American Samoa does not necessarily preclude an important role for marine biogenic sources.