Key Points (2020-01-24)What we learned What we will learn today

(1) Atmospheric nanoparticles• Direct radiative forcing• Indirect radiative forcing

(2) Chemical reactions of atmosphericNanoparticles

• Polar Stratospheric Clouds• Phase changes in atmospheric

nanoparticles• Chemical reactivity

(3) Air and mineral surface reactionsat low relative humidity

Course Introduction

• Nanochemistry at air-mineral interfaces- Aerosol chemistry at nanoscale- Air and mineral surface reactions at low relative humidity (RH)

Today’s Lecture (Jan 22nd, 2008)

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Aerosol Chemistry at Nanoscaleatmospheric nanoparticles

Image from http://books.nap.edu/books/0309057825/xhtml/images/img00021.gif

windows2universe.org

Image from http://asd-www.larc.nasa.gov/erbe/

Aerosol Chemistry at Nanoscaleatmospheric nanoparticles

Figures from Anastasio C and Martin ST, Atmospheric nanoparticles in “Nanoparticles and the environment” REVIEWS IN MINERALOGY & GEOCHEMISTRY 44: 293-349

2001

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ELEMENTS DECEMBER 2008390

the U.S. Environmental Protection Agency use PM1, PM2.5, and PM10 to indicate particulate matter (PM) plus their cutoff sizes in micrometers (µm); for example, PM2.5 signi-fi es particles smaller than 2.5 µm.

Whatever the name, each size mode has distinctive chem-ical and optical properties, transformation mechanisms, and deposition pathways (Seinfeld and Pandis 2006). The relationships are complex, and thus FIGURE 2 is also. We will discuss the fi gure fi rst in terms of the curves in its upper part and then the information below them.

The uppermost curve (solid line) refl ects the number of aerosol particles in an idealized volume of the atmosphere. By far the largest number is in the smallest size range, called the nucleation mode, in which sizes are smaller than a few tens of nanometers. These particles are important for the formation and growth of larger particles. Their surface to volume ratio is high, but their total surface area and volume are small relative to the other size groups.

In the atmosphere, most nucleation-mode particles consist of sulfates, nitrates, and organic compounds. They form by homogeneous nucleation processes that occur at ambient temperatures and involve the nucleation of relatively nonvolatile products of photochemically initiated gas-phase reactions (Kulmala et al. 2004; Seinfeld and Pandis 2006). During this process, nanoparticles ~3 nm in diam-eter form at a rate of about 102 particles/cm3 per second in urban areas. The rate can rise to 104 to 105 particles/cm3 per second in coastal areas and industrial plumes; typical particle growth rates range from 1 to 20 nm per hour (Kulmala et al. 2004). Clearly, nanoparticles form a highly dynamic system in the atmosphere.

Hot, supersaturated gases from manufacturing facilities, automobiles, and biomass burning (FIG. 3) also form new particles upon rapid cooling. They are considered primary particles, which grow through aggregation as well as by adhesion of gas particles onto their surfaces.

Overlapping with the nucleation mode are particles in the Aitken-mode size range (~10 to 100 nm), named after the person who fi rst recognized the importance of these par ticles. Aitken-mode particles form by coagulation and condensation of nucleation-mode particles (Anastasio and Martin 2001; McMurry et al. 2004; Seinfeld and Pandis 2006). Depending on their nature, they may get larger as they form chains (e.g. soot). “Nanoparticles” as used here refers to the combination of these fi rst two size modes.

The accumulation and coarse modes make up the remainder of the size ranges of aerosol particles. They are notable for their signifi cant volumes and masses and include the parti-cles that produce many objectionable visible effects, such as dirty surfaces. The seeming mismatches among number concentrations, surface areas, volumes, and masses across this large size range are striking and contribute to the importance of defi ning the problems of interest before embarking on a study of aerosol particles.

Particles typically grow in the atmosphere, whether by clumping, condensation of other materials onto their surfaces, or the common process of deliquescence, whereby hygroscopic particles such as salts grow by collecting water (Wise et al. 2007). When large enough, they are removed by washout during rains. The largest particles, in the coarse mode, consist of advected minerals (mineral particles are collectively called dust in the atmospheric literature). These coarse particles are lost through dry deposition under the infl uence of gravity or through washout.

EHF T QD 2 Schematic of the size distribution of aerosol particles for various parameters (number, N; mass, M; volume,

V; surface area, S) in an idealized atmospheric sample, together with illustrations of their formation mechanisms. The number, surface area, volume, and mass distributions, as well as the principal modes, sources, and particle formation and removal mechanisms, are adapted from Whitby (1978). The nucleation mode was defi ned by Whitby (1978) as containing particles with diameters less than ~100 nm, and Seinfeld and Pandis (2006) further divided particles into nucleation and Aitken modes for those smaller than 10 nm and between 10 to 100 nm, respectively.

