Karlsruhe 290104

Spectroscopic studies of fundamental properties of atmospheric aerosols  : 

Spectroscopic studies of fundamental properties of atmospheric aerosols Cathrine Lund Myhre Department of Chemistry University of Oslo

Outline: 

Outline Fundamental physical properties of the stratospheric sulphate aerosols Optical constants of the liquid systems H2SO4/H2O and HNO3/H2O Solid phase transitions in the HNO3/H2O system New results of the optical properties of organic acids present in tropospheric aerosols

Stratospheric aerosol systems:PSC’s and SSA’s!: 

Stratospheric aerosol systems: PSC’s and SSA’s! PSC: Polar Stratospheric Clouds Solid nitric acid water hydrates: HNO3/H2O Liquid ternary solution droplets of nitric acid, sulphuric acid and water: H2SO4/HNO3/H2O Solid water ice: H2O SSA: Stratospheric sulphuric acid aerosols Supercooled liquid aerosols of H2SO4/H2O Globally distributed

The system H2SO4/H2O: 

The system H2SO4/H2O The degree of the dissociation of the first step: a1=1 The degree of the dissociation of the second equilibrium The dynamics of the equilibrium is important in the interpretation of the optical constants of H2SO4/H2O

Raman spectrum of aqueous H2SO4 solution with the spectral deconvolution of the bands: 

Raman spectrum of aqueous H2SO4 solution with the spectral deconvolution of the bands C. L. Myhre et al., J. Phys. Chem. A, 107, 1979-1991 (2003)

Raman spectra of aqueous H2SO4: 

Raman spectra of aqueous H2SO4 C. L. Myhre et al., J. Phys. Chem. A, 107, 1979-1991 (2003)

The degree of dissociation of H2SO4/H2O: 

The degree of dissociation of H2SO4/H2O C. L. Myhre et al., J. Phys. Chem. A, 107, 1979-1991 (2003) a2, maximum 30-50 wt%, low temperature a2, minimum: 75-80 wt%- independent of temperature

The surface tension of H2SO4/H2O: 

The surface tension of H2SO4/H2O C. L. Myhre et al., J. Chem. Eng. Data 43, 617-622 (1998) Surface tension = Surface free energy Surface tension of water at 293 K g = 72.75 mJ m-2

Why study optical properties?: 

Why study optical properties? IPCC (2001): There are large uncertainties connected to the impact of aerosols on climate the optical properties: extinction coefficient and single scattering albedo are of major importance in the quantification of the climate impact of aerosols Interpretation of remote sensing observations of aerosols, both in the stratosphere and in the troposphere the optical constants are crucial parameters in the retrieval of the remote sensing data The complex index of refraction Ñ(n)= n(n) + ik(n)

Method, specular reflectance: 

Method, specular reflectance

From reflectance measurements to absorption and refractive index: 

From reflectance measurements to absorption and refractive index Kramers-Kronig transformation

The influence of low wavenumber spectral region on the calculation of the optical constants: 

The influence of low wavenumber spectral region on the calculation of the optical constants C. L. Myhre et al., J. Phys. Chem. A, 107, 1979-1991 (2003)

H2SO4/H2OT=293 K: 

H2SO4/H2O T=293 K C. L. Myhre et al., J. Phys. Chem. A, 107, 1979-1991 (2003)

H2SO4/H2O48 wt%: 

H2SO4/H2O 48 wt% C. L. Myhre et al., J. Phys. Chem. A, 107, 1979-1991 (2003)

HNO3/H2OT=293 K: 

HNO3/H2O T=293 K

HNO3/H2O : 

HNO3/H2O 30 wt% 54 wt%

Quantitative tests of the available datasets performed by Dr. R. Wagner: 

Quantitative tests of the available datasets performed by Dr. R. Wagner Significant discrepancies particularly for HNO3/H2O To few data of the ternary system H2SO4/HNO3/H2O R. Wagner et al, Atm. Chem. Phys., 3, 1147-1164, 2003

Conclusions; Optical constants of stratospheric systems: 

Conclusions; Optical constants of stratospheric systems H2SO4/H2O The absorption and refractive index varies significantly with concentration and temperature, particularly in the concentration region 40-65 wt%. HNO3/H2O The optical properties are mainly determined by the concentration. To a good approximation n(n) and k(n) is independent of the temperature below ~253 K. Lack of data particularly of the ternary system H2SO4/HNO3/H2O, but also of the HNO3/H2O system.

