A method for sizing submicrometer particles in air

and Physics

s

ss

and Physics

Discussions

Discussions

Discussions

Discussions

Discussions

|

Ocean Science

Open Access

Open Access

Ocean Science 5401

Discussion Paper

Earth System Sciences

Open Access

Earth System Sciences

Open Access

HydrologyUnion. and Hydrologyon and Published by Copernicus Publications behalf of the European Geosciences

|

Correspondence to: E. Hamacher-Barth ([email protected])

Discussion Paper

Open Access

Open Access

Geoscientific Model Development Model Development Received: 21 May 2013 – Accepted: 27 May 2013 – Published: 19 June 2013Discussions

Open Access

Open Access

Geoscientific Geoscientific Instrumentation 2 1 Instrumentation E. Hamacher-Barth , K. Jansson , and C. Leck Methods and Methods and 1 Data Systems Data Systems Department of Meteorology, Stockholm University, Stockholm, Sweden 2 Discussions Department of Materials and Environmental Chemistry, Stockholm University, Geoscientific Stockholm, Sweden 1

AMTD 6, 5401–5446, 2013

A method for sizing submicrometer particles in air E. Hamacher-Barth et al.

Title Page Abstract

Introduction

Conclusions

References

Tables

Figures

J

I

J

I

Back

Close

|

Discussions

Open Access

Open Access

A method for sizing submicrometer particles in air collected on formvar films Earth System Earth System and imaged byDynamics scanning electronDynamics microscope

Discussion Paper

Climate of the Past

Open Access

Open Access

Climate of the Past

|

Open Access

Open Access

Biogeosciences This discussion paper is/has been under review for the journal Atmospheric Measurement Biogeosciences Discussions Techniques (AMT). Please refer to the corresponding final paper in AMT if available.

Discussion Paper

Atmospheric Measurement Techniques

Open Access

Open Access

Atmos. Meas. Tech. Discuss., 6, 5401–5446, 2013 Atmospheric www.atmos-meas-tech-discuss.net/6/5401/2013/ Measurement doi:10.5194/amtd-6-5401-2013 Techniques © Author(s) 2013. CC Attribution 3.0 License.

Full Screen / Esc

Printer-friendly Version Interactive Discussion

5

Discussion Paper |

25

Aerosol particles are ubiquitious in the troposphere and have a major influence on global climate and climate change. The direct influence of particles on climate is through scattering and absorption of both incoming light from space and thermal radiation emitted from Earth’s surface. The indirect effect of airborne aerosols is related to their role as cloud condensation nuclei (CCN). The number, size, morphology and

|

5402

Discussion Paper

1 Introduction

AMTD 6, 5401–5446, 2013


Title Page Abstract

Introduction

Conclusions

References

Tables

Figures

J

I

J

I

Back

Close

|

20

Discussion Paper

15

|

10

Here we present a method to systematically investigate single aerosol particles collected on formvar film supported by a copper grid, with Scanning Electron Microscopy (SEM) operating at low accelerating voltage. The method enabled us to observe the surface of the sample grid at high resolution. Subsequent processing of the images with digital image analysis provided a statistically and quantitative size resolved information on the particle population including their morphology on the film. The quality of the presented method was established using polystyrene nanospheres as standards in the size range expected for ambient aerosol particles over remote marine areas (20– 900 nm in diameter). The sizing was found to be critically dependent on the contrasting properties of the particles towards the collection substrate. The relative standard deviation of the diameters of polystyrene nanospheres was better than 10 % for sizes larger than 40 nm and 18 % for 21 nm particles compared with the manufacturer’s certificate. The size distributions derived from the microscope images of airborne aerosols collected during a research expedition to north of 80◦ N in the summer of 2008 were compared with simultaneously collected number particle size distributions seen by a Twin Differential Mobility Particle Sizer. We captured a representative fraction of the aerosol particles with SEM and were able to causally relate the determined morphological properties of the aerosol under investigation to aerosol transformation processes in air being advected from the marginal ice edge/open sea south of 80◦ N.

