Aerosol Physical Properties and Processes in the Lower Marine Boundary Layer: A Comparison of Shipboard Sub-Micron Data from ACE 1 and ACE 2



 
 

Timothy S. Bates(1,2,3) , Patricia K. Quinn(1,2),
David S. Covert(3,2), Derek J. Coffman(2,1), James E. Johnson(2,1),
and Alfred Wiedensohler(4)


(1) NOAA/Pacific Marine Environmental Laboratory (PMEL), 7600 Sand Point Way NE, Seattle, Washington, 98115 USA

(2) The Joint Institute for the Study of the Atmosphere and Ocean (JISAO), University of Washington, Seattle, Washington, USA

(3) Department of Atmospheric Sciences, University of Washington, Seattle, Washington, USA

(4) Institute for Tropospheric Research, Leipzig, Germany
 
 

Running title: MBL aerosol properties and processes
 
 

Resubmitted to Tellus, 5 June 1999

Abstract. The goals of the IGAC Aerosol Characterization Experiments (ACE) are to determine and understand the properties and controlling processes of the aerosol in a globally representative range of natural and anthropogenically perturbed environments. ACE 1 was conducted in the remote marine atmosphere south of Australia while ACE 2 was conducted in the anthropogenically modified atmosphere of the Eastern North Atlantic. In-situ shipboard measurements from the RV Discoverer (ACE 1) and the RV Professor Vodyanitskiy (ACE 2), combined with calculated back trajectories can be used to define the physical properties of the submicron aerosol in marine boundary layer (MBL) air masses from the remote Southern Ocean, western Europe, the Iberian coast, the Mediterranean and the background Atlantic Ocean. The differences in these aerosol properties, combined with dimethylsulfide, sulfur dioxide and meteorological measurements provide a means to assess processes that affect the aerosol distribution.

The background submicron aerosol measured over the Atlantic Ocean during ACE 2 was more abundant (number and volume) and appeared to be more aged than that measured over the Southern Ocean during ACE 1. Based on seawater DMS measurements and wind speed, the oceanic source of non-sea-salt sulfur and sea-salt to the background marine atmosphere during ACE 1 and ACE 2 was similar. However, the synoptic meteorological pattern was quite different during ACE 1 and ACE 2. The frequent frontal passages during ACE 1 resulted in the mixing of nucleation mode particles into the marine boundary layer from the free troposphere and relatively short aerosol residence times. In the more stable meteorological setting of ACE 2, a significant nucleation mode aerosol was observed in the MBL only for a half day period associated with a weak frontal system. As a result of the longer MBL aerosol residence times, the average background ACE 2 accumulation mode aerosol had a larger diameter and higher number concentration than during ACE 1. The submicron aerosol number size distributions in the airmasses that passed over Western Europe, the Mediterranean, and coastal Portugal were distinctly different from each other and the background aerosol. The differences can be attributed to the age of the airmass and the degree of cloud processing.

1. Introduction

Atmospheric aerosol size distributions impact the radiative and cloud nucleating properties of aerosols. A better understanding of the regional variations in these distributions and the processes controlling them is fundamental to improving aerosol process and climate models. In recent years, measurements and models have identified a number of key features in unperturbed marine boundary layer (MBL) size distributions and the processes that control them (Hoppel et al., 1986, 1990; Hegg et al., 1990, 1992; Covert et al., 1992; Raes and Van Dingenen, 1992; Sievering et al., 1992; Clarke, 1993; Gras, 1993; Ito, 1993; ODowd and Smith, 1993; Hoppel et al., 1994; Perry and Hobbs, 1994; Russell et al., 1994; Raes, 1995; Clarke et al., 1996; Covert et al., 1996a & b; Wiedensohler et al., 1996; Gong et al., 1997; Bates et al., 1998a; Brechtel et al., 1998; Clarke et al., 1998a & b; Kreidenweis et al., 1998; Weber et al., 1998). However, continentally influenced air masses substantially alter these background size distributions and thus affect the overall aerosol radiative and cloud nucleating properties. Measurements in a globally representative range of natural and anthropogenically perturbed environments are needed to assess the extent of this perturbation, the resulting changes in aerosol properties, and the atmospheric processes that determine the extent, timing, and magnitude of the perturbation.