Chemical conversionof gases to low-volatility vapors

Hot vapor

Primaryparticles

Chainaggregates

1 10 100 1000

10-3

10-2

10-1

100

101

102

103

104

105

Parti

cle

num

ber

Diameter (nm)

Droplets

Nucleationmode

Aitkenmode

Accumulationmode

Wind-blown dust+

Emissions+

Sea spray+

Volcanoes+

Plant particles

Sedimentation

Massor

Volume

Surface area

10

20

30

Volu

me

(μm

3 cm

-3)

Surf

ace

area

( × 2

0μm

2 cm

-3)

Number

Condensation Coagulation

Coarse-particle

mode

Secondaryparticles

Rainoutand

washout

Low-volatilityvapor

10000

Mas

s (×

2 μg

cm

-3)

Diffusion

Homogeneousnucleation

3 nm

105

104

103

102

101

Des

ert

Mar

ine

Rem

ote

cont

inen

tal

Fore

st

Urb

an

Traf

fic

Rur

alPart

icle

s cm

-3

Free

trop

osph

ere

Pola

r

EHF T QD 1 Number concentrations of nanoparticles from various environments. The concentrations in the free

troposphere, polar, marine, desert, remote continental, rural, and urban areas are from Seinfeld and Pandis (2006). Other data are for marine (Weber et al. 1998), urban (Shi et al. 2001; Smith et al. 2005), forest (O’Dowd et al. 2002), and traffi c (Zhu et al. 2002; Charron and Harrison 2003) sources. Bars indicate minimum and maximum concentrations given in the references. The cubes at the tops of some bars are schematic representations of the number concentrations of particles.

ELEMENTS DECEMBER 2008390

the U.S. Environmental Protection Agency use PM1, PM2.5, and PM10 to indicate particulate matter (PM) plus their cutoff sizes in micrometers (µm); for example, PM2.5 signi-fi es particles smaller than 2.5 µm.

Whatever the name, each size mode has distinctive chem-ical and optical properties, transformation mechanisms, and deposition pathways (Seinfeld and Pandis 2006). The relationships are complex, and thus FIGURE 2 is also. We will discuss the fi gure fi rst in terms of the curves in its upper part and then the information below them.

The uppermost curve (solid line) refl ects the number of aerosol particles in an idealized volume of the atmosphere. By far the largest number is in the smallest size range, called the nucleation mode, in which sizes are smaller than a few tens of nanometers. These particles are important for the formation and growth of larger particles. Their surface to volume ratio is high, but their total surface area and volume are small relative to the other size groups.

In the atmosphere, most nucleation-mode particles consist of sulfates, nitrates, and organic compounds. They form by homogeneous nucleation processes that occur at ambient temperatures and involve the nucleation of relatively nonvolatile products of photochemically initiated gas-phase reactions (Kulmala et al. 2004; Seinfeld and Pandis 2006). During this process, nanoparticles ~3 nm in diam-eter form at a rate of about 102 particles/cm3 per second in urban areas. The rate can rise to 104 to 105 particles/cm3 per second in coastal areas and industrial plumes; typical particle growth rates range from 1 to 20 nm per hour (Kulmala et al. 2004). Clearly, nanoparticles form a highly dynamic system in the atmosphere.

Hot, supersaturated gases from manufacturing facilities, automobiles, and biomass burning (FIG. 3) also form new particles upon rapid cooling. They are considered primary particles, which grow through aggregation as well as by adhesion of gas particles onto their surfaces.

Overlapping with the nucleation mode are particles in the Aitken-mode size range (~10 to 100 nm), named after the person who fi rst recognized the importance of these par ticles. Aitken-mode particles form by coagulation and condensation of nucleation-mode particles (Anastasio and Martin 2001; McMurry et al. 2004; Seinfeld and Pandis 2006). Depending on their nature, they may get larger as they form chains (e.g. soot). “Nanoparticles” as used here refers to the combination of these fi rst two size modes.

The accumulation and coarse modes make up the remainder of the size ranges of aerosol particles. They are notable for their signifi cant volumes and masses and include the parti-cles that produce many objectionable visible effects, such as dirty surfaces. The seeming mismatches among number concentrations, surface areas, volumes, and masses across this large size range are striking and contribute to the importance of defi ning the problems of interest before embarking on a study of aerosol particles.

Particles typically grow in the atmosphere, whether by clumping, condensation of other materials onto their surfaces, or the common process of deliquescence, whereby hygroscopic particles such as salts grow by collecting water (Wise et al. 2007). When large enough, they are removed by washout during rains. The largest particles, in the coarse mode, consist of advected minerals (mineral particles are collectively called dust in the atmospheric literature). These coarse particles are lost through dry deposition under the infl uence of gravity or through washout.