Discussion Paper

Abstract

Full Screen / Esc


5403

|

Discussion Paper | Discussion Paper

25

AMTD 6, 5401–5446, 2013


Title Page Abstract

Introduction

Conclusions

References

Tables

Figures

J

I

J

I

Back

Close

|

20

Discussion Paper

15

|

10

Discussion Paper

5

chemical composition of the CCN will determine the number and size of the cloud drops and thus be important factors in determining the optical properties of clouds. The process of cloud formation however is still not fully understood and modeling these processes has a large degree of uncertainty, mainly due to a lack of observational data (IPCC, 2007). To improve our understanding of cloud droplet activation and evolution we need a much better understanding on how the chemistry of the aerosol multiphase system differs with size. The atmosphere contains significant concentrations of aerosol particles with a diameter size span ranging from a few nanometers to around a few micrometers. It is therefore convenient to describe particle size with one single number. However, unless the particle is a perfect sphere, which is rare in natural atmospheric samples (Bigg and Leck, 2001), there are many ways, including shape/morphology, degree of compactness, state of mixture and particle texture, to describe an aerosol particle (Coz and Leck, 2011). The only method at present to derive size resolved mapping of single particles including the above parameters, down to sizes in the lower nanometer range, is electron microscopy (Coz et al., 2008; Coz and Leck, 2011; Laskin et al., 2006; Kandler et al., 2007; Capanelli et al., 2011; Leck and Bigg, 2008; Bigg et al., 2004). When particles collected onto a thin film of carbon or polyvinyl formal (“formvar”), supported by a 3 mm transmission electron microscopy (TEM) copper grid, are imaged using the electron microscope the electron beam scatters more in the thicker and denser particulate material compared with the uniform grid film. The images usually have good contrast and excellent resolution in two dimensions. Three-dimensional (3-D) resolution of particles can be obtained by shading the grid surface film with deposited particles by fine metal coatings under a low angle (Bigg and Leck, 2001). However, by shadowing the particles they will end up being slightly larger in size which in turn will result in a false size number distribution and move the size limit of detection to larger sizes. Without shading, parts of the sample and the formvar film might on the other hand be destroyed by the high energy electron beam (80–200 kV) usually in operation.

Full Screen / Esc


| Discussion Paper |

5404

Discussion Paper

25

AMTD 6, 5401–5446, 2013


Title Page Abstract

Introduction

Conclusions

References

Tables

Figures

J

I

J

I

Back

Close

|

20

Discussion Paper

15

|

10

Discussion Paper

5

Scanning electron microscopy (SEM) has been used for more than fifty years to image surfaces and particles (Junge, 1952; Parungo et al., 1986; Stevens et al., 2009) and can be operated at an accelerating voltage as low as 1 kV. This enables highly detailed imaging of beam sensitive particles without radiation damage, and in abandonment of metal coating information on size, morphology and chemical (elemental) composition is available without any distortion from any additional chemical constituent. Particle size and morphological parameters can be a resolved down to 2–5 nm in diameter while chemical mapping can be made down to about 50–100 nm in diameter, but at a higher accelerating voltage. In any case, microscopic methods are not completely quantitative, and obtaining statistical information on an aerosol population is very time consuming. The availability of an objective and quantitative method for the evaluation of aerosol samples and microscope images is therefore a prerequisite for a systematic investigation. Moreover, to assess the net aerosol effects on cloud formation and regional climate it has to be assured that the aerosol samples form a representative fraction of the collected ambient aerosol. Here we present a quantitative method to systematically investigate single aerosol particles collected on formvar film supported by a copper grid, with a SEM operating at low accelerating voltage. The method enables us to observe the surface of the sample grid at high resolution. Subsequent processing of the images with digital image analysis has provided statistically size resolved information on the particle population collected on the film. The use of certified polystyrene latex sphere standards to be used in calibration of surface screening inspection systems is discussed. Furthermore, the size distributions derived from evaluating microscope images of the spheres and natural samples were compared with simultaneously collected number particle size distributions seen by a Twin Differential Mobility Particle Sizer (TDMPS; Heintzenberg et al., 2012). Some of the problems associated with the size determination of natural samples are identified and discussed.