The gas and aerosol measurements made aboard the RV Discoverer during ACE-1 (Bates et al., 1998b) provided a means to characterize aerosol properties and assess processes that affect the aerosol distribution in the lower MBL south of Tasmania (Bates et al., 1998a). In this paper we present similar measurements from ACE 2 (Raes et al., this issue) and compare background submicrometer aerosol size distributions in the two regions and different continentally influenced air masses in the ACE 2 region. Additional complementary ACE 2 shipboard measurements, including aerosol chemical and optical properties (Quinn et al., this issue), hygroscopic properties (Swietlicki et al., this issue), and organic characterization (Novakov et al., this issue), are included in this special issue.

2. Methods

The aerosol and gas sampling methods were similar in ACE 1 and 2. Aerosol particles were sampled at 18 m (ACE 1) or 10 m (ACE 2) above sea level (asl) through a heated mast (Bates et al., 1998b). The mast extended 6 m above the aerosol measurement container and was capped with a cone-shaped inlet nozzle that was rotated into the relative wind. Air was drawn through this 5 cm diameter inlet nozzle at 1 m3 min-1 and down the 20 cm diameter mast. The lower 1.5 m of the mast were heated to dry the aerosol to a relative humidity (RH) of 55 ± 5%. Fifteen 1.9 cm diameter electrically conductive polyethylene tubes extending into this heated zone were used to subsample the air stream for the various aerosol sizing/counting instruments and impactors at flows of 30 l min-1. Comparisons of the total particle count (Dp > 3 nm) during intercomparisons with the NCAR C-130 and ACE 1 ground stations agreed to within 20% suggesting minimal loss of particles in the inlet system (Weber et al., 1999).

Total particle number was measured with condensation particle counters (CPC, TSI model 3010 and UCPC, TSI model 3025). The CPC and UCPC measure all particles larger than roughly 12 and 3 nm respectively. The total particle counts from each instrument were recorded each minute. The number size distribution between 5 and 600 nm was measured every 10 min with two differential mobility particle sizers (DMPS, TSI model 3071 mobility analyzer plus CPC and Hauke mobility analyzer plus UCPC) at an RH of approximately 10% (ACE 1) or 55% (ACE 2). The data were filtered to eliminate periods of calibration and instrument malfunction and periods of ship contamination (based on relative wind and high CN counts). The filtered mobility distributions from the DMPSs were converted to number-size distributions using the inversion routine of Stratman and Wiedensohler (1997). The data were corrected for diffusional losses (Covert et al., 1997) and size dependent counting efficiencies (Wiedensohler et al., 1997) based on pre-ACE 1 and ACE 2 intercalibration exercises. The ten minute number distributions were averaged into 30 minute periods centered on the hour and half-hour. An interactive routine was used to fit lognormal curves to the several modes of the number size distribution. Assuming 1) an external mixture of seasalt and non-sea-salt sulfate and 2) the coarse mode(s) consisted of sea-salt and the accumulation and Aitken modes consisted of the non-sea-salt sulfate, the number size distribution was shifted from 55% RH to dry (10% RH) using growth factors of 1.4 for seasalt and 1.15 for non-sea-salt (nss) sulfate aerosol. The integral of the number size distribution was 69 ± 4.5% of the total number concentration measured with the TSI 3025. The difference is most likely a result of losses in Aitken mode and smaller particles in the DMPS in excess of the typical losses in a model 3071 mobility analyzer.

