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The Characterization of Atmospheric Particulate Matter Richard F. Niedziela DePaul University 16 May 00 The atmosphere Have you thought about your atmosphere today? Physical dimensions – matm 5.2 1018 kg 10-6 mearth – hatm 100 km – Vatm 1.0 1011 km3 10-1 Vearth Thermal profile – Several different thermal gradients The atmosphere The atmosphere The atmosphere is made out of... – 78% N2 (3.9 1018 kg) – 21% O2 (1.2 1018 kg) – 1% trace gases and suspended matter, or aerosols (0.1 1018 kg) Aerosols Aerosols Aerosols are small particles of condensed matter that are found throughout the environment, from the surface of the Earth to the upper reaches of the atmosphere. Brilliant red sunsets Blue hazes in forests Fog Aerosol characteristics An aerosol is characterized by Composition Size Phase Shape Aerosol composition Organic materials Long-chained hydrocarbons Large carboxylic acids Inorganic materials Mineral acids Metals Organic/inorganic mixtures Aerosol size Particle diameters range from submicron to tens of microns 10-4 10-3 .01 .1 1 10 100 micron = 1 mm = 10-4 cm = 10-6 m 103 104 Aerosol phase Liquids Oil droplets from vegetation Sulfuric acid aerosols Solids Suspended crust material Water ice particles in cirrus clouds Liquid/solid mixtures Aerosol shape Liquids: spherical droplets Solids: crystals and complex structures Shape can impact physical, chemical, and optical properties of aerosols Some actual aerosols Sulfate particle Aluminum particle T. Reichhardt, Environ. Sci. Tech., 29(8), 360A, (1995). Aerosol sources Natural sources Vegetation Oceans Volcanoes Anthropogenic sources Vehicle and industrial emissions Agricultural practices Aerosol production Mechanical action Abrasion of plant leaves Sea spray Wind Nucleation and condensation Cloud formation Aerosols and the Environment Aerosols and the Environment Ozone depletion Global climate change The atmosphere thermosphere upper atmosphere mesopause altitude (km) 80 mesosphere 60 stratopause middle atmosphere 40 stratosphere 20 tropopause troposphere lower atmosphere Ozone O O Pungent gas (named after the Greek word ozein, “to smell”) “Good” vs. “Bad” O – 90% of all ozone – 10 ppmv peak concentration – UV screening O3 Stratosphere Troposphere – 10 ppbv peak concentration – Disinfectant – Respiratory stress Ozone O2 + h O + O2 + M O3 + h O3 + O O+O O3 + M O2 + O O2 + O2 Chapman mechanism Proposed in 1930 Qualitative prediction of atmospheric ozone profile Ozone depletion There has been a recent overall decrease in the stratospheric ozone concentration. CF2Cl2 + h Cl + O3 ClO + O O3 + O Ozone measured over Payerne, Switzerland CF2Cl + Cl ClO + O2 Cl + O2 2 O2 Polar ozone depletion The loss of ozone over the South Pole is more dramatic Polar ozone depletion theories Atmospheric motions Stratospheric air replaced with tropospheric air Discounted due to lack of tropospheric trace gases in the stratosphere Polar ozone depletion theories Reactive nitrogen species chemically destroy ozone Discounted due to low concentrations of nitrogen species during depletion events Polar ozone depletion theories Chlorine compounds are responsible for the ozone depletion Produced from CFCs Persist for up to 100 years Polar ozone depletion cycle 2ClO + M Cl2O2 + h ClOO + M 2Cl + 2O3 2O3 + h Cl2O2 + M ClOO + Cl Cl + O2 + M 2ClO + 2O2 3O2 These reactions are thought to be responsible for 70% of the observed ozone depletion Homogeneous reactions CFCs h ClONO2 ClO h NO2 Polar stratospheric chemistry Homogenous chemistry cannot provide all of the ClO needed to deplete ozone Ozone depletion occurs in the presence of polar stratospheric clouds or PSCs Polar stratospheric clouds Type I Formed near 195 K Composed of nitric acid and water Exist in different phases – Type Ia: Solid nitric acid particles – Type Ib: Supercooled liquid