Chem. 253 – 2/25 Lecture Announcements I • Return HW 1.3 + Group Assignment • Last Week’s Group Assignment – most did reasonably well • New HW assignment (1.5 – posted on website) • Next Wednesday – – – – Will have HW due No Group Assignment Exam 1 Justin will take over (covering water chemistry) Announcements II • Exam 1 – On all topics covered through today – Will review topics to know at end of lecture – Exam will be mix of short answer questions (multiple choice or fill in the blank) + work out problems – I may post an example exam (if I can find a relevant copy) • Today’s Lecture Topics – Tropospheric Chemistry – Finish up cloud chemistry (Chapter 3 and 4) – Atmospheric Effects (Chapter 4) Sulfur, Aerosol, and Cloud Chemistry Review of Main Concepts I • Aerosols – suspension of particles in a gas – particle size range is related to formation and growth – three main sizes (ultrafine mode – new particles from gas phase, accumulation mode – processed particles, and coarse mode – from mechanical production) – distributions are log normal and can be defined based on number, surface area, or mass – four main chemical classes (sea-salt, soil dust, sulfate, and organic) – both primary and secondary sources Sulfur, Aerosol, and Cloud Chemistry Review of Main Concepts II • Sulfur Chemistry – both natural and anthropogenic sources – mostly emitted as SO2, but reduced S is also important – predominant pathway is oxidation to H2SO4 – gas phase oxidation occurs through 2-step OH reaction – gas phase H2SO4 production can lead to new particle formation (although mostly leads to growth of existing particles) – aqueous phase SO2 oxidation adds mass to accumulation mode sized particles Sulfur, Aerosol, and Cloud Chemistry Review of Main Concepts III • Cloud/Precipitation Chemistry – Will review + add new material Cloud/Precipitation Chemistry - Incorporation of Pollutants Cloud Chemistry - Incorporation of Pollutants • Main mechanisms - Nucleation of cloud droplets on aerosol particles - Scavenging of gases - Reactions within the droplet 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 (d < 50 nm) 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) • While larger particles are more efficient at nucleation, there are a lot more small particles, so number of droplets formed is dominated by the accumulation mode (100 nm < d < 2.5 mm) Cloud Chemistry - Nucleation of Cloud Droplets aerosol size distribution (mass based) 0.01 100 0.1 log dp (mm) 1 10 Nucleation efficiency Soot, soil hygroscopic aerosol 0 0.01 0.1 log dp (mm) 1 10 Cloud Chemistry - Nucleation of Cloud Droplets • The concentration of constituents incorporated from nucleation depends on the efficiency of nucleation and on the liquid water content (or LWC). • LWC = g liquid H2O/m3 of air • The higher the LWC, the lower the concentration (dilution effect) • Cloud nucleation leads to heterogeneous cloud droplet composition – Ignored here for calculations Cloud Chemistry Nucleation Example Problems • Why is an RH over 100% required for cloud droplet nucleation? • Why is nucleation efficiency higher in less polluted regions (for a given particle size)? • An ammonium bisulfate aerosol that has a concentration of 5.0 μg m-3 is nucleated with 50% efficiency (by mass) in a cloud that has a LWC of 0.40 g m-3. What is the molar concentration? What is the cloud pH? Cloud Chemistry - Scavenging of Gases • Also Important for covering water chemistry (e.g. uptake of CO2 by oceans) • For “unreactive” gases, the transfer of gases to cloud droplets depends on: the Henry’s law constant (always) • In special cases, transfer can depend on LWC (if high), or can be limited by diffusion (if reacting very fast in droplets) • Henry’s Law: KH X PX where KH = constant (at given T) and X = molecule of interest - Cloud Chemistry Scavenging of Gases: “unreactive” gases • When LWC and KH are relatively low, we can assume that PX is constant (good assumption for SO2 and CO2) Then [X] = KH∙PX where PX comes from mixing ratio • When KH is high (>1000 M/atm), conservation of mass must be considered (PX decreases as molecules are transferred from gas to liquid) • We will only consider 2 cases (low KH case and 100% gas to water case) - Cloud Chemistry Scavenging of Gases “unreactive” gases • For compounds with high Henry’s law constants, a significant fraction of compound will dissolve in solution • fA = 10-6KHRT(LWC) where fA = aqueous fraction (not used in assigned problems) • When fA ~ 1, can use same method as for cloud nucleation From Seinfeld and Pandis (1998) - Cloud Chemistry Scavenging of Gases: “reactive” gases • Many of the gases considered are acidic and react further • Example: Dissolution of SO2 gas Reaction: Equation: SO2(g) + H2O(l) ↔ H2SO3(aq) H2SO3(aq) ↔ H+ + HSO3HSO3- ↔ H+ + SO32- KH = [H2SO3]/PSO2 Ka1 = [H+][HSO3-]/[H2SO3(aq)] Ka2 = [H+][SO32-]/[HSO3-] Note: concentration of dissolved SO2 = [S(IV)] = [H2SO3] + [HSO3-] + [SO32-] = [H2SO3](1 + Ka1/[H+] + Ka1Ka2/[H+]2) “Effective” Henry’s law constant = KH* = KH(1 + Ka1/[H+] + Ka1Ka2/[H+]2) = function of pH Cloud Chemistry Some Example Problems • Example Problem (low KH case): What is the concentration of CH3OH in cloud water if the gas phase mixing ratio is 10 ppbv and a LWC of 0.2 g/m3? The Henry’s law constant is 290 M/atm (at given