Air samples for DMS analysis were collected through a Teflon line which ran approximately 60 m from the top of the aerosol sampling mast to the analytical system. One hundred ml min-1 of the 4 L min-1 flow were pulled through a KI solution at the analytical system to eliminate oxidant interferences. The air sample volume ranged from 0.5 to 1.5 L depending on the DMS concentration. Seawater samples were collected from the ship's seawater pumping system at a depth of approximately 5 m (bow inlet) during ACE 1 and 3 m (mid-ship inlet) during ACE 2. The seawater line ran to the analytical system where 5.1 ml of sample were valved into a Teflon gas stripper. The samples were purged with hydrogen at 80 ml min-1 for 5 min. Water vapor in either the air or purged seawater sample stream was removed by passing the flow through a -25° C Teflon tube filled with silanized glass wool. DMS was then trapped in a -25° C Teflon tube filled with Tenax. During the sample trapping period, 6.2 pmoles of methylethyl sulfide (MES) were valved into the hydrogen stream as an internal standard. At the end of the sampling/purge period the coolant was pushed away from the trap and the trap was electrically heated. DMS was desorbed onto a DB-1 mega-bore fused silica column where the sulfur compounds were separated isothermally at 50° C and quantified with a sulfur chemiluminesence detector. The system was calibrated using gravimetrically calibrated DMS and MES permeation tubes. The precision of the analysis, based on both replicate analyses of a single water sample and replicate analyses of a standard introduced at the inlet of the air sample line, was typically ± 8%. The detection limit was approximately 0.8 pmole. The performance of the system was monitored regularly by running blanks and standards through the entire system. Values reported here have been corrected for recovery losses. Losses within the system were <10%. System blanks were below detection limit. Air samples are reported in units of parts-per-trillion by volume (ppt). The mixing ratios were calculated at standard temperature (25° C) and pressure (1013 mbar) such that 1 nmole/m3 equals 24.5 ppt.

Sulfur dioxide (SO2) was measured with a standard TECO SO2 analyzer. Ambient air was drawn into the instrument through a 0.63 cm OD, 30 cm Telfon inlet-tube, a Teflon filter, a nafion drier, and finally 3 m of Teflon tubing at a flow rate 450 ml/min. The inlet tube was located 4 m above sea level at the front of the aerosol sampling container. The filter and drier were located in the aerosol sampling container and were heated to maintain the inlet air at 50% RH. The system was calibrated daily using gravimetrically calibrated SO2 permeation tubes. The standard gas (3.8 or 1 ppb) and SO2 free gas (generated by scrubbing ambient air with activated charcoal) were introduced to the system up-stream of the Teflon filter. The detection limit during ACE-2 was 0.1 parts-per-billion by volume (ppb). The mixing ratios were calculated at standard temperature (25° C) and pressure (1013 mbar) such that 1 nmole/m3 equals 24.5 ppt.

The signal from the TECO SO2 analyzer was recorded as one minute averages. The data were filtered to eliminate periods of calibration and zero, periods when the wind was abaft the beam and thus possibly subject to ship contamination, and during spikes of high SO2 from passing ships (identified by spikes of high CN counts). The filtered one minute data were averaged into 30 minute periods centered on the hour and half-hour.

Aerosol chemical sampling was conducted with Berner-type multijet cascade impactors. Air was sampled only when the relative wind direction was forward of the beam, the relative wind speed was greater than 3 m s-1, and the total particle count indicated the air was free of ship contamination. Further details of the sampling and ion chromatographic and gravimetric analysis are reported elsewhere (Quinn et al., 1998; Quinn et al., this issue). The 50% aerodynamic cutoff diameters of the impactors (Quinn et al., 1998) were converted to geometric diameters by dividing by the square root of the particle density. The densities were calculated using the analyzed chemical mass during the experiments and the chemical equilibrium model AeRho (Quinn and Coffman, 1998). The mass size distributions were dried to 10% RH using growth factors of 1.4 for seasalt and 1.15 for non-sea-salt sulfate aerosol. Values reported in this paper as submicron refer to the first four impactor stages which translates to a 50% geometric dry cutoff diameter of 0.8 m m. The mixing ratios were calculated at standard temperature (25° C) and pressure (1013 mbar) such that 1 nmole/m3 equals 24.5 ppt.

Additional measurements made aboard the ship included atmospheric temperature, pressure, and humidity, surface seawater temperature, salinity and nitrate concentrations, wind speed and direction, rainfall rates, solar radiation, atmospheric radon (Whittlestone and Zahorowski, 1998), and vertical profiles of temperature, dewpoint temperature, and wind speed and direction from radiosondes. The back trajectories, calculated by the Royal Netherlands Meteorological Institute (KNMI), are based on ECMWF 3-D windfields with a resolution of 1° x1° (Scheele et al., 1996). All references to time are reported here in UTC. Dates are given as Day of Year (DOY) where noon on February 1 equals DOY 32.5
 
 