Full Screen / Esc


2.1 Scanning Electron Microscopy (SEM

5

Discussion Paper |

5405

|

25

Discussion Paper

20

With the purpose to objectively photograph the particle population collected on the formvar film, each grid was screened systematically. To achieve this each grid was divided into four quadrants using the grid marker in the center of the grids, as displayed in Fig. 1a. The screening of a grid started from the grid marker and went outwards towards the edge of the grid following a quadrant’s (a, b, c or d) diagonal (i.e. 1,1; 2,2; 3,3, etc). In total between 7 and 9 squares were scanned, and between 300 and 500 images were obtained for each grid. To reduce the screening time and to obtain a sufficient number of images for digital image analysis the scanning of the grid started from a low magnification (80×). The whole grid was scanned in order to determine the orientation and to identify damaged areas, and proceeded via magnification of 1100×

AMTD 6, 5401–5446, 2013


Title Page Abstract

Introduction

Conclusions

References

Tables

Figures

J

I

J

I

Back

Close

|

2.2 Grid screening protocol

Discussion Paper

15

|

10

The samples were investigated with a high-resolution SEM (JEOL JSM-7401F) under −5 high vacuum conditions, less than 9.63 × 10 Pa (Stevens, 2009). The electron beam was generated by a cold field emission gun operating with an extracting voltage of 6.8 kV yielding an emission of 20 µA. The focused electron probe current was 16 pA and secondary electrons emitted from the sample surface were detected with an “in-lens” column detector. A negative bias (−2 kV) was applied to the sample stage to decelerate incident electrons before entering the sample surface. To generate a landing energy corresponding to 1 kV a column accelerating voltage of 3 kV was used. This so called Gentle Beam mode was used to minimize radiation damage, surface charge-up and electron beam diameter. Images were recorded after correction of stigmatism, focusing and automatic grey scale adjustment (contrast and brightness). Particle samples under investigation were collected directly onto 3 mm TEM copper 300 mesh grids, coated with a 30–60 nm thick formvar film (TED PELLA INC.).

Discussion Paper

2 Methods for image recording and digital analysis

Full Screen / Esc


2.3 Digital image analyses

5


5406

Discussion Paper

25

To reduce the background noise of the digital image obtained with SEM an Automedian filter and subsequently a Gauss filter (3 × 3) were applied to each image. The following separation of the imaged aerosol particles from their background requires the determination of the intensity of their neighbouring background. Therefore an image section of the particle background was produced at a size of about 250 × 250 pixels by cutting out the aerosol particle (Fig. 2 left, area inside the dashed rectangle) determing the maximal background intensity (see Fig. 2, left). Exactly the same image but including the aerosol particle enabled us to separate the particle and determine its area (in pixel; see Fig. 2, right) by setting 40 % of the intensity difference between background and aerosol maximal intensity as a threshold (see Fig. 3).

AMTD 6, 5401–5446, 2013


Title Page Abstract

Introduction

Conclusions

References

Tables

Figures

J

I

J

I

Back

Close

|

20

2.3.1 Separation of particles from the background

Discussion Paper

15

|

10

Digital image analysis encompasses all processes that are necessary to extract information from a digital image: image processing, segmentation of the object (the aerosol particle) from the background, and measurement of the desired parameters. Microscope images at 40 000× were evaluated using an optimized commercially available ™ image processing software (Aphelion Dev 4.10). Quantitative description of a particle is often a more complex matter than it first appears and measuring particle size alone is sometimes insufficiently sensitive to identify subtle differences between single particles. In addition to size we therefore use morphological parameters to extend the description of the mapped particles (standards and natural samples). These descriptors (elongation and circularity) give a measure on the deviation from spherical geometry and can be directly addressed by the image processing software.