EHF T QD 2 Schematic of the size distribution of aerosol particles for various parameters (number, N; mass, M; volume,

V; surface area, S) in an idealized atmospheric sample, together with illustrations of their formation mechanisms. The number, surface area, volume, and mass distributions, as well as the principal modes, sources, and particle formation and removal mechanisms, are adapted from Whitby (1978). The nucleation mode was defi ned by Whitby (1978) as containing particles with diameters less than ~100 nm, and Seinfeld and Pandis (2006) further divided particles into nucleation and Aitken modes for those smaller than 10 nm and between 10 to 100 nm, respectively.

Chemical conversionof gases to low-volatility vapors

Hot vapor

Primaryparticles

Chainaggregates

1 10 100 1000

10-3

10-2

10-1

100

101

102

103

104

105

Parti

cle

num

ber

Diameter (nm)

Droplets

Nucleationmode

Aitkenmode

Accumulationmode

Wind-blown dust+

Emissions+

Sea spray+

Volcanoes+

Plant particles

Sedimentation

Massor

Volume

Surface area

10

20

30

Volu

me

(μm

3 cm

-3)

Surf

ace

area

( × 2

0μm

2 cm

-3)

Number

Condensation Coagulation

Coarse-particle

mode

Secondaryparticles

Rainoutand

washout

Low-volatilityvapor

10000

Mas

s (×

2 μg

cm

-3)

Diffusion

Homogeneousnucleation

3 nm

105

104

103

102

101

Des

ert

Mar

ine

Rem

ote

cont

inen

tal

Fore

st

Urb

an

Traf

fic

Rur

alPart

icle

s cm

-3

Free

trop

osph

ere

Pola

r

EHF T QD 1 Number concentrations of nanoparticles from various environments. The concentrations in the free

troposphere, polar, marine, desert, remote continental, rural, and urban areas are from Seinfeld and Pandis (2006). Other data are for marine (Weber et al. 1998), urban (Shi et al. 2001; Smith et al. 2005), forest (O’Dowd et al. 2002), and traffi c (Zhu et al. 2002; Charron and Harrison 2003) sources. Bars indicate minimum and maximum concentrations given in the references. The cubes at the tops of some bars are schematic representations of the number concentrations of particles.

308  Anastasio & Martin 

Figure 10. Schematic diagram of nanoparticle formation and growth.

organic compounds to form low-volatility products such as H2SO4 or polar organics. At sufficient supersaturations, these products condense to form a critical germ (also called a critical nucleus), which is a polymeric unit just sufficient in size to begin growth into a larger particle. Growth of the critical germ by condensation of low-volatility gases yields a nanoparticle (diameter of ~1 nm). These fresh nanoparticles can eventually join the accumulation mode via further growth, e.g., through continued condensation of vapors.

Nucleation is the science investigating the kinetics and thermodynamics of the formation of a new phase of a material at a size just sufficient to be stable. In addition to their role in new particle formation, nucleation processes are also critical to an accurate understanding of a number of other atmospheric events, including cloud droplet activation on CCN, ice formation, and the deliquescence/efflorescence of particles. In this section we focus on the nucleation of new particles through homogeneous nucleation, i.e., from gaseous precursors. The theoretical treatment of new particle nucleation, as well as field and laboratory measurements of nanoparticle formation, are addressed.

Theoretical treatment of critical germ formation

The process of new particle formation is a first-order transition from a disordered to

297Atmospheric Nanoparticles

dominant removal mechanism for nanoparticles (Dp < 50 nm) because of their small sizes. Nanoparticles can be removed by diffusing to the Earth’s surface (dry deposition), diffusing and colliding with larger particles (intermodal coagulation), or by growing out of the nanoparticle size range (through condensation of gases). Nanoparticle residence times thus depend both upon diameter (which affects the diffusion rate) and upon the atmospheric aerosol surface area. Under typical conditions, nanoparticles are believed to have residence times ranging from minutes to a few days. Coarse particles also have atmospheric residence times that vary from minutes to days, but due to their relatively large sizes, gravitational settling is their dominant removal mechanism. Intermediate between these sizes are the accumulation mode particles. These particles are removed efficiently neither by diffusion nor by settling, and thus they tend to have the longest atmospheric lifetimes (typically days to weeks). The dominant sink for accumulation

Table 1. Global emission estimates for major aerosol types in the 1980s. From Seinfeld and Pandis (1998). Used by permission of Wiley-Interscience.