3. Results and Discussion

3.1 Oceanographic Settings and Ocean-Atmosphere DMS Flux

The intensive ACE 1 campaign (November 15 to December 14, 1995) covered three distinct water masses in the region south and west of Tasmania, Australia. Seawater DMS concentrations and ocean-atmosphere fluxes were highest in the Subtropical Convergence Zone to the north and lowest in the Polar Water to the south (Bates et al., 1998a). ACE 2 (June 16 to July 24, 1997) was conducted in one water mass on the eastern edge of the North Atlantic gyre (Figure 1). Sea surface temperature and salinity in the ACE-2 region varied linearly with latitude ranging from 23° C, 36.8 PSU in the south (29° N) to 18° C, 35.8 PSU in the north (41° N). Nitrate concentrations in these waters were less than 0.05 m M and chlorophyll concentrations were generally less than 0.07 m g/L. Surface seawater DMS concentrations during ACE 2 were relatively uniform during the first 3 weeks of the campaign averaging 1.3 ± 0.2 nM (data from distances greater than 40 km from the coast). During the final week, concentrations in the offshore region west of Portugal began to increase, reaching 2.7 nM by the end of the campaign. Using shipboard measured winds, surface water DMS concentrations and the wind speed transfer velocity relationship of Liss and Merlivat (1986), the calculated average ocean to atmosphere flux of DMS was 4.7 ± 5.0 m mol/m2/d and 5.4 ± 3.2 m mol/m2/d, in ACE 1 and ACE 2, respectively (Table 1). Although we can not rule out regions of higher/lower DMS fluxes upwind of the study areas, the comparable fluxes within the study areas suggest a similar natural source of sulfur to the atmospheric aerosol in the two regions during the time of the experiments.

3.2 Meteorological Settings

During the ACE 1 intensive campaign, the synoptic weather pattern was very active with 14 cold fronts passing through the area and six low-pressure systems distinctly cut off from the prevailing westerlies (Hainsworth et al., 1998). The air masses sampled at the ship were typical of background marine conditions in the midlatitudes with very low radon concentrations (<100 mBq m-3) and a mean total (Dp > 3 nm) particle number concentration of 500 cm-3 (Bates et al., 1998a).

In sharp contrast to the active weather patterns during ACE 1, weather patterns in the ACE 2 study area were dominated by the Azores high producing winds from the northwest to northeast 90% of the time. In general, the atmosphere was thermally stable with boundary layer heights less than one km. Based on observations from the ship, the sky was > 50% cloud covered 60% of the time. Frontal systems usually passed to the north of the ACE 2 area. The passage of these frontal systems and the thermal lows that occasionally developed over the European continent, affected the air mass trajectories reaching the ACE 2 region. The air masses sampled at the ship during ACE 2 can be classified into five groups (Figure 1) based on calculated back trajectories (Verver et al., this issue). The ship encountered background Atlantic/Arctic marine airmasses approximately 38% of the time, Mediterranean flow 6% of the time, Western European air flow 18% of the time and air flowing along the Portuguese coast 38% of the time. Airflow along the Portuguese coast resulted in strong gradients in gaseous and aerosol species (Figure 2), making comparisons between platforms in this region difficult.
 
 

3.3 Comparison of the ACE 1 and ACE 2 Background Marine Aerosol

The number size distributions of the background aerosol in the Southern Ocean (ACE 1) and Atlantic Ocean (ACE 2) MBL can be described by 4 distinct modes: a nucleation mode (referred to in Bates et al. (1998a) as the ultrafine mode) (Dp = 5-20 nm), an Aitken mode (Dp = 20-80 nm), an accumulation mode (Dp = 80-300 nm), and a coarse mode (Dp > 300 nm). We will limit our discussion in this paper to the dominant (by number) 3 smaller modes (Table 1).

During ACE 1 the nucleation mode was a major feature in the number size distribution 50% of the time (Figure 4). The observed periods of high nucleation mode particle concentrations coincided with the passage of cold fronts and regions of high convective activity associated with cumulus clouds (Bates et al., 1998a). In the more stable meteorological setting of ACE 2, a nucleation mode was observed at the ship only for a one half-day period (DOY 178.9 179.5, 27-28 June, Figure 3) in a region of broken convective clouds behind a weak frontal system. Boundary layer heights before the passage of this front were < 500 m but rose to 1500-2000 m during and after the frontal passage. Although we have no measurements in this region at this time to confirm the presence or absence of nucleation mode particles in the free troposphere (FT), the presence of nucleation mode particles in the MBL during this period of convective mixing is consistent with our observations from ACE 1 and suggests a FT source for these particles.