Discussion Paper

to take an image of a square in the copper grid, and finally images at 10 000× and 40 000× were recorded (Fig. 1). The latter were used in the digital imaging procedure.

Full Screen / Esc


The size of an aerosol particle can be expressed by calculating the diameter (Dpa ) of an equivalent sphere which covers the same area according to Eq. (1) q Dpa = Area/π (1)

Discussion Paper

2.3.2 Particle size

|

5

Discussion Paper

10

Dpa is the diameter of a sphere that comprises the same area as the projected dry vacuum particle. The value for the area is calculated from the number of pixels counted for each particle (Allen, 1997; Hinds, 1999). From the particle area (in pixel) the diameter of an equivalent sphere for each aerosol particle was calculated according to Eq. (1). A number size distribution for the respective aerosol sample was obtained by calculating a histogram using MATLAB 2011a and the free available software package EasyFit.

AMTD 6, 5401–5446, 2013


Title Page Introduction

Conclusions

References

Tables

Figures

J

I

J

I

Back

Close

|

Abstract

The morphological properties of the particles have been quantified by the following parameters: 15

– Circularity is sensitive to both overall form and surface roughness, showing values in the range from 0 until 1. A perfect circle has a circularity of 1 while a very irregular object has a circularity value closer to 0 (see Fig. 4).

Discussion Paper

2.3.3 Morphological properties

|

20

4πArea CP2

(2)

|

Circularity uses the Crofton perimeter (CP), which is calculated according to Eq. (3) Xn CP = π N (X , ki ) (3) i =1 i 5407

Discussion Paper

Circularity =

Full Screen / Esc


Discussion Paper

5

with Ni (X , ωi ) the intercept of X in the orientation ωi and ki coefficient to correct for the bias depending on the orientation of the particle on the image. Aphelion™ ◦ ◦ ◦ ◦ 4.10 uses 4 orientations (ω = 0 , ω1√= 45 , ω2 = 90 and ω3 = 135 ); k and k2 equal 1, whereas k1 and k3 equal 1/ 2. An intercept is the number of transitions from intensity to non-intensity in a given orientation (see Fig. 5; Meyer, 1990; Serra, 1988).

|

Elongation = 10

(4)

Dmax is the longest projection of the particle, Dmin is the shortest projection. Elongation shows values to zero and 1; for compact particles the value is closer to zero whereas for open branched ones elongation is closer to 1. The higher the ratio Dmax /Dmin the closer to 1 the value for elongation becomes (see Fig. 4); for a particle with Dmax /Dmin = 1000 : 1 the elongation is calculated to 0.998. 3 Methods for sampling of airborne aerosol particles during ASCOS


The approach using scanning electron microscopy with subsequent image mapping described above (Sect. 2) and below (Sect. 4) was applied to two ambient aerosol samples (see Sect. 5.1 for details) collected onboard the Swedish icebreaker Oden in the course of the Arctic Summer Cloud Ocean Study (ASCOS) in 2008. ASCOS focused on the study of the formation and life cycle of low-level Arctic clouds with linkages to the microbiological life in ocean and ice. ASCOS departed from Longyearbyen on Svalbard on 2 August and returned on 9 September. After traversing the pack ice ◦ northward an ice camp was set up on 12 August at 87 N and remained in operation ¨ et al. (2013) have documented through 1 September, drifting with the ice. Tjernstrom 5408

Discussion Paper

20

(Dmax + Dmin )

6, 5401–5446, 2013


Title Page Abstract

Introduction

Conclusions

References

Tables

Figures

J

I

J

I

Back

Close

|

15

(Dmax − Dmin )

Discussion Paper

– Elongation is sensitive to the overall form of a particle and is calculated according to Eq. (4).