droplets (sulfuric acid, nitric acid, water) Type II Formed near 185 K Water ice particles Heterogeneous reactions ClONO2(s) + HCl(s) PSCs Cl2(g) + HNO3(s) ClONO2(s) + H2O(s) PSCs HOCl(g) + HNO3(s) Chlorine is released into the gas phase Nitrogen is chemically removed Nitrogen is physically removed Heterogeneous reactions h HCl HNO3 CFCs ClONO2 Polar Stratospheric Clouds PSCs H2O Cl2 h HOCl h Cl Sedimentation Cl Polar stratospheric chemistry h HCl CFCs ClONO2 HNO3 h ClONO2 h NO2 H2O PSCs HOCl ClO Cl2 h ClO + ClO h Cl2O2 h ClO Cl Sedimentation O3 O2 Polar stratospheric chemistry Heterogeneous reaction rates are dependent on PSC phase, composition, and size Need to characterize PSCs to fully investigate depletion process PSC characterization Collect infrared spectra of PSCs Mie scattering theory Spherical particles Complex refractive indices for proposed PSC components Complex refractive indices N n ik n is the real component of the refractive index k is the imaginary component of the refractive index determines how fast light moves through material n=c/v determines how light is absorbed by material k = al / 4p Optical constants PSC spectra Ice NAD NAT O.B.Toon and M.A. Tolbert, Nature, 375, 218, (1995). Polar stratospheric clouds Good fits were not obtained using known optical constants for Water ice Nitric acid monohydrate (NAM): HNO3H2O Nitric acid dihydrate (NAD): HNO3H2O Nitric acid trihydrate (NAT): HNO33H2O Polar stratospheric clouds PSCs are not pure water or nitric acid aerosols Ternary mixtures with sulfuric acid Determine optical constants for ternary mixtures Retrieving optical constants Retrieve optical constants from infrared spectra of model PSC aerosols Frequency Temperature Optical constants for NAD Aerosol flow cell II Flow Injection Port 2 1 3 4 MCT 6 Flow Exhaust 5 Cooling Coils FTIR Spectrometer Aerosol flow cell II Aerosol flow cell II Retrieving optical constants Collect many scattering spectra representing different particle sizes Scattering spectra 1.0 Nitric Acid Dihydrate at 180 K 0.8 Extinction 0.6 0.4 0.2 0.0 700 1200 1700 2200 2700 3200 -1 Wavenumber (cm ) 3700 4200 4700 Retrieving optical constants Collect many scattering spectra representing different particle sizes Collect a non-scattering spectrum to estimate k k() = Ka() Non-scattering spectrum 0.20 Extinction 0.15 Nitric Acid Dihydrate at 180 K 0.10 0.05 0.00 700 1200 1700 2200 2700 3200 -1 Wavenumber (cm ) 3700 4200 4700 Retrieving optical constants Collect many scattering spectra representing different particle sizes Collect a non-scattering spectrum to estimate k k() = Ka() Select a scattering spectrum and guess the particle size Retrieving optical constants Collect many scattering spectra representing different particle sizes Collect a non-scattering spectrum to estimate k k() = Ka() Select a scattering spectrum and guess the particle size Use Kramers-Kronig relationship to calculate n() Retrieving optical constants Collect many scattering spectra representing different particle sizes Collect a non-scattering spectrum to estimate k k() = Ka() Select a scattering spectrum and guess the particle size Use Kramers-Kronig relationship to calculate n() Use Mie scattering theory to calculate scattering spectrum Retrieving optical constants Collect many scattering spectra representing different particle sizes Collect a non-scattering spectrum to estimate k k() = Ka() Select a scattering spectrum and guess the particle size Use Kramers-Kronig relationship to calculate n() Compare calculated and experimental spectra Use Mie scattering theory to calculate scattering spectrum Retrieving optical constants Collect many scattering spectra representing different particle sizes Collect a non-scattering spectrum to estimate k k() = Ka() Select a scattering spectrum and guess the particle size Use Kramers-Kronig relationship to calculate