Other measurements from ACE 2 are consistent with our shipboard observations. Measurements aboard the CIRPAS Pelican found no evidence of nucleation mode particles during ACE 2 in the MBL (Collins et al., this issue). Conversely, measurements in the upper FT aboard a Cessna Citation aircraft found evidence for new particle formation and growth (M. deReus, personnel communication). The shipboard and aircraft data from ACE 1 and ACE 2 are consistent with the theory that nucleation mode particles are rarely formed over the ocean in background MBL airmasses (Hegg et al., 1990; Raes and Van Dingenen, 1992; Clarke, 1993; Hegg et al., 1993; Hoppel et al., 1994; Raes, 1995; Covert et al., 1996a & b; Clarke et al., 1998a & b). Concentrations of precursor gases are generally too small to promote new particle production in this region of high sea-salt particle surface area (Hegg et al, 1992; Raes et al., 1995). Sea-salt was always a major component of the aerosol in ACE 1 and ACE 2 in both background marine and continentally modified air-masses (Neusess et al., this issue; Putaud et al., this issue; Quinn et al., 1998, this issue). Average aerosol surface areas during the periods of background marine airmasses of ACE 1 (18m asl) and ACE 2 (10m asl) were 40± 22 and 82± 66 m m2 cm-3, respectively.

The ubiquitous features in marine aerosol number size distributions are the well-defined Atiken and accumulation modes that result from cloud processing (Hoppel et al., 1986, 1990). These modes were clearly evident in both ACE 1 and ACE 2 (Figure 4) although the mean diameters and number concentrations differed. The number concentrations of the Aitken and accumulation modes were higher during ACE 2 than ACE 1 (Table 1, Figure 4). In addition, the mean diameter of the accumulation mode was higher in ACE 2 than ACE 1. The distributions are consistent with our current understanding of the sources and sinks of these particles and the meteorological conditions during the two experiments. Aitken mode particles in the remote MBL originate from the condensational growth and coagulation of nucleation mode particles (Hoppel et al., 1990) and more commonly from entrainment from aloft (Clarke et al., 1996, 1997). The large-scale subsidence that accompanied the background marine ACE 2 air masses (Verver et al., this issue; Johnson et al., this issue a) are the most likely source of these MBL Aitken mode particles. Note the average concentration of Aitken mode particles observed during Arctic marine flow (420 cm-3) was appreciably higher than during mid-Atlantic marine flow (235 cm-3) which is consistent with a stronger subsidence during the periods of Arctic flow (Verver et al., this issue). Aircraft measurements during the first ACE-2 Lagrangian experiment, which was conducted in a background marine air mass (Johnson et al., this issue b), indicated periods of elevated concentrations of aerosols with diameters < 100 nm above the MBL. An alternate source for these particles is from long range transport from continental sources in either North America or Western Europe. However, calculated backtrajectories (Verver et al., this issue; Johnson et al., this issue a) and shipboard atmospheric 222Rn measurements suggest the air masses identified here as background marine had not been in contact with the continent for greater than 5 days.

