Aerosol Dynamics

State-of-the-Art Chamber Facility for Studying Atmospheric Aerosol Chemistry

4.7 Aerosol Dynamics

96

the final aerosol water content cannot be independently determined, the total aerosol yield, including water and organic material, is reported.

97

interpretation is particle-particle coagulation that also reduces the particle number concentration. In seeded experiments, at typical seed aerosol size (about 100 nm in diameter) and number concentrations (about 1

o4

^cmm3), the time scale for coagulation is about 74 hours; consequently, coagulation can be neglected as a cause of the decrease in the aerosol number concentration when seed aerosol is used.

In the absence of seed aerosol, initial particle formation occurs by nucleation.

Typically, nucleation occurs in a brief burst that produces 1

o4

^{to 1}

o7

particles cm-l. At number concentrations of 1 07, 1 06, 1 05, and 1

o4

particles cmm3, the time scale for coagulation is order 200 s, 33 min, 5.6 h, and 56 h respectively. Therefore, for large nucleation bursts, coagulation becomes a significant process to reduce the number

concentration Because coagulation is a second order process, the rate of reduction in the number concentration by coagulation decreases much more rapidly than does particle loss to the walls.

Table 4.3 gives characteristic times for chamber processes important in gas-to- particle conversion. Note that the only significant change in particle number

concentration occurs through wall deposition processes (except when nucleation occurs).

Therefore, the number of particles in the chamber can be described by the first-order wall loss rate as N= ~ , e - ~ ~ . As the mass transfer rate to suspended particles greatly exceeds that of those deposited on the wall, the amount of organic material associated with each particle at the time of deposition is assumed to be constant. Thus, the total organic material produced is estimated to be the sum of that still suspended in the aerosol phase plus that deposited to the wall. The particle size increases due to condensation of organic vapors (and any nucleation products). The final size of the particles less the initial size

98

of the seed particles provides an estimate of the volume of organic material suspended in the chamber. Figure 4.1 1 illustrates the competing processes. Adsorption of gas-phase compounds on the wall might possibly decrease their concentrations since the

characteristic time for diffusion to the wall of the present chamber is 1 min. However, FEP Teflon film is non-absorptive to most hydrocarbons; measured losses of C6F6 and m- xylene are 0.00 1 hr-' (loss rate includes potential leaks).

4.8 References

Bowman F. M., Odurn J. R., Seinfeld J. H., and Pandis S. N. (1997) Mathematical model for gas-particle partitioning of secondary organic aerosols. Atmos. Emiron. 31:392 1

-

393 1.

Carter W. P. L., Luo D., Malkina I. L., and Pierce J. A. (1995) Environmental chamber studies of atmospheric reactivities of volatile organic compounds. Effects of varying chamber and light source. Final report to National Renewable Energy Laboratory.

Collins D. R., Flagan R. C., and Seinfeld J. H. (2001) Improved inversion of scanning DMA data. Aerosol Sci. Technol. In press.

Cruz C. N., and Pandis S. N. (2000) Deliquescence and Hygroscopic Growth of Mixed Inorganic-Organic Atmospheric Aerosol. Environ. Sci. Technol. In press.

Liu B. Y. H., and Lee K. W. (1975) An aerosol generator of high stability. Amer. Ind Hyg. J 861-865.

Rader D. J., and McMurry P. H. (1 986) Application of the tandem differential mobility analyzer to studies of droplet growth or evaporation. J Aerosol Sci. 17:771-787.

Nenes A., Pandis S. N., and Pilinis C. (1 998) ISORROPIA: A new thermodynamic equilibrium model for multiphase multicomponent inorganic aerosols. Aquat.

99 Geochem. 4: 123- 152.

Wang S. C., and Flagan R. C. (1989) Scanning electrical mobility analyzer. J. Aerosol Sci. 8:1485-1488.

Table 4.1. Instrumentation Summary

Instrument Measures LDL/Range Accuracy Flow Rate Dedicated}

(LPM) Alternating Gas chromatograph Reactive 1 P P ~ " 2% 0.4 alternating

Flame Ionization Detector Organic HP 5890 series I1 Gas (ROG) Chemiluminescent NO, NO, NO2

analyzer

Thennoenvironmental Instruments, Model 42

S P P ~ 7% 0.7 alternating

Ozone Ozone 1 P P ~ 4% 1.0 alternating

Dasibi Environmental

Hygrometer (capacitance probe) Temperature 5%

-

95% 0.50%

"

alternating Vaisala HMP 233 Relative -20 C to 50 0.1 C

humidity C

Condensation particle counter Volume 0.01 l%d 1.0 dedicated TSI 3010 CPC Concentration particles/cc

Scanning electrical mobility Size 25-700 nmb 0.3%

"

2.75 dedicated

spectrometer Distribution and

Number 1%"

Concentration

Tandem differential mobility Hygroscopic 1.003 +/-.003

"

2.75 alternating

analyzer Growth Factor

Portable spectroradiometer Light Spectrum 280

-

850 0.1 nm N/A N/ A

Licor

-

1800 nm

Gas chromatograph mass ROG1ga.s-phase 1 ppta 5% 5.0 dedicated

spectrometer oxidation

HP GCD products

"

will be a function of hydrocarbon as currently configured

"

in series with ozone and NOx instruments 500

-

30000 cmJ as configured

"

measured precision

Table 4.2*. Calculated ratios of rate constants for photolysis reaction for selected light sources.