Estimated flux (Tg yr -1) Particle size Source Low High Best categorya

NATURAL Primary Soil dust (mineral aerosol) 1000 3000 1500 Mainly coarseb

Sea salt 1000 10000 1300 Mianly coarseb

Volcanic dust 4 10000 30 Coarse Biological debris 26 80 50 Coarse Secondary Sulfates from biogenic gases 80 150 130 Fine Sulfates from volcanic SO2 5 60 20 Fine

Organics from biogenic gases 40 200 60 Fine Nitrates from NOx 15 50 30 Fine and coarse

Total natural 2200 23500 3100

ANTHROPOGENIC Primary Industrial dust, etc. (except soot) 40 130 100 Fine and Coarse Soot 5 20 10 Mainly Coarse Secondary Sulfates from SO2 170 250 190 Fine Biomass burning 60 150 90 Fine Nitrates from NOx 25 65 50 Mainly Coarse Organics from anthropogenic gases 5 25 10 Fine

Total anthropogenic 300 650 450

TOTAL 2500 24000 3600 a Coarse and fine size categories refer to mean Dp above and below 1 m, respectively. b By mass, coarse particles dominate. By number the fine mode may dominate. Note that sulfates and nitrates are assumed to occur as ammonium salts. Flux units: 1012 g yr -1 (dry mass).

Sinks of Atmospheric Particles

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Removal by diffusion (Brownian motion):dominant removal mechanism of Dp < 50 nm (1) dry deposition and colliding with larger particles (intermodal coagulation) or (2) glowing out of the nanoparticle size range through condensation of gases)

Wet deposition: dominant removal mechanism of intermediate sized particle (accumulation mode, 100-500 nm)

Gravitational settling : dominant removal mechanism of the coarse particles

Figures from Anastasio C and Martin ST, REVIEWS IN MINERALOGY & GEOCHEMISTRY 44: 293-349 2001

Direct Radiative Forcing

Figures from Anastasio C and Martin ST, REVIEWS IN MINERALOGY & GEOCHEMISTRY 44: 293-349 2001

Indirect Radiative Forcing

Indirect effect: changing optical properties, frequencies, and lifetimes of clouds (IPCC 2001)Anthropogenic increases in the number of cloud condensation nuclei (CCN).

Hygroscopic particles serve as centers for water condensation and form cloud droplets.With a fixed cloud liquid water content, an increase in the number of CCN yield smaller cloud drops.

Increase albedo (reflectivity) of the cloud and enhance its efficiency for scattering incoming solar radiation.

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The chemical composition of atmospheric ultra¯ne particles 2589

stationary fuel use32.2%

on-road vehicles43.1%

petroleum industry0.5%

other industrial processes

7.2%

miscellaneous processes

5.2%

surface coating0.2%

waste burning1.2%

other mobile sources10.4%

organic compounds

64.8%

chloride0.078%

ammonium0.14%

nitrate0.83%

sodium0.031%

EC7.1%

sulphate7.3%

trace species4.2%

unknown15.5%

(a)

(b)

Figure 3. Ultra¯ne particle emissions in California’ s South Coast Air Basin (1996) that surroundsLos Angeles: (a) source contributions to primary ultra¯ne particle emissions; (b) chemical com-position of primary ultra¯ne particle emissions. Total PM0 :1 emissions are 13.25 tonnes perday.

The emissions inventory for ultra­ ne particles (PM0:1) constructed for the LosAngeles area indicates a mass emissions rate of 13 tonnes per day in particle sizessmaller than 0.1 m m aerodynamic diameter. This emissions rate is approximatelyconsistent with the measured 0.8 m g m¡3 ambient ultra­ ne particle concentration,indicating that most of the ultra­ ne particle mass in the Southern California atmo-sphere could well be due to primary particle emissions from sources. As shown in­ gure 3a, the largest sources are on-road motor vehicles (43%), stationary source fuelcombustion (32%), non-highway mobile sources (10%, particularly diesel engines usedin o¬-road vehicles and in mobile equipment such as refrigeration units), and other

Phil. Trans. R. Soc. Lond. A (2000)

Chemical reactions of atmospheric particles

• Particle reactions in the troposphere are also significant and affect concentrations of important gaseous pollutants such as ozone, nitric acid and sulfur dioxide.

• Mineral dusts: The mode of the number size distribution of mineral dusts = 100 nm. The mass mode lies between diameter of 2 - 5 um.Dominant clays-kaolinite, montmorillonite, and chlorite.Surface Coating- Iron oxide and aluminum oxides and hydroxides.Mineral dust can be coated by sulfate, nitrate, and other species.

hygroscopic dusts acts as CCN and further oxidation reaction can occurs in aqueous medium.

Transmission electron micrographs of collected Fe2O3 particles with (NH4)2SO4 loadings.