The major sink for Aitken mode particles is coagulation with other Aitken mode particles and cloud processing which leads to growth by liquid phase conversion of trace gases to nonvolatile solute compounds (Hoppel and Frick, 1990; Hoppel et al., 1996). Based on the water supersaturation level in the cloud, some particles will eventually grow sufficiently large to activate and form cloud droplets. When the cloud dissipates, the interstitial (unactivated) particles and residual (from evaporated cloud droplets) particles remain. This process is thought to differentiate the Aitken and accumulation modes with the MBL cloud supersaturation and aerosol chemistry defining the diameter of the intermode minimum (Hoppel et al., 1986; 1994). The larger diameter and higher average number concentration of the accumulation mode during ACE 2 (Figure 4 and Table 1) are consistent with enhanced condensational growth and liquid phase oxidation of soluble trace gases during cloud processing (Hoppel et al., 1996) and are similar to previous measurements in the Azores high (Hoppel et al., 1990; Jensen et al., 1996; Clarke et al., 1997). The sulfur gases available for condensational growth appear to be of similar magnitude in ACE 1 and ACE 2. Although SO2 concentrations measured at the ship during the background conditions of ACE 2 were below the instrumental detection limit (100 ppt), MBL SO2 measurements taken at Punta del Hidalgo on Tenerife (J.P. Putaud, personnel communication) and aboard the Meteorological Research Flightss C-130 Aircraft (Johnson et al., this issue b) during background marine conditions show average concentrations of approximately 20 ppt (range from a few ppt to 100 ppt). This compares with an average background SO2 concentration during ACE 1 of 12 ± 7.6 ppt (DeBruyn et al., 1998). Similarly, the natural source of sulfur (DMS) from the ocean to the atmosphere appears to be similar in magnitude in ACE 1 and ACE 2 (Table 1), although we can not rule out an enhanced DMS source upwind of the ACE 2 area in the higher latitude regions often dominated by coccolithophore blooms (Malin et al., 1993). Although the concentration of condensable species may not have been significantly higher in ACE 2 than ACE 1, enhanced condensational growth also can result from longer MBL residence times. The frequent frontal passages during ACE 1 resulted in relatively short MBL aerosol residence times due to convective mixing and precipitation. Conversely, the more stable meteorological conditions during ACE 2 resulted in longer MBL aerosol residence times (several days based on back trajectory analysis). Similar results have been reported for the tropical regions of the Pacific (Covert et al., 1996a) where long MBL airmass residence times (>5 days) lead to enhanced accumulation mode number and larger modal diameters.

An additional source of accumulation mode particles is direct production from the sea surface (ODowd and Smith, 1993). Sea-salt particles dominated the accumulation mode number and mass during ACE-1 (Murphy et al., 1998; Quinn et al., 1998). Although the sea-salt sub-micron mass was similar in ACE-1 and ACE-2 (Quinn et al., this issue), it did not dominate the sub-micron mass in the background MBL during ACE 2 (Putaud et al., this issue; Quinn et al., this issue) due to the presence of higher concentrations of other species. The importance of sea-salt particles to accumulation mode number was not directly quantified during ACE-2. However, a time-scale analysis of background MBL aerosol evolution during the first Lagrangian experiment suggests that the enhancement of accumulation mode aerosol during the course of the Lagrangian, as wind speed increased, could be ascribed to enhanced sea-salt aerosol flux (Hoell et al., this issue). Although the direct measurements of ACE 1 and the time-scale analysis of ACE 2 show that sea salt particles can be an important component of the accumulation mode number concentration, there was no correlation between wind speed and accumulation mode number concentration during ACE 1 or ACE 2. Even though an increase (decrease) in wind speed increases (decrease) the flux of sea-salt particles to the atmosphere, a change in wind speed does not necessarily change the MBL number or mass concentration of sea-salt particles since the concentration is also a function of the MBL height and sink processes.

3.4 Comparison of ACE 2 Modified Continental Aerosols

The modified continental aerosols reaching the ship during ACE 2 were divided into three regions of origin. Although the submicron aerosol did not show the clear bimodal structure ubiquitous in the background marine distributions, the number size distribution was usually best-fit by the sum of two lognormal modes, an Aitken and an accumulation mode (Table 2 and Figure 5). A significant nucleation mode was not present in any of the modified continental air masses and appeared only when ships passed upwind (Figure 6). The submicron aerosol mass in the continentally modified air masses during ACE 2 was dominated by the nss sulfate aerosol (nss sulfate + ammonium) (Quinn et al., this issue) with average values ranging from 6.6 to 7.9 m g m-3 (1.7 to 2.0 ppb) (Table 2). Total Aitken and accumulation mode aerosol volume in the air masses from the three continental regions covered a limited range from 5.8 to 6.7 m m3cm-3 despite the markedly different number size distributions (Table 2).

The air masses that advected along the coast of Portugal were subjected to local point sources (Verver et al., this issue) and were thus the least aged. The measured concentrations of gases and particles were highly variable (Figures 2 and 6) depending on the trajectory and the distance between the ship and the coast. Being close to the source, SO2 concentrations were higher than in the other air masses (Table 2). The dominant mode in the number size distribution was centered at 76 nm while the secondary accumulation mode was centered at 230 nm. (Figure 5). The modal parameters of these size distributions are very similar to those measured off the coast of Portugal by Hoppel et al. (1990).