Stratospheric solid phase aerosols: 

Stratospheric solid phase aerosols PSC type Ia: PSC’s may crystallise if the temperature stays below ~190 K for hours or even days The study of phase transitions in solid HNO3/H2O is initiated by Dr. Hinrich Grothe, TU, Wien

The composition of the solid phases: 

The composition of the solid phases NAT: HNO3:3H2O corresponding to 54 wt% NAD: HNO3:2H2O: corresponding to 64 wt% NAM: HNO3:H2O: corresponding to 78 wt% Presented here Raman study of the phase transitions IR spectra of the two NAD modifications recorded by Hinrich and his co-workers in Vienna

The structures of the a and b NAD: 

The structures of the a and b NAD a-NAD: low temperature modification b-NAD: high temperature modification Phonon bands rising from lattice vibrations are located in the FAR-IR region

a-NAD and b-NAD: 

a-NAD and b-NAD H. Grothe et al., Vibrational Spectroscopy, 34, 55-62 (2004)

FTIR- spectra of a-NAD (+ NAM) and b-NAD: 

FTIR- spectra of a-NAD (+ NAM) and b-NAD H. Grothe et al., Vibrational Spectroscopy, 34, 55-62 (2004)

Conclusions; Solid phase changes in HNO3/H2O: 

Conclusions; Solid phase changes in HNO3/H2O The infrared band regions particularly suited to distinguish a-NAD and b-NAD: Bands characteristic for NAD, not observed in NAT or NAM: Sym. NO stretch: a-NAD: 1025 cm-1 b-NAD: 1040 cm-1 Torsion mode: a-NAD: 738 cm-1 b-NAD: 754 cm-1 In addition differences are observed in the following regions: ~3490-3250 cm-1, 1737-1850 cm-1 1450 cm-1 characteristic for a-NAD Phase changes a-NAD crystallises at 170-180 K a-NAD to b-NAD at approximately 200 K b-NAD decomposes to NAT at approximately 220 K H. Grothe et al., Vibrational Spectroscopy, 34, 55-62 (2004)

The impact of aerosols on climate: 

The impact of aerosols on climate In a detailed understanding of the influence of different aerosol types on the radiative balance, the key parameter to be explored is the single scattering albedo SSA (Hansen et al., 1997). The single scattering albedo is defined as where Qscat and Qabs is the scattering and absorption efficiency of the aerosols calculated by Mie theory assuming spherical aerosols and adopting the aerosol complex index of refraction and size distribution. v=1 the aerosols are totally scattering, low values of v indicate that the aerosols will rise the global mean surface temperature. Turnover value: v= 0.85-0.91 Complex index of refraction of black carbon: Ñ(550 nm) = 1.75+i0.44 Hansen, J., et al, Radiative forcing and climate response. J. of Geophys. Res.,102, 6831-6864, 1997

Optical properties of organic acids and fog samples in the UV and visible region : 

Optical properties of organic acids and fog samples in the UV and visible region Selected organic acids present in the Water-Soluble Organic Carbon (WSOC) fraction Dicarboxylic acids COOH-(CH2)n-COOH : n=0-4 oxalic, malonic, hydroxymalonic, succinic, glutaric acid Other carboxylic acids Fog samples from the Po Valley, collected at the field station of S. Pietro Capofiume The area is characterised by high levels of pollution from industrial trading and agricultural activities. Results for the most polluted sample presented here

Absorption Index of organic acids, 1 wt% : 