Image from S.T. Martin, J.H. Han, Journal of Crystal Growth 219 (2000) 290-299

• Polar Stratospheric Clouds compose of sulfuric and nitric acid and water present.

• H2SO4 aerosols (50 nm) condensation of gaseous H2O and HNO3 to form larger particles (several micrometer).

• The surface of PSC particles can activate chlorine (non-ozone-destroying chlorine to ozone-destroying chlorine compounds)

• ClONO2 + H2O HOCl + HNO3

• ClONO2 + HCl Cl2 + HNO3

• The PSC also slow down the chlorine deactivation pathway by decreasing nitric acid.

ClO + less NO2 less ClONO2

Polar Stratospheric Clouds (PSC)

Cl radical formation by photolysis on PSC

Image from http://www.jpl.nasa.gov/releases/2004/108.cfm

Reactions on PSC

TEM images of sea salt

(a and b) Subhedral halite (NaCl) and euhedral sulfate crystals. The particle in b belongs to the smallest sea-salt particles that occur in the ACE-1 samples. (Southern Ocean, Cape Grim, ACE-1); (c) Halite particles in various stages of conversion to sulfate and nitrate. Grain A is partly converted, whereas C has been completely converted to nitrate and grains B to sulfates. (Azores, North Atlantic, ASTEX/MAGE.),

images from Proc Natl Acad Sci U S A. 1999 March 30; 96(7): 3372–3379.

NaCl (s, aq) + HNO3(g) NaNO3 (s, aq) + HCl

After photolysis, chlorine radials form. This affects the chemistry of troposphericozone, dimethyl sulfide, and hydrocarbons.

Figures from Anastasio C and Martin ST, REVIEWS IN MINERALOGY & GEOCHEMISTRY 44: 293-349 2001

2.7-4 nm

Idaho Hill, Colorado

SO2 + ·OH + O2 +H2O H2SO4 + HO2 ·Photochemical reactionsSunrise (UV irradiation) ·OH formation

H2SO4 concentration increases New particle formation

This new particle model can be explained By H2SO4, H2O, and NH3 ternary systemrather than H2SO4 and H2O systems.

The presence of pptv levels of NH3 greatlyReduce the critical concentration of H2SO4required to form new particles.

No diurnal trend

Sx

Aqueous Particle

Crystalline Particle

5

4

3

2

1

Rel

ativ

e Vo

lum

e

90807060504030Relative Humidity

0.5125102030400.050.10.150.20.30.4

-10123579∆G

HomogeneousNucleation

(NH4)2SO4/H2O

Upper Loop

Lower Loop

Aqueous Particles

CrystallineParticles

*

(NH4 )2 SO

4NH4NO3

H2S

O 4HNO

3

Hysteresis Effect: Particle Phase Depends on Relative Humidity History

g(80%) from 5 to 1000 nm. The growth factor drops from1.84 for an infinitely large particle to 1.46 for a particle of5-nm mobility diameter.

3. Observations of the Nanosize Effect on theHygroscopic Growth Factor

[14] Hygroscopic growth factors of NaCl nanoparticleswere measured both for increasing and for decreasingrelative humidity (i.e., ‘‘deliquescence-mode’’ and ‘‘efflo-rescence-mode’’ experiments, respectively). Details of theexperimental apparatus were given by Biskos et al. [2006].Aerosol nanoparticles were generated using two indepen-dent methods, namely by vaporizing and condensing gran-ular NaCl and by electrospraying a high-purity NaClaqueous solution. Nanoparticles are especially susceptible

to significant impurities because of their small mass, andself-consistent results from redundant sources are an impor-tant test for ruling out the influence of chemical impuritiesin the observations.[15] The hygroscopic growth curves for the particles

generated by the two methods and for both deliquescence-and efflorescence-mode experiments are the same withinmeasurement uncertainties (Figure 2). In agreement with thepredictions of Figure 1, the observations show that thehygroscopic growth of NaCl nanoparticles decreases withdecreasing particle size. For example, the mobility-diametergrowth factor of 6-nm particles at 80% RH is approximately1.55 compared to 1.75 for 60-nm particles. The decrease inthe growth factor is observed for particle sizes smaller than40 nm.[16] The predictions of models 1 to 4 are shown as lines