AMTD

Full Screen / Esc


5409

|


25

AMTD 6, 5401–5446, 2013


Title Page Abstract

Introduction

Conclusions

References

Tables

Figures

J

I

J

I

Back

Close

|

20

Discussion Paper

15

|

10

Discussion Paper

5

the ASCOS instrument suite in detail. Key methods for sampling used in this study are described below. The sampling manifold. A PM10 -inlet (diameter < 10 µm at ambient relative humidity) was utilized upstream of all aerosol measurements located in a laboratory container on the 4th deck of the icebreaker. The aerosol inlet was designed to optimize the distance from the sea and from the ship’s superstructure: located ∼ 25 m above the sea surface, 3 m above the roof of the laboratory container. Direct contamination from the ship was ◦ excluded by using a pollution controller. Provided that the wind was within ±70 of the −1 direction of the bow and stronger than 2 m s , no pollution reached the sample inlets (Leck et al., 1996). To maximize the sampling time it was necessary to face the ship into the wind. More details of the set up for the sampling of aerosol particles are given in Leck et al. (2001). Collection of particles for subsequent single particle mapping by SEM. Airborne aerosol particles were collected directly onto the formvar surfaces of the 3 mm TEM grids with an electrostatic precipitator as described earlier (Dixkens and Fissan, 1999; Leck and Bigg, 2008). In brief, particle charges were imparted at the aerosol inlet by 63 −1 a Ni beta-emitting radioactive source and particles were precipitated by a 12 kV cm electric field between the inlet and the collecting surface. Flow rate was kept very −1 low (0.17 mL s ) in order to collect particles up to ∼ 1 micrometer. The collection efficiency of the electrostatic precipitator was inter-compared with the TSI 1236 Nanometer Aerosol Sampler (63 Ni beta-emitting radioactive source and sample flow of 1 L m−1 ) mounted side-by-side with the electrostatic precipitator. Both collected a small, but statistically significant number of particles < 25 nm. The precipitator took samples for 6 to 12 h. Before and after sampling the grids were placed in a gridholder box in a sealed dry plastic bag, together with silica gel packets, and stored in a desciccator at a constant temperature of 20 ◦ C before they were investigated. Number size distributions using a TDMPS. The sampling system deployed to measure the number size distributions of dry sub-micrometer particles used pairs of differential mobility analyzers (DMAs). The TSI 3010 counters used in the DMAs were

Full Screen / Esc


| Discussion Paper

10

Discussion Paper

5

size and concentration calibrated against an electrometer and the TSI 3025 counters for particle sizes below 20 nm diameter in the standard way (Stolzenburg, 1988). This set up yielded a complete number size distribution from 3 to 800 nm diameter in 45 intervals of diameter every 10 min. Further details of the TDMPS system can be found in Heintzenberg and Leck (2012). Using NIST (National Institute of Standards Technology) traceable calibration standards of polystyrene latex (PSL) spherical particles the deviation in determination of the mobility diameter has been determined to ±5 % (W. Birmili, personal communication, 2013). Number size distributions obtained from TDMPS measurements were adapted to SEM measurements by taking the median value for each of the 45 TDMPS size intervals by assuming the same particle diameter in the respective interval. The particle diameters were then merged to form a complete set of diameters across the TDMPS measuring interval. The data were treated in the same way as the SEM derived data to obtain a number size distribution.

AMTD 6, 5401–5446, 2013



Conclusions

References

Tables

Figures

J

I

J

I

Back

Close

|

Abstract

|

5410

Discussion Paper

To determine the error on sizing with the SEM a range of NIST PSL spherical particles was investigated. The diameter values obtained with SEM/image maping were compared with the values obtained by the manufacturer, and morphological parameters (circularity, elongation) were determined. The sizes of the standards were chosen to cover the size distribution of a remote marine atmospheric aerosol population in the submicrometer size range (Covert et al., 1996; Heintzenberg and Leck, 2012) as a superposition of more or less distinguishable modes, and we used PSL spheres of the following diameters: 21 ± 1 nm, 40 ± 1 nm, 57 ± 4 nm, 81 ± 3 nm, 200 ± 6 nm, and 903 ± 12 nm.