Absorption Index of organic acids, 1 wt% The results of the low concentrations are similar to water in the region from 310-1100 nm, except for pyruvic acid ~ 980 nm: H2O combination/overtone band

Comparison of absorption indexes of common organic and inorganic aerosol constituents: 

Comparison of absorption indexes of common organic and inorganic aerosol constituents

Absorption Index of the fog samples compared with the results from the organic acids: 

Absorption Index of the fog samples compared with the results from the organic acids The chemical composition of the fog samples are similar to the composition of the fine mode aerosols Black carbon: 1.75+i0.44 at 550 nm

Single scattering albedo of aerosols containing a mixture of organic acids and water: 

Single scattering albedo of aerosols containing a mixture of organic acids and water 5 wt% oxalic acid, 5 wt% malonic acid, 5 wt% succinic acid, 5 wt% glutaric acid, 20 wt% pyruvic acid, 1 wt% benzoic acid and H2O Mie calculations assuming spherical aerosols Two different size distributions; r=0.05 mm, r=0.25 mm

Conclusions; Optical properties of common organic acids: 

Conclusions; Optical properties of common organic acids UV-region The maximum of the UV absorption bands are all at l<325 nm and little solar radiation is received at this wavelengths. Near infrared region It is only the amount of water present which is important for the spectral intensity Atmospheric implications The organic acids in WSOC have single scattering albedo ranging from 0.995-1 in the entire wavelength region. They scatter solar radiation similar to sulphate aerosols and have a cooling effect on the climate.

Conclusions; The fog radiative interactions in the Po Valley: 

Conclusions; The fog radiative interactions in the Po Valley The observed spectral characteristics of the organic acids and the common inorganic species do not explain the spectral structure of the fog samples from the Po Valley The single scattering albedo of fog and clouds indicate that the fog can be treated as pure scattering particles in radiation calculations similar to pure water, even in polluted regions like here.

Acknowledgements: 

Acknowledgements Prof. Claus Nielsen Dr. Hinrich Grothe Dr. Robert Wagner Prof. Sandro Fuzzi This work has received financial support from the Norwegian Research Council through grant no. 123289/ 410.

Summary: 

Summary Optical constants in the infrared region H2SO4/H2O: The optical constants varies significantly with concentration and temperature HNO3/H2O: The optical properties are mainly determined by the concentration and to a good approximation independent of the temperature, below ~250 K Phase changes in the solid HNO3/H2O system a-NAD crystallisation at 170-180 K a-NAD to b-NAD at approximately 200 K b-NAD decomposes to NAT at approximately 220 K Optical properties of organic acids They scatter solar radiation similar to sulphate aerosols and they have cooling effect on the climate. Optical properties of polluted fog The fog can be treated as pure scattering particles in radiation calculations similar to pure water

References: 

References Myhre, Cathrine E. L.; Nielsen, Claus J.; Saastad, Ole W. Density and Surface Tension of Aqueous H2SO4 at Low Temperature. Journal of Chemical and Engineering Data (1998), 43(4), 617-622 Myhre, Cathrine E. Lund; Christensen, Daniel H.; Nicolaisen, Flemming M.; Nielsen, Claus J. Spectroscopic Study of Aqueous H2SO4 at Different Temperatures and Compositions: Variations in Dissociation and Optical Properties. Journal of Physical Chemistry A (2003), 107(12), 1979-1991 C. E. Lund Myhre and C. J. Nielsen, Optical Properties of fog and cloud droplets collected in the Po Valley, Italy. Asian Chemistry Letters, 7, Nr, 2&3, 113, 2003 Grothe, Hinrich; Myhre, Cathrine E. Lund; Tizek, Heinz. Vibrational spectra of nitric acid dihydrate (NAD). Vibrational Spectroscopy (2004), 34(1), 55-62 Myhre, C. E. Lund and C. J. Nielsen, Optical properties in the UV and visible spectral region of organic acids relevant to tropospheric aerosols Manuscript in prep.