temp.). Assume an atmospheric pressure of 0.9 atm and 20°C. • Example problem (high KH case): Determine the pH and aqueous NO3- concentration (in M) if air containing 1 ppbv HNO3 enters a cloud with a pressure of 0.90 atm, a T = 293K, and a LWC of 0.50 g/m3. Assume 100% scavenging. Break for Group Activity Cloud Chemistry - Overview of Scavenging • Gases scavenged are almost always in Henry’s law equilibrium • We will assume one of two cases occurs: – so little scavenging that Px(pre-cloud) = Px(incloud) – or 100% scavenging (complete transfer from gas phase to aqueous phase) • Aerosol scavenging depends on size and type of particles (with typical lower end of around 100 nm) Cloud Chemistry - What determines pH? • It is complicated • Strong acids (HNO3(g) and H2SO4(l)) provide [H+], tempered by NH3 and other bases • Both SO2 and CO2 can add acidity through reaction of H2XO3 with water • In many senarios, including “background” locations, neither SO2 nor CO2 significantly contribute to pH Cloud Chemistry - What determines pH? 4 Independent Senerios Source pH 400 ppmv CO2 5.61 1 ppbv SO2 5.38 1 ppbv HNO3; LWC = 0.5 g/m3 4.09 5 mg/m3 NH4HSO4 aerosol; LWC = 0.5 g/m3 4.06 Cloud Chemistry - A Modeled Example example including ammonium bisulfate, sulfur dioxide and carbon dioxide Equilibrium pH where sum of anion charge = sum of cation charge Calculation method is fairly complex (uses systematic method) Cloud Chemistry - Reactions in Clouds • Cloud reactions are important for water soluble species because of higher concentrations in clouds • Only sulfur chemistry covered here Cloud Chemistry - Reactions in Clouds • Reaction of S(IV) and H2O2 - HSO3- + H2O2 → HSO4- + H2O (acid catalyzed) Rate = k[HSO3-][H+][H2O2] Rate = k’[H2O2]PSO2 Effectively pH independent Cloud Chemistry - Reactions in Clouds • Reaction of S(IV) and Ozone - Two main reactions: HSO3- + O3 → HSO4- + O2 moderately fast SO32- + O3 → SO42- + O2 fast reaction is faster at high pH because more S(IV) is present in reactive forms Cloud Chemistry - Reactions in Clouds 1 E -0 2 S(IV) loss rate (M/s) 1 E -0 4 1 E -0 6 1 E -0 8 1 E -1 0 1 E -1 2 1 E -1 4 2 3 4 5 6 pH O3 H CH OH 2 O 2 7 Cloud Chemistry - Reactions in Clouds • Oxidation of S(IV) – H2O2 is more important oxidant in acidic clouds – O3 can be important in cleaner air – Bulk models underestimate O3 reaction – Under certain conditions, reactions can be diffusion limited drop 1 pH = 4.0 drop 2 pH = 6.0 pH of combined drop = 4.30 rate ratio (H2O2/O3) at combined pH ~ 1000 rate ratio (H2O2/O3) from independent reactions in two drops ~ 0.5 Cloud/Precipitation Chemistry - Incorporation of Pollutants Precipitation Chemistry • Precipitation Formation – Cloud droplets are collected by collisions with rain droplets or snow crystals and transfer their contents – Snow crystals also can form mainly through diffusion from water vapor and are very clean From Mosimann ETH Dissertation diffusion growth (top) to high degree of riming (bottom) Precipitation Chemistry • Precipitation Formation – In addition to in-cloud transfer, pollutants can be incorporated from below cloud scavenging – This tends to be best for aerosols by snow and gases by rain – Precipitation pollutants are typically somewhat lower than low-level cloud concentrations Chapter 4: Consequences of Polluted Air • Effects Covered in Chapter 4 Include: Haze, Acid Precipitation, and Health Effects • We will cover health effects when covering toxicology Chapter 4: Consequences of Polluted Air - Haze • How do aerosols affect visibility and what factors contribute to reduce visibility? • Loss of light transmission (as in spectroscopy) can occur due to scattering or absorption – usually aerosol scattering is most important – NO2 absorption and soot absorption contribute to a lesser extent Chapter 4: Consequences of Polluted Air - Haze • Light scattering is most mass efficient (most scattered light per g of aerosol) for dp ~ l • Thus accumulation or fine aerosol mass is good indicator for poor visibility • High humidity also makes problem worse due to hygroscopic growth of aerosol particles • Meteorological conditions trapping pollutants or contributing to photo-oxidation also make visibility worse Chapter 4: Consequences of Polluted Air – Acid Rain • Main contributors are strong acids HNO3 and H2SO4 • These species form slowly (e.g. relative to ozone), so worst places are downwind of major NOx and SO2 sources • Besides pollution sources, two other factors are important: – atmospheric neutralization – soil chemistry Chapter 4: Consequences of Polluted Air – Acid Rain • Neutralization by Atmospheric Bases – NH3 (from fertilizers and animal excretions) – CaCO3 (in soil dust) • Soil Also Allows Run-off Neutralization – Occurs in soils containing carbonates (limestone, marble, etc.) • Acid Rain More Strongly Affects Soils with Weak Buffer Capacity – Granite or quartz bedrock regions can’t buffer acidic precipitation – This results in acidic lakes Chapter 4: Consequences of Polluted Air – Acid Rain • Problems with Acidified Water and Soils • Plant growth in lakes is reduced, which can affect whole ecosystem • Additionally, Al and other metals are mobilized at lower pH due to shift in: Al(OH)3(s) ↔ Al3+ + 3OH• Many such metals are toxic to fish at higher concentrations