Chem. 250 – 10/7 Lecture Updated 10/30 Instructor: Roy Dixon My Website for Course:

Announcements - I A.Lecture Format 1. Powerpoint – overview + graphics + some concepts 2. Blackboard – mainly problem solving B. Topics I’m Covering 1. Aerosol and Cloud Chemistry (10/7) 2. Precipitation/Water Chemistry (11/18) 3. Metals in the Environment (12/2) 4. Toxicology (12/9) Note:- See syllabus for pages from text to read

Announcements - II C.Exam based on understanding of main concepts and ability to solve relevant problems Note: Today’s lecture is on material I understand well – so exam questions will have greater emphasis on lecture than on text (also text emphasis is out-dated)

Atmospheric Aerosols Assigned homework (due next Wed.): Chapter 2 (12, 14, 15, 17, 18, 19) Chapter 6 (44a,b, 45, 48)

Atmospheric Aerosols Rationale for Studying 1. Important in biogeochemical cycles (e.g. S cycle) 2. Direct Effects on Visibility and Climate 3. Effects on Clouds and Precipitation Formation 4. Effects on Human Health

Atmospheric Aerosol Visibility Effects View from my window on typical day Aerosol particles reduce visibility by scattering light Picture on unusually clear day from CSUS internet site View of mountains blocked by particle scattering

Introduction – Climate Effects Direct Effect of aerosols - aerosols scatter more light back to space, leading to cooling at the earth’s surface. Example: Star Fire, Aug., 2001 smoke region looks lighter due to light scattered back to space

Atmospheric Aerosols Climate Effects Example of clouds modified by ship exhaust

Atmospheric Aerosols Health Effects High aerosol concentrations correlate with hospital visits Brauer and Hisham- Hashim, ES&T, 32, 1998

Atmospheric Aerosols – Size Matters Many Properties of Aerosol Particles Depend on Their Size Most Aerosols have Log-Normal Size Distributions Common Types of Size Distributions –Number (number of particles of given size) –Mass (or Volume) –Surface Area

Atmospheric Aerosols – Normal Distribution Normal Distribution (not very common) Mean diameter = 34 nm; Standard deviation (σ) = 5 nm

Atmospheric Aerosols – Log Normal Distributions Log normal distribution – appears as a normal distribution when x-axis is plotted on log scale Geometric Mean Diameter = 23 nm; Geometric Standard Deviation (σ) = 1.8

Atmospheric Aerosols – Calculation Example How many 10 nm particles would have the same volume as nm particles? –N*[  (10 nm) 3 /6] = 1*[  (100 nm) 3 /6] –N = (100/10) 3 = 1000 How many 10 nm particles would have the same surface area as nm particle? –N*[  (10 nm) 2 ] = 1*[  (100 nm) 2 ] –N = 100

Atmospheric Aerosols – N and Mass Distributions Same aerosol, number distribution is dominated by smaller particles, mass distribution is dominated by larger particles For Number: Geometric Mean Diameter = 23 nm; Geometric Standard Deviation (σ) = 1.8 For Mass: Geometric mean = 65 nm

Atmospheric Aerosols – Sources of Aerosols Major Classes (Based on Composition) –Soil Dust (coarse particles) –Sea Salt (coarse particles) –Sulfate (fine particles) –Carbonaceous or Organic (fine particles) Classes (Based on Sources) –Primary Sources –Secondary Sources (typically from oxidation of gaseous precursors) Note: particle “aging” and physical processes make distinction of particle classes more difficult

Atmospheric Aerosols – Sizes of Various Aerosols Surface Area Distribution (3 modes) (Whitby, 1978)

Atmospheric Aerosols – Sulfate Originates from Oxidation of SO 2 –Gas Phase Reaction: 1)SO 2 + OH + O 2 → SO 3 + HO 2 (2 steps) 2)and SO 3 + H 2 O(g) → H 2 SO 4 (g) 3)H 2 SO 4 (g) → H 2 SO 4 (s) Step 3 can occur through a) addition to existing particles (growth of particles) or b) formation of new particles (one of very few ways to form new particles via atmospheric reactions)

Atmospheric Aerosols – Sulfate -Aqueous Phase Reactions -Reactions occur in cloud droplets -Specific reactions covered later -Reactions add sulfate only to particles big enough to nucleate cloud droplets (>50 nm) -Properties -Acidic, unless neutralized by NH 3 (g) -Water Soluble

Atmospheric Aerosols – Carbonaceous Primary Sources –Biomass combustion (forest fire smoke) –Inefficient Fossil Fuel Combustion –Mechanically Produced (e.g. from tires) Secondary Sources (generally richer in O) –Photooxidation of gaseous precursors (e.g.  -pinene to pinonic acid) –Other (cloud, aerosol reactions)

Atmospheric Aerosols – Carbonaceous - Composition Rogge et al., ES&T, 1993; Los Angeles Samples

Atmospheric Aerosols – “ Aging” of Aerosols 1.Sea-salt and soil dust particles -Acids affect particle composition -Examples: -CaCO 3 (s) + 2HNO 3 (g) → Ca(NO 3 ) 2 (s) + CO 2 (g) + H 2 O(g) -2NaCl(s) + H 2 SO 4 (aq) → Na 2 SO 2 + 2HCl(g) 2.Fine particles -Neutralization of sulfuric acid -H 2 SO 4 (aq) + 2NH 3 (g) → (NH 4 ) 2 SO 4 (s) -Oxidation/Nitration of Organic Compounds -Aggregation/Growth of particles