in Figure 2. The measurements agree best with model 4,suggesting that the inclusion of a size-dependent shapefactor is critical when interpreting or predicting growthfactors of nonspherical nanoparticles. Moreover, model 4predicts the observations within experimental uncertaintiesof 2% for RH and 1% for mobility diameter. Therefore, thephysical parameters of saq, raq, rs, aw, and c are knownaccurately enough to predict nanosize behavior, thus imply-ing no observable size dependence of saq, raq, or rs for thenanosizes investigated. The sole exception is that thegrowth factor of 6-nm particles (the smallest size studied)deviates in a statistically significant way from the predic-tions of model 4. Either nanosize effects on saq, raq, or rs oruncertainties in saq, raq, rs, aw, or c therefore becomequantitatively important at approximately 6 nm. Measure-ments at even smaller sizes would be highly warranted toexpand on this finding.[17] Hameri et al. [2001] also carried out measurements

of the hygroscopic growth of NaCl nanoparticles. Thoseresults are included in Figure 2 as black squares. The twostudies have excellent qualitative agreement. There are,however, some nonnegligible differences both in the mag-nitude of the growth factors (approximately 0.05 higher forHameri et al. [2001] at lower RH values) and on thepromptness of the phase transitions at efflorescence anddeliquescence. The measurements of the current studysuggest that deliquescence and efflorescence are promptwhen a critical RH is passed, whereas the measurements ofHameri et al. [2001] show a nonprompt response, which ismore evident for the smaller particle sizes.[18] A credible explanation of the nonpromptness of

particle deliquescence and efflorescence observed byHameri et al. [2001] is the presence of hygroscopic impu-rities in the nanoparticles. Precipitation nanoparticles gen-erated by atomizing an aqueous solution (i.e., the methodused by Hameri et al. [2001]) may concentrate impuritiesbecause the sizes of the primary aqueous droplets prior toevaporation are large [Kinney et al., 1991]. Although little isknown about the nature of the impurities, mixed particleshave an initial deliquescence relative humidity (DRH) at theeutonic and a final DRH at full solution, i.e., apparantnonprompt deliquescence [Martin, 2000]. Moreover, theefflorescence relative humidity (ERH) can decrease if theimpurities inhibit critical germ formation.[19] The level of impurities is lower when a solution is

electrosprayed (one of the methods used in this study) rather

Figure 2. Mobility-diameter growth factors of NaClnanoparticles. Experimental growth factors are shown bythe data points (equation (1)): (open circles) particlesgenerated by electrospray and (open squares) particlesgenerated by vaporization-condensation. Red points arerecorded during efflorescence-mode experiments, and bluepoints are recorded during deliquescence-mode experiments.Black squares show the data of Hameri et al. [2001] forparticles generated by atomization. Lines show the growthfactors of four models (see x10): model 1 (hatched line),model 2 (dot-dashed line), model 3 (dashed line), and model4 (solid line). Models are evaluated for 0 < wt < 45%.

L07801 BISKOS ET AL.: HYGROSCOPICITY OF AEROSOL NANOPARTICLES L07801

3 of 4

Sx

Aqueous Particle

Crystalline Particle

5

4

3

2

1

Rel

ativ

e Vo

lum

e

90807060504030Relative Humidity

0.5125102030400.050.10.150.20.30.4

-10123579∆G

HomogeneousNucleation

(NH4)2SO4/H2O

Upper Loop

Lower Loop

Aqueous Particles

CrystallineParticles

*

(NH4 )2 SO

4NH4NO3

H2S

O 4HNO

3

Hysteresis Effect: Particle Phase Depends on Relative Humidity History

Effect of Size on Deliquescence RH (DRH)q ( )Light scattering is affected by both changes in refractive indices and particle sizes that accompany cycling between aqueous and crystalline states

H ra

tio Surface tension at air-mineral interfaces

DR

H

dyne/cm at T=300K

NaCl crystal size, # of solvent molecules in crystal and drop

JOURNAL OF CHEMICAL PHYSICS VOLUME 113 NUMBER 18 2000 8200

Plots of DRH relative to bulk crystal (75%) vs. crystal size of various air-NaCl surface tension.

JOURNAL OF CHEMICAL PHYSICS VOLUME 113, NUMBER 18, 2000, 8200

Chemical Reactivity1. Chemical reaction can alter the chemical composition and thus

the physicochemical properties of nanoparticles

2. Chemical reactions can change the composition of the gas-phase.

Example:Atmospheric heterogeneous chemistry is generally oxidative (e.g., ozone and hydroxyl radical). Particles evolve from hydrophobic to hydrophilic.

“Aged” Organic particles are better cloud condensation nuclei than freshly formed particles.

Courtesy by Dr. Katrib Y.and Martin Group at Harvard

ELEMENTS DECEMBER 2008392

the smallest particles can be used to infer initial sizes by using a photodetector with a PHA, which detects them as voltage pulses that depend on their size. This technique is called CNC-PHA, and it is used to measure post-CNC-treatment sizes of individual nanoparticles with diameters of 3 to 10 nm.