|

20

4 Polystyrene latex spheres used for SEM calibration

Discussion Paper

15

Full Screen / Esc


5

| Discussion Paper

10

To prepare the grid standards for subsequent screening and evaluation the bottle containing the microspheres (all polymer microsphere suspensions were purchased from Thermo Scientific) were gently inverted several times to disperse the microspheres and then immersed in an ultrasonic bath at 45 Hz. Two droplets of the 1 % microsphere suspension were further suspended in 250 ml deionized water (Millipore Alpha-Q, resistivity 18 MΩ cm) and stirred gently. A TEM formvar grid was placed on a 0.8 µm pore size Millipore filter (type AA), which was lying upon a nutrient pad (the porous filter material used to hold nutrients in bacterial cultures) in contact with deionized water. 2 µL of the microsphere suspension were placed on the surface of the grid with a hypodermic syringe. The grid, filters and water were kept in a fume hood for 24 h to allow the water and soluble material to diffuse through the plastic film into the water phase. The grid was subsequently stored in a desiccator until further SEM imaging.

Discussion Paper

4.1 Sample preparation

AMTD 6, 5401–5446, 2013



Conclusions

References

Tables

Figures

J

I

J

I

Back

Close

|

Abstract

15

|

5411

Discussion Paper

25

|

20

The measured values for the diameters of the PSL particles were in the range of the manufacturer’s size specifications; for the 21±1 nm spheres a diameter of 22.5 nm with a standard deviation of ±4.1 nm was measured. The larger spheres showed successively smaller relative standard deviations: 42.7 ± 4.1 nm, 60.8 ± 4.5 nm, 81.9 ± 4.6 nm, 199.7±5.2 nm and 903.9±13.1 nm (see Table 1). Generally the diameter for each of the PSL sphere sizes shows a good agreement with the size the manufacturer states in the NIST certificate following with each PSL sphere size: the number size distributions obtained for the PSL spheres are shown in Fig. 6, and examples for intensity histograms are shown in Fig. 7. The size distributions obtained from SEM imaging are monomodal with median values and standard deviations listed in Table 1. The magnitude of the relative standard deviation in our measurements decreased with the increase of the sphere diameter, from 18 % for the 20 nm spheres to 1.3 % for the 900 nm spheres. The same trend was observed for the elongation and circularity of the spheres, as these values

Discussion Paper

4.2 Sizing results

Full Screen / Esc


Discussion Paper

20

|

25

Within the Arctic pack ice region studies of atmospheric aerosol composition resolved over size are sparse. The in-situ data archives include essentially only the observations of ASCOS and three previous research expeditions onboard the Swedish icebreaker ¨ et al., 2004). Oden in the summers of 1991 (Leck et al., 1996c, 2001, 2004; Tjernstrom

|

5412

Discussion Paper

5 Airborne aerosol samples imaged by scanning electron microscope

AMTD 6, 5401–5446, 2013


Title Page Abstract

Introduction

Conclusions

References

Tables

Figures

J

I

J

I

Back

Close

|

15

Discussion Paper

10

At a magnification of 40 000× the size of each pixel in the digital images is 2 nm and by assuming that the electron beam diameter is optimized with respect to focus and stigmatism, a diameter of 1 nm will be obtained. Under this condition the observed size limit of detection in the images will be about 5–10 nm in diameter. By use of 3 × 3 pixels to define a particle from the background noise in the Digital Image Analysis the theoretical minimum size of detection will then be particles with a diameter of 6 nm. At lower magnification the image pixel size and thus particle size that is possible to determine, increases. At 10 000× the pixel size is 8 nm and the theoretical minimum size is 24 nm. The real size limitation of detection of a particle also depends on the element composition of both particle and substrate, and the magnitude of accelerating voltage that generates the electron beam interaction volume. Summarizing the contributing factors the size limit of detection using the PSL particles was found to be around 15 nm by using a magnification of 40 000×.