The aerosol coming from the Mediterranean had a dominant mode in the number size distribution centered at 105 nm (Figures 5 & 7). SO2 and O3 concentrations in this air mass were high (610 ppt and 54 ppb, respectively). The total aerosol number concentration was the highest measured during ACE-2. The size distribution was unique in that there was no identifiable accumulation mode. The average cloud cover during this period was only 6% as opposed to approximately 60% during the other time periods (Table 2) which would suggest a near absence of cloud processing. It is also interesting to note that this aerosol contained the lowest fraction of nss sulfate + ammonium and the highest concentrations of organic and black carbon (Quinn et al., this issue).

The aerosol that originated over Western Europe and advected 1500 km over the ocean (Figure 1) was well aged with primary and secondary modes in the number size distribution centered at 93 and 220 nm (Figures 5 & 8). SO2 concentrations in this air mass were generally below the detection limit of 100 ppt.

We attribute the differences in the size distributions in these three air masses to the time spent in the MBL and the degree of cloud processing. The least aged aerosol had the smallest diameter particles and was in an airmass with the highest concentration of SO2 (coastal Portugal). Condensational growth and coagulation with time increase the Aitken modal diameter while cloud processing creates an accumulation mode. This evolution was clearly seen in the Lagrangian experiment that followed this "coastal" airmass (Johnson et al., this issue B; Osborne et al., this issue). On the other hand, little evolution was seen in the Lagrangian experiment that followed the more "aged" Western European aerosol (Johnson et al., this issue b; Wood et al., this issue) which suggests that the "processing" had largely taken place by the time the air mass reached the ACE 2 study area.

Aerosols in the continentally modified air masses completely dwarfed the background MBL aerosol (Figure 5). The total submicron volume of the continentally modified air masses was an order of magnitude higher than the background marine atmosphere (Tables 1 and 2).

4. Conclusions

Measurements of aerosol number size distributions during ACE 1 and 2 were used to define the physical aerosol properties in the different air masses of the two regions. The background submicron aerosol measured over the Atlantic Ocean during ACE 2 was more abundant (number and volume) and appeared to be more aged than that measured over the Southern Ocean during ACE 1. Based on seawater DMS concentrations and wind speed, the marine source of DMS and sea salt to the atmospheric aerosol was similar within the two regions. The differences in the average size distributions were most likely a result of the regional meteorology. ACE 1 occurred in a region of frequent frontal passages while ACE 2 occurred in a region dominated by a subtropical anticyclone. The "fair-weather" conditions during ACE 2 favored a longer MBL aerosol residence time.

The submicron aerosol number size distributions in the airmasses that passed over Western Europe, the Mediterranean, and coastal Portugal were distinctly different. The modal characteristics suggest that the Western European aerosol was the most aged and the coastal Portugal aerosol was the least aged. Although the number size distributions of the different continentally influenced air masses were different, the total aerosol volumes were surprisingly consistent.

5. Acknowledgments

We thank Frank Stratmann, Sylvia Henning and the Institute of Tropospheric Research, Leipzig Germany, for the DMPS data inversions, Rita Van Dingenen and Frank Raes for their reviews of this manuscript and the officers and crew of the Institute of Biology of Southern Seas (Sebastopol, Ukrania) Research Vessel Professor Vodyanitiskiy. This research was funded by the Aerosol Project of the NOAA Climate and Global Change Program and the NOAA Office of Oceanic and Atmospheric Research. Financial support for ship time was provided by the European Commission DG XII (Environment and Climate) and the NOAA Office of Oceanic and Atmospheric Research. This is a contribution to the International Global Atmospheric Chemistry (IGAC) Core Project of the International Geosphere-Biosphere Programme (IGBP) and is part of the IGAC Aerosol Characterization Experiments (ACE). This is NOAA/PMEL contribution 2046 and JISAO contribution 651.