Atmospheric Aerosols – Presence of Water At relative humidity (RH) less than 100%, many aerosol particles exist at concentrated solutions Concentration of solute is related to RH through Raoult’s law (provided particles are large enough): Where: P H2O = the vapor pressure of water, P H2O = the saturated vapor pressure of water; P H2O / P H2O = RH X H2O = the mole fraction of water in the solution i = number of species following dissociation (e.g. for NaCl, i = 2)

Atmospheric Aerosols – Removal of Aerosols Dry deposition particles –Most important for coarse particles (D>1 μm) –Settling rate larger for larger particles –Calculation in book (terminal velocity type) is misleading because mixing in boundary layer is fast (~ hours) –Very small particles (<30 nm) can be removed efficiently to surfaces because they have faster diffusion rates

Atmospheric Aerosols – Removal of Aerosols Wet Deposition –Removal in precipitation processes –Major pathway for fine particles but inefficient for particles with D<50 nm –In-cloud scavenging (1) nucleation of cloud droplets on aerosol particles and 2) formation of precipitation from cloud droplets) –Below-cloud scavenging

Cloud Chemistry - Incorporation of Pollutants Removal in Precipitation for larger particles

Atmospheric Aerosols – Some Example Problems/Questions If SO 2 gas is present at 10 ppbv at P = 0.9 atm and T = 20°C, what would be the mass concentration (in μg m -3 ) of a resultant sulfuric acid aerosol? What if it is converted to ammonium sulfate? –n/V = C = P/RT= (10 x )(0.9 atm)/( L atm/mol K)(293K) –C = (3.74 x mol/L)(1 mol ammonium sulfate/1 mole SO 2 )* (132 g/mol)(10 6  mol/mol)(1000 L/m 3 ) = 49  g/m 3 The element sodium would be expected to exist primarily in which sized particles? –Coarse particles. This is because Na would be expected to mostly originate from sea-salt, which is predominantly in the course mode (since sea-salt has a high % Na and few other sources of Na are significant). As an organic aerosol ages, what should happened to the ratio of the mass of carbon to mass of organics? –As an organic aerosol ages, it becomes oxidized so that O makes up a more significant fraction of the mass. This will decrease the ratio of the mass of C to mass organics. Under what conditions will aerosol particles become more acidic or less acidic as they age? –Aerosol particles will become more acidic if there are significant sources of SO 2 (which oxidizes to sulfuric acid). They will become neutralized (more basic) if there is a significant source of ammonia gas. Which sulfur dioxide oxidation process leads to ultra-fine (D<50 nm) particles? –Gas phase oxidation (Aqueous phase oxidation requires large enough particles for nucleation of cloud droplets).

Atmospheric Aerosols Some Example Problems/Questions – cont. What is the mole fraction of ammonium sulfate in an aerosol particle present at a RH of 95%? At 95% RH, P H2O /P̽ H2O = 0.95 = X H2O (mole fraction of water) 0.95 = n H2O /(n H2O + 3n AS ) note: n AS = moles ammonium sulfate) 0.95n H2O n AS = n H2O 0.05n H2O = 2.85n AS n H2O /n AS = 57 or n AS /n H2O = X AS = n AS /(n AS + n H2O ) = (n AS /n H2O )/[(n AS /n H2O ) + 1] X AS = /( ) = (or 1.7 % by mole) What is the mass fraction of water in the above aerosol particle? mass ratio = m AS /m H2O = ( n AS /n H2O )(132/18) = % H 2 O = m H2O *100/(m H2O + m AS ) = (m H2O /m AS )*100/[(m H2O /m AS ) + 1] = 89%

Cloud Chemistry Rationale for Studying - Cloud reactions can be important (e.g. formation of H 2 SO 4 ) - Precipitation composition depends on cloud composition - Provide introduction to aqueous chemistry

Cloud Chemistry - Incorporation of Pollutants Main mechanisms - Nucleation of cloud droplets on aerosol particles - Scavenging of gases - Reactions within the droplet Incorporation into precipitation

Cloud Chemistry - Incorporation of Pollutants

Cloud Chemistry - Nucleation of Cloud Droplets Cloud droplets can not form in the absence of aerosol particles unless RH ~ 300%. Cloud droplets nucleate on aerosol particles at RH of ~100.1 to ~101%. Cloud droplets should nucleate when RH = 100% except that the vapor pressure over a curved surface is less than that over a flat surface (due to water surface tension) Smaller particles have more curved surfaces and are harder to nucleate

Cloud Chemistry - Nucleation of Cloud Droplets Nucleation more readily occurs with: - Larger particles - Particles with more water soluble compounds (due to growth according to Raoult’s law) - Compounds that reduce surface tension - Smaller aerosol number concentrations (less competition for water so higher RH values)

Cloud Chemistry - Nucleation of Cloud Droplets The concentration of constituents incorporated from nucleation depends on the efficiency of nucleation and on concentration of liquid water in the cloud (also called liquid water content or LWC). The higher the LWC, the lower the concentration (dilution effect) Cloud nucleation leads to heterogeneous cloud droplet composition – Ignored here for calculations