n() Correct k() if necessary Compare calculated and experimental spectra Use Mie scattering theory to calculate scattering spectrum Retrieving optical constants Collect many scattering spectra representing different particle sizes Collect a non-scattering spectrum to estimate k k() = Ka() Select a scattering spectrum and guess the particle size Use Kramers-Kronig relationship to calculate n() Vary k() scaling factor, K Correct k() if necessary Compare calculated and experimental spectra Use Mie scattering theory to calculate scattering spectrum Retrieving optical constants Collect many scattering spectra representing different particle sizes Collect a non-scattering spectrum to estimate k k() = Ka() Select a scattering spectrum and guess the particle size Vary particle size Use Kramers-Kronig relationship to calculate n() Vary k() scaling factor, K Correct k() if necessary Compare calculated and experimental spectra Use Mie scattering theory to calculate scattering spectrum Retrieving optical constants Collect many scattering spectra representing different particle sizes Collect a non-scattering spectrum to estimate k k() = Ka() Select a scattering spectrum and guess the particle size Vary particle size Use Kramers-Kronig relationship to calculate n() Vary k() scaling factor, K Correct k() if necessary Compare calculated and experimental spectra Use Mie scattering theory to calculate scattering spectrum Final fit results 1.0 Nitric Acid Dihydrate at 180 K 0.8 Extinction 0.6 0.4 rmed = 0.33 mm 0.2 0.0 700 1200 1700 2200 2700 3200 -1 Wavenumber (cm ) 3700 4200 4700 Final optical constants 2.6 2.4 2.2 Nitric Acid Dihydrate at 180 K 2.0 Refractive index 1.8 1.6 n 1.4 1.2 1.0 0.8 0.6 0.4 0.2 k 0.0 700 1200 1700 2200 2700 3200 -1 Wavenumber (cm ) 3700 4200 4700 NAD optical constants Overall good agreement with thin-film results Some discrepancies do exist Comparison of several aerosol and thinfilm spectra suggest substrate interference Aerosol vs. thin-film spectra NAD thin-film spectra NAD aerosol spectra Wavenumber (cm-1) Aerosol optical constants Optical constants derived from aerosols are better suited for analyzing atmospheric particles Aerosol composition NAD aerosols have a fixed composition Composition of liquid sulfuric acid aerosols can vary Tunable diode laser Window to Cell TDL Focusing Objective Alignment Pinhole Beamsplitter HeNe Ocular Bypass Optics Slit Monochromator To Vacuum Vacuum Jacket Tunable diode laser Tunable diode laser Diode laser beam samples the same aerosol stream as the FT-IR spectrometer Determines water vapor pressure by applying Beer’s law to a single water absorption line Tunable diode laser 1.0 0.9 Transmission (I/Io) 0.8 0.7 0.6 0.5 0.4 Cell Pressure (Torr) 9.7 49.3 99.5 200.0 300.0 0.3 0.2 1751.39 1751.40 1751.41 1751.42 1751.43 -1 Wavenumber (cm ) 1751.44 1751.45 Aerosol flow cell II Flow Injection Port 2 1 3 4 6 5 MCT Detectors Flow Exhaust TDL and Optics Box Cooling Coils FTIR Spectrometer Sulfuric acid optical constants One optical constant study by Palmer and Williams in 1975 Bulk data for a few concentrations at room temperature Widely used by atmospheric scientists Spectra change substantially at low temperatures Sulfuric acid optical constants 2.6 2.4 2.2 75 wt% Sulfuric Acid/Water 2.0 Refractive Index 1.8 1.6 n 1.4 1.2 1.0 0.8 0.6 0.4 k 0.2 0.0 800 1300 1800 2300 2800 3300 -1 Wavenumber (cm ) 3800 4300 Sulfuric acid optical constants 2.5 38 wt% Sulfuric Acid/Water Refractive index 2.0 n 1.5 1.0 0.5 k 0.0 800 1300 1800 2300 2800 3300 -1 Wavenumber (cm ) 3800 4300 Sulfuric acid optical constants 300 Temperature (K) 280 260 240 220 200 30 40 50 60 Weight % H2SO4 70 80 90 Sulfuric acid optical constants The Palmer and Williams optical constants should not be used at low temperatures Temperature and composition dependence indicate interesting ion equilibrium chemistry