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ACE 1 & ACE 2

Background Marine Aerosol

Table 1

   
ACE 1
ACE 2
DMS, seawater

(>40 km from coast)

Mean, nM 
1.7± 1.1
1.6± 0.5
Median
1.3
1.4
DMS, flux

(>40 km from coast)

Mean, m mol/m2/d
4.7± 5.0
5.1± 2.9
Median
3.4
4.4
DMS, atmosphere

(background marine conditions)

Mean, ppt
110± 61
79± 45
Median
96
68
     
 
Atlantic Flow
Arctic Flow
Nucleation mode 

(dry)

N, cm-3
190± 180
60± 70
120± 150
Dgn, nm
16± 8.2
14± 3.3
14± 3.4
s g
1.45± 0.55
1.19± 0.16
1.22± 0.13
Aitken mode

(dry)

N, cm-3
210± 160
235± 125
420± 220
Dgn, nm
33± 6.8
37± 9.3
36± 6.0
s g
1.40± 0.16
1.43± 0.13
1.38± 0.14
Accumulation mode

(dry)

N, cm-3
74± 35
110± 70
110± 36
Dgn, nm
110± 25
170± 24
170± 17
s g
1.41± 0.09
1.44± 0.06
1.42± 0.05
Submicron volume

(<0.8 m m dry geometric diameter)

m m3cm-3  
0.9± 0.54
1.0± 0.22
Submicron nss sulfate+ammonium m g3m-3
0.19± 0.06
0.74± 0.20
0.67± 0.12
(<0.8 m m dry geometric diameter)

Means are given with ± one standard deviation

ACE 2 Continentally Modified Aerosol

Table 2

   
Coastal Portugal
Mediterranean W. Europe
SO2 ppt
840± 760
610± 410
160± 90
Ozone ppb
39± 7.6
54± 10
45± 5.3
DMS, atmosphere ppt
54± 44
40± 26
51± 25
Average cloud cover %
62
6
56
         
Aitken mode

(dry)

N, cm-3
2800± 1900
4600± 1300
650± 310
Dgn, nm
76± 15
105± 15
93± 21
s g
1.52± 0.13
1.51± 0.06
1.60± 0.11
Accumulation mode

(dry)

N, cm-3
350± 160
 
420± 190
Dgn, nm
230± 41
 
220± 39
s g
1.36± 0.14
 
1.37± 0.08
Submicron volume

(<0.8 m m dry geo dia)

m m3cm-3
6.7± 2.1
5.8± 1.7
5.8± 1.8
Submicron nss sulfate + ammonium m g3m-3
7.9± 2.4
6.6± 1.8
7.7± 1.9
(<0.8 m m dry geo dia)

Means are given with ± one standard deviation
 
 
 
 
 
 

Figure 1 ACE 2 study area showing the region sampled by the ship and approximate 5-day isentropic back trajectories for the air mass types discussed in the text. The background Atlantic trajectory (not shown) was similar to the Portuguese Coastal Flow trajectory but did not contact the coast. Figure 2 Total aerosol number, radon and trace gas concentrations plotted as functions of day of year (DOY for 12 July to 20 July) during a period of Portuguese coastal flow showing the highly variable concentrations.
 
 

Figure 3 ACE 2 submicron aerosol dry number size distribution (dN/dlogD) during a period of background marine flow (26-30 June 1997). The legend values refer to the upper dN/dlogD for that color shade.
 
 
 
 

Figure 4 Background MBL aerosol dry number size distributions for ACE 1 and ACE 2. The ACE 1 distribution, showing a large nucleation mode shoulder on the Aitken mode, was obtained from a log-normal fit to the average fitted data (Table 1). The ACE-2 data are an average of the size distributions during the time periods of Atlantic and Arctic flow. Figure 5 Average aerosol dry number size distributions during periods when the MBL was influenced by flow over the continent. The average size distribution during periods of background Arctic flow (Figure 4) is shown for comparison.

 

Figure 6 Aerosol dry number size distribution (dN/dlogD) during a period of Portuguese coastal flow (12-14 July 1997). The legend values refer to the upper dN/dlogD for that color shade.
 


 

Figure 7 Aerosol dry number size distribution (dN/dlogD) during period of Mediterranean flow (7-8 July 1997). The legend values refer to the upper dN/dlogD for that color shade.

Figure 8 Aerosol dry number size distribution (dN/dlogD) during period of Northern European flow (9-11 July 1997). The legend values refer to the upper dN/dlogD for that color shade.