|

4.3 Determination of size limit of detection

Discussion Paper

5

decrease continuously from 16 % deviation from the elongation for a perfect sphere for the 20 nm spheres to 0.13 % deviation for the 900 nm spheres. The values for circularity reveal measurements of spheres closer to the value of ideal circularity for a perfect sphere for the 900 nm spheres (0.2 % deviation) rather than for the 20 nm spheres (7 % deviation), see Table 1.

Full Screen / Esc


5413

|


25

AMTD 6, 5401–5446, 2013


Title Page Abstract

Introduction

Conclusions

References

Tables

Figures

J

I

J

I

Back

Close

|

20

Discussion Paper

15

|

10

Discussion Paper

5

Based on the above efforts we have arrived at the following knowledge concerning the high Arctic aerosol sources and transformation while airborne: in summer the air in the −3 Arctic is generally very clean and aerosol numbers are low (ca. 100 cm ) because the air mass transport from strong mid-latitudinal polluted regions into the Arctic is reduced (Lannefors et al., 1983; Leck et al., 1996c; Heintzenberg et al., 2006). Nevertheless, during the four expeditions, the size distribution of aerosol particles have shown high variability, but generally presented itself as a superposition of more or less distinguishable modes (Covert et al., 1996; Heintzenberg and Leck, 2012). Following the original work of Whitby (1978) and Hoppel (1988) each of these modes can be represented by a log-normal distribution. Particles with sizes less than 10 nm diameter are generally rarely observed over remote oceans of temperate and tropic climates (Heintzenberg et al., 2004), but occasionally may suddenly increase dramatically for a brief period. In contrast to the warmer oceans, new particle formation events in the boundary layer (BL) of the central Arctic Ocean are relatively common during summer (Wiedensohler et al., 1996; Leck and Bigg, 1999; Leck and Bigg, 2010). At D > 10 nm number concentrations increase up to a maximum somewhere between 25 nm and 80 nm, depending on the location (Heintzenberg and Leck, 2012; Kerminen and Leck, 2001; Leck and Bigg, 2005a, b). This so called Aitken mode results from condensation on, and selfcoagulation of nucleation mode particles as well as from primary marine emissions (Leck and Bigg, 2010; Orellana et al., 2011). A concentration minimum, referred to as the Hoppel minimum, follows at about 80 nm. Another maximum, the so called the accumulation mode, occurs at about 100–200 nm. OOne further maximum appears at diameters larger than 1000 nm (the “coarse mode”), often inconspicuous in a number distribution but contributing substantially to aerosol volume/mass. The accumulation mode typically results from condensation of gases such as the oxidation products of dimethyl sulfide (sulfuric and- methane sulfonic acids), on pre-existing Aitken mode particles (Karl et al., 2012; Kerminen and Leck, 2001) and from the formation of particle mass by in-cloud chemistry via sulfur dioxide. In addition to creation of accumulation mode particles through growth processes, primary particles can be directly injected into

Full Screen / Esc


|

5414

Discussion Paper

The first sample (Sample A) was collected from 15 August, 15:55 to 22:46 UTC (total sampling time of 6:51 h). This period was dominated by air masses originating from the

|

5.1 The approach applied to two aerosol samples collected during ASCOS

Discussion Paper

25

AMTD 6, 5401–5446, 2013


Title Page Abstract

Introduction

Conclusions

References

Tables

Figures

J

I

J

I

Back

Close

|

20

Discussion Paper

15

|

10

Discussion Paper

5

this mode by bubble bursting at the sea–air interface (Bigg and Leck, 2008; Leck and Bigg, 2005a; Leck et al., 2002). Over the summer pack ice near surface wind speeds −1 ¨ et al., 2012), and the extent of open water in are typically low (