Another common approach to particle sizing is to use a DMA to sort particles before they enter the CNC. A DMA classifi es particles according to their mobility in an electric fi eld. The mobility is a measure of the velocity of a charged particle in a known fi eld and depends on charge and aero-dynamic drag, which in turn depend on size and shape but not on density. The combination DMA-CNC instru-ment is called a scanning mobility particle spectrometer (SMPS). It can be used to measure size distributions from 3 to 700 nm and can be operated at either discrete or continuously changing voltages, depending on whether accuracy or statistics is paramount.

The SMPS has better sizing resolution than the CNC-PHA. On the other hand, the CNC-PHA is more sensitive and detects nanoparticles faster. CNCs in their various confi gu-rations are extremely powerful tools for particle sizing. Operating two in parallel, each with different size cutoffs, allows rapid measurement of nanoparticle nucleation in the remote troposphere, where concentrations are low and changes rapid (McMurry 2000).

Aerosol Mass SpectrometryAerosol mass spectrometry has become a major technique for studying aerosol particles, both individually and in size-classifi ed groups, and it is increasingly being used by the atmospheric-science community (see Nash et al. 2006 for a review). Several types of instruments are used. They consist of an inlet to accept the particles, an optical or aerodynamic size analyzer, followed by a mass spectrom-eter. If groups of size-classifi ed particles travel to the mass spectrometer, it measures their bulk compositions. When nanoparticles are being measured, they are focused, e.g. with an electrostatic lens, prior to detection by the mass spectrometer.

During single-particle analysis, particle sizes are deter-mined by a combination of aerodynamics and light-scat-tering techniques (Wang et al. 2006; Johnston et al. 2006). For particles with diameters between ~7 and 30 nm, aero-dynamic and electrodynamic properties are used for focusing particles into the measuring chamber (Wang et al. 2006; Johnston et al. 2006). After sizing, the particles are decomposed and ionized by an intense laser pulse, and the masses of the resulting fragments are determined by a mass spectrometer that provides information regarding elemental and molecular identities. Sizes and compositions are measured online, i.e. in real time (Noble and Prather 2000). Tens of particles per second can be analyzed, and both organic and inorganic species can be detected (Murphy et al. 1998). Combined with statistical analysis, the procedure provides time-resolved information regarding single particles.

EHF T QD 4 Schematic of the nanoparticle cycle in the atmosphere. CCN = cloud condensation nuclei

AutomobileIncinerators

SmeltersPower plant

Commercially produced

Meteoritic smoke

Soot SootMetalSea salt

CCNCloud

SulfatesNitrates

Organic aerosolOrganic aerosol

Particles larger than 50 nm

Mesosphere and Thermosphere (80 to 100 Km)

Nanoparticle cycle in the atmosphere

AnthropogenicNatural

Biomass burning

Breaking waves

Volcano

Biogenesis

NH3 Organic compoundsNOxSO2

Organic aerosol

Condensation

Mixed or coated particles

Coagulation

Scattering (cooling)

Absorption (warming)

Nanotube

Ti oxideFullerenes

Gases

Homogeneous nucleation

Sources

Single phase

particlesAggregated

particlesSedimentation

Dry

dep

ositi

on

DiffusionNanoparticles

Wet

dep

ositi

on

Rainout

Washout

Primary particle

formation

Secondary particle

formation

Coagulation and

accumulation processes

Air and mineral surface reactions at low relative humidity (RH)

Bulk mineral structure-based surface model

(10-14) MnCO3 Surface Structure in Ambient ConditionsIn response of the surface truncation, the atomic structure changes to minimize the surface energy by relaxation.

Relaxed surface structure (Top view)

Relaxed surface structure (side view)

Jun et al, ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 11, 2007, 3918

MnCO3 (Rhodochrosite)

have done a study on the surface roughness calculationsincontact-modeandtapping-modeatomic forcemicroscopyand concluded that tapping-mode AFM provides a bettermeasure of surface roughness.39 Using the 1 µm × 1 µmimages shown in the middle panel of Figure 1a, R iscalculated from the following equation

where N is the number of pixels in the image, zi is theheight of the ith pixel, and zm is the mean image height.38

The calculated value of R is 0.42 nm and is on the orderof one monatomic step height and one unit cell.34,35,40

The remaining images shown in Figure 1b-e wereobtained over a 3-week time period. Because of therelatively long time period, the samples were moved outof the AFM head between measurements. When thesurfaces were imaged again, attempts were made to putthe sample back in the AFM in such a way that imagesof the same or nearby regions could be obtained. Theremaining AFM data shown in Figure 1b-e clearly showthat the surface changes as it is exposed to the ambientenvironment. After 2 days at ca. 15% RH, there is evidenceof “white spots” in the image. As seen in both the 3 µm× 3 µm and 1 µm × 1 µm images, the number and size ofthese spots grow over the time. It is evident that the spotsare nucleating at the steps and the number of spots growsas a function of time. Rows of white spots decorating

(39) Simpson, G. J.; Sedin, D. L.; Rowlen, K. L. Langmuir 1999, 15,1429.