Emphasize the need to perform similar studies on ternary systems Aerosols and the Environment Ozone depletion Global climate change The atmosphere thermosphere upper atmosphere mesopause altitude (km) 80 mesosphere 60 stratopause middle atmosphere 40 stratosphere 20 tropopause troposphere lower atmosphere Global climate change Climate depends on the chemical composition of the atmosphere Forecasting how the climate will change Will our current coastlines disappear? Will there be another ice age? Over time, incoming solar energy is balanced by energy radiated from Earth Energy balance Sun Eath Earth Climate Change 1994: Radiative Forcing of Climate Change and An Evaluation of the IS92 Emission Scenarios (Cambridge University Press, Cambridge, 1995). Energy imbalance Anything which causes a change in the energy balance is known as a forcing Climate responds to forcing by reestablishing energy balance A forcing example Doubling CO2 concentration Forcing of 4 Wm-2 Surface must warm up 1 K to restore balance Positive forcing warms the planet, while negative forcing cools the planet Forcing sources Solar output Surface characteristics of the Earth Greenhouse gases H2O, CO2, O3, CH4, N2O, and halocarbons Direct interaction with energy radiated from the Earth Forcing sources Aerosols “Direct” forcing – Direct interaction with incoming or outgoing light “Indirect” forcing – Affecting other components of the climate Forcing contributions S.E. Schwartz and M.O. Andreae, Science, 272, 1121, (1996). Aerosol forcing uncertainties Interaction with light is largely unknown Lack of optical constant information Hygroscopic properties are unknown Important gauge of indirect effects Complex spatial and temporal distributions throughout the atmosphere Aerosol forcing effects Aerosol forcing could offset greenhouse forcing Cooling of 2 - 3 K due to “background aerosols” Mt. Pinatubo eruption forcing of -4.5 Wm-2 A temporary, calculated and observed cooling of 0.5 K Peak Tropospheric aerosols Materials: soil dust, sulfates, sea salt, soot, and organics Only sulfates have been “characterized” Soot and organic aerosols are perhaps the most important Present laboratory work Apply optical constant retrieval method to organic aerosols Study hygroscopic properties of organic aerosols Characterize multi-component organic aerosols Organic aerosols Primary organic aerosols (POAs) Emitted from source as an aerosol Secondary organic aerosols (SOAs) Condensation of gas-phase species on preexisting particles Composed of terpenes, PAHs, alkanes, and carboxylic acids Organic aerosols - terpenes Organic aerosols - terpenes Natural sources are nearly ten times greater than anthropogenic sources C=C bonds are susceptible to attack by O3, NO3, and OH Model organic aerosols Determine optical constants for singlecomponent organic aerosols Start with easily obtained materials that closely represent actual organic aerosols Model organic aerosols 2.0 o Absorbance 1.5 1.0 0.5 0.0 1000 2000 3000 4000 -1 Wavenumber (cm ) 5000 Carvone Aerosol flow cell III Aerosol flow cell III Aerosol flow cell III First spectra 0.40 0.35 0.30 Absorbance 0.25 0.20 0.15 0.10 0.05 0.00 1000 2000 3000 4000 -1 Wavenumber (cm ) 5000 Humidity dependence Add water vapor along with organic aerosols Optical constants as a function of relative humidity Hygroscopic vs. hygrophilic Evaluate the indirect effect of organic aerosols Multi-component aerosols Prepare known mixed organic and mixed organic/inorganic aerosols Use single-component optical constants to determine refractive index mixing rules Test rules on unknown aerosols Apply rules to real tropospheric aerosols Acknowledgments PSCs (UNC - Chapel Hill) R.E. Miller, D.R. Worsnop, and M.L. Norman NASA Upper Atmosphere Research Program Organic aerosol studies (DePaul University) Elena Lucchetta LA&S Summer Research Program (1999)