Wavelength regions affecting the various photolysis reactions are also shown.

Reaction or Speciesa k , l ( ~ o ) ~ ~~" Ad,,d (kdZ=60) (kdblack) k ~ ( z = O ) ) ~ k l ( Z = ~ > ) ~

0 3 4 , + OiD 0.43 320 3 04 2 8 3 3

CH3CHO-CH3 +HCO 0.067 330 3 10 46 49

CH3COCH3+Products 0.0093 335 3 10 43 49

Higher ketones: products 0.018 340 3 13 53 60

Higher aldehydes ^-+ products 0.24 345 313 53 60

HCHO-+H + HCO 0.33 340 3 17 62 68

H202+2 OH 0.084 355 322 69 1 04

CH300H (absorption) 0.082 3 60 324 70 109

HCHO-+H2 + CO 0.48 360 329 80 133

Acrolein (absorption) 5 -2 3 80 339 87 165

Benzaldehyde-+products 0.48 385 345 89 159

CH30N0 (absorption) 24.0 410 350 93 157

H O N O 4 H + NO 18.0 390 356 97 156

N02-+N0 + 03P 100.0 425 3 69 100 100

Glyoxal+products 1.23 460 3 83 95 37

Methyl Glyoxal+products 1.7 470 417 112 15

N03-+N02 + 03P 1900.0 63 5 548 124 2

N03-+N0

+

0, 207.0 640 592 126 0

0 3 - + 0 2 + 03P 5.2 900 647 114 13

*Adopted from Carter et al. (1995)

"Absorption cross sections and quantum yields.

b~hotolysis rates relative to NO2, expressed as

kl

⁼ 100 x (Photolysis rate for reaction) / (Photolysis rate for NO2).

Zongest wavelength where product of absorption cross sections and quantum yields are nonzero.

d ~ v e r a g e wavelength weighed by Jnon Q.

"Ratios of kEl calculated for the spectral distribution for zenith angle of 60 (2;60), to the calculated using the ground level solar spectral distribution for zenith angle of zero.

f ~ a t i o s of k,, calculated for the spectral distribution for blacklights, to the

kel

calculated using the ground level solar spectral distribution for zenith angle of zero.

Table 4.3. Characteristic time scales for processes occurring in the chamber.

*If transport to particle >> formation of condensable gases

KOH

⁼ 2 x 10-I cm3/molecule~s [OH] = 7 x lo6 molecule/cm3 N = 1 x 10' cm-3

Dp ⁼ 100 nrn a ⁼ 1.0 a, = 1.0

(&Nc) ⁼ 1.0 m-' KMG ⁼ 1.0 d m i n Km = 1.5 x mlmin A ~ W , = 2 x lo4

Kij for N ⁼ 1 x 10' em", D,= 100 nm

Typical chamber values 2 hrs

2min

5 hrs

3.5 days

3.1 days

200 sec at 1

o7

particle cm-3 to 56 hours at lo4 particle

~ m - ~ Process

Formation of condensable

&as

Gas-particle transport (condensation)

Particle-wall transport (wall deposition) Gas-deposited particle transport (absorption to deposited aerosol) Coagulation (no nucleation) Coagulation (nucleation)

Time constant

1

ZF = KoH[OH]

I +8hlaDe

ZGP = 2~ND,hc

1

ZPW = ( A J V ~ ) K M P

1

ZGD =

a(Aflc)KM~

-

²

TCOAG = KN

TCOAG=

-

2

KN

0 & --- ^--- 7 - ^{- --} 7 - -7--- - T -

200 300 400 500 600 700 800 900 1000

Wavelength (nm)

Figure 4.1. Absorption spectra for 2 mil FEP Teflon film.

Figure 4.2. Block diagram of gas- and aerosol-phase instrumentation.

5m 5LPM

-

m t h

Genersatc4;.

South

^vent

,

port port

Aerosol

-

Blacklights (Same NO2 photolysis rate)

--- Solar Z=O

...

Solar Z=60

300 350 400 450 500 550 600 650 700

Wavelength (nm)

Figure 4.3. Light spectrum

corn

Carter et al. (1995). Solid line represents Sylvania 350BL; dashed line is solar light intensity at zenith angle ⁼ 0; dotted line is solar light intensity at zenith angle ⁼ 60.

Dry compressed air (0-40 psi) - I

Activated Carbon (ROG)

Purafil (NO,)

Molecular Sieve

Filter (aerosol)

Dry, NO,-fiee, ROG-fiee Particle- fiee air

Figure 4.4. Flow scheme for clean-air system.

Bypass for aqueous

210Po

Figure 4.5. Seed particle generation schematic.

Chamber air

@

Pressure transducer Solenoid valve H HEPA filter

2.75 LPM +-.

4 ( I I

I I I