Air Quality, Atmospheric Deposition, and Lake Tahoe October 15, 2003 Western Regional Pollution Prevention Network Grannlibakken, Lake Tahoe Jim Pederson Research Division, California Air Resources Board

Outline of Topics Air Quality Terminology Air Quality Trends Atmospheric Deposition Lake Tahoe Atmospheric Deposition Study

Air Chemistry Sampler Atmospheric Reactions of NO X and VOCs –NO X and VOCs + S unlight  O 3 + PM –Hydroxyl radical (OH) and ozone from sunlight- initiated reactions of NO X and VOCs –NO 2 + OH  HNO 3 (nitric acid, 5-30% per hour) –HNO 3 + NH 3  NH 4 NO 3 (ammonium nitrate) at low sulfuric acid, low temperatures, and wet conditions –Other N species: PAN, HONO, NO 3, N 2 O 5

“Criteria” Pollutants AQ Standards -- Acute Exposures Human Health or Welfare “Non-Attainment” Areas Control Strategies in State Implementation Plans (SIPs) e.g. Particle Mass -- PM10, PM2 Ozone, Nitrogen Dioxide, Sulfur Dioxide, Carbon Monoxide,

“Toxic” Air Contaminants Formal Identification Risk Assessment Long-Term Health Effects –e.g., cancer, birth defects Control Reduction of Exposures

Direct Emissions and Secondary Pollutants Directly Emitted Pollutants –CO, NO, NO2, VOCs,, NH3, some PM Products Formed in Atmosphere: –Ozone, some PM, HNO3 NOx and VOCs regulated as precursors of ozone or PM

Why Regulate Particles? l HEALTH  Health effects are significant  Premature death and cardiorespiratory disease  Body of evidence is substantial l WELFARE  Reduced visibility

HOW SMALL IS PM? Human Hair (60  m diameter) PM10 (10  m) PM2.5 (2.5  m) Hair cross section (60  m)

10  m0.1  m Sulfates Nitrates Ammonia Carbon Organics Soil Dust Silica Salts Pollen Tire Rubber 2.5  m PM10 AND PM2.5 SIZE VS. COMPOSITION “Coarse” “Fine”

Size Distributions of Several Particulate Source Emissions

Chemicals From Different Particle Emissions Sources

California Emission Trends CO 2 NO x SO x VOC CO

Ozone and NO X Trends

Historical Perspective on Ozone 1959 Haagen-Smit paper 1970s Ozone frequently 600 ppb Today Ozone rarely exceeds 200 ppb –Pop., Vehicles/person, Miles/vehicle all up –Reduced Both NOx and VOC Emissions Health Based NAAQS is 120 ppb

What Sets Deposition Rates? Concentration Largest Particles: –Settling velocity (PM size, density) Gases and Smaller Particles: –Multiple Rate Limiting Steps Deposition Velocity –Deposition Rate/Concentration –Normalized Rate - Not Process –Differentiate from Setting Velocity

Deposition of Gases and PM 1. Turbulence mixes pollutants toward “sink” –Atmospheric turbulence set by wind speed, surface roughness (decreased by thermal stratification) –Aerodynamic Resistance 2. Diffusion across very thin laminar layer –Depth of layer (wind speed. surface elements) –Rate of diffusion (particle size, molecular weight) –Quasi-laminar Resistance 3. Capture by surface –Pollutant solubility, chemical reactivity –Surface type, biophysical factors (stomatal opening) –Surface Resistance

Three-Step Deposition Model Resistance Analogy –Aerodynamic Resistance –Laminar Layer Resistance –Surface Resistance

Plants Hasten Removal of Some Pollutants Atmospheric mixing controls removal of highly reactive gases PM size, meteorological variables, shape and nature of surfaces Plants increase removal of O 3 and NO 2 –Leaf area, open stomata

Leaf Area in California OctoberJuly

Rate of Deposition of Gases to Water Highly Reactive or Soluble? –Surface Resistance ~ 0 –Aerodynamic Resistance Sets Rate What determines turbulence? –Wind speed, Direction, Fetch –Thermal Stratification Relatively Insoluble Gas? –Surface Resistance Sets Rate

PM Deposition to Water Surface Resistance ~ 0 for PM Quasi-Laminar Resistance –Wind Speed –Particle Size Presence of Water May Modify Processes and Resistances –Hygroscopic particle growth –White caps and spray

Lake Tahoe Atmospheric Deposition Study

Objectives Methods and Equipment Special Studies Calculations

LTADS Primary Objectives Characterize Deposition to Lake –Pollutants affecting Lake clarity –Phosphorus, Nitrogen, and Particles Characterize Emission Source Types Clarify Relative Contributions of Local and Upwind Sources

LTADS Methods Two-Week Concentrations –Nitric Acid, Ammonia –PM Chemistry: PM2.5, PM10, TSP Hourly PM mass - PM2.5, PM10, TSP PM Size Observations –Size Counts (in 6 “bins”) –( , , , 2.5-5, 5-10, and  m) –Spatial & Diurnal Patterns

LTADS Meteorological Measurements Wind, temperature, and humidity Surface and Aloft (remote sensing) Uses of Meteorological Data –Vertical Mixing –Transport Trajectories –Deposition Velocity

Spatial and Temporal Variations

Calculation of Deposition Estimate Deposition Velocities Spatial and Temporal Variation of –Concentration –Deposition Velocity Calculate Deposition Rate –(Concentration x Deposition Velocity) Analysis of Uncertainty Bounding Calculations