(40) Sangwal, K.; Gorostiza, P.; Sanz, F.; Borc, J. Cryst. Res. Technol.2000, 35, 959.

Figure 1. MgO(100) surface cleaved and imaged with tapping-mode AFM under ambient conditions: left panel, 3 µm × 3 µm AFMimages (600 nm scale bar); center panel, 1 µm × 1 µm images (210 nm scale bar); and right panel, line profiles. Boxes in the imagesshown in the left panel are the 1 µm × 1 µm images shown in the center panel. Lines drawn in the images shown in the centerpanel correspond to the profile plots shown in the right panel. Images are shown after cleaving the MgO crystal to expose the (100)surface to the ambient environment for different amounts of time: (a) 2 h (0 days), (b) 2 days, (c) 6 days, (d) 13 days, and (e) 21days.

R ) |∑i)1

N (zi|zm)2

N|1|1/2 (1)

H2O, HNO3 Adsorption on MgO(100) and CaCO3(101h4) Langmuir, Vol. 21, No. 19, 2005 8795

after reaction with nitric acid vapor at 15% RH, respec-tively. The image taken after reaction was recorded atlow relative humidity under a dry nitrogen purge over thesample. There are clearly new structures present on thesurface after reaction with nitric acid vapor. On the basisof previous infrared studies,44 these surface features areconcluded to be microcrystals of magnesium nitrate thatappear to nucleate on steps that are still evident on thesurface. The microcrystals show evidence for polygon-typesymmetry and are between 0.2 and 1 µm in width and∼45 nm in height. The outline of a hexagon has beensuperimposed on one of the microcrystallites to see thissymmetry better.

The other AFM images, shown in Figure 3c-e, havebeen recorded of the nitric acid MgO(100) surface but athigher relative humidity. The line profiles shown next tothe AFM images in c and d indicate that as the relativehumidity increases the height of the microcrystals in-

creases by approximately 20%. This increase in height isconsistent with infrared studies that have shown that asthe relative humidity increases, crystalline magnesiumnitrate hydrate can incorporate water into its structure.44

The infrared data suggest that the crystalline magnesiumnitrate hydrate goes from a tetra- to a penta- to ahexahydrate salt. The AFM data indicate that as wateris incorporated into the structure it swells along the surfacenormal.

At the highest relative humidity (Figure 3e), the AFMimage shows an increase in the number of smaller featureson the surface. Besides an increase in the number of smallsurface features, there is a nearly a doubling in the heightof the larger surface structure as seen in the line profiles.The surface begins to become difficult to image, and there

(44) Al-Abadleh, H. A.; Grassian, V. H. J. Phys. Chem. B 2003, 107,10829.

Figure 2. CaCO3(101h4) surface cleaved and imaged with tapping-mode AFM under ambient conditions: left panel, 3 µm × 3 µmAFM images (600 nm scale bar); center panel, 1 µm × 1 µm images (210 nm scale bar); and right panel, line profiles. Boxes in theimages shown in the left panel are the 1 µm × 1 µm images shown in the center panel. Lines drawn in the images shown in thecenter panel correspond to the profile plots shown in the right panel. Images are shown after cleaving the CaCO3 crystal to exposethe (101h4) surface to the ambient environment for different amounts of time: (a) 5.5 h (0 days), (b) 3 days, (c) 7 days, (d) 13 days,and (e) 21 days.

H2O, HNO3 Adsorption on MgO(100) and CaCO3(101h4) Langmuir, Vol. 21, No. 19, 2005 8797

Take Home MessageNanochemistry at air-mineral interfaces- Nanoparticle synthesis under air conditions.

A l h i t t l- Aerosol chemistry at nanoscale(1) New particle formation and growth (homogeneous and heterogeneous

reactions) affects CCN formation.(2) Particle composition and morphology( ) p p gy(3) Chemical and photochemical reactions(4) Unique size-dependent properties

* R d A t i C d M ti ST “At h i ti l ” i* Read Anastasio C and Martin ST, “Atmospheric nanoparticles” inNanoparticles and the environment, REVIEWS IN MINERALOGY &GEOCHEMISTRY 44: 293-349 2001.

* Buseck and Adachi, “Nanoparticles in the Atmosphere,” Element, 4, 389-3942008.

- Air and mineral surface reactions at low relative humidity (RH)The structure of nanostructured islands depends on the quantitaty of surface

adsorbed water.