Air Quality, Atmospheric Deposition, and Lake Tahoe

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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

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Outline of Topics

• Air Quality Terminology
• Air Quality Trends
• Atmospheric Deposition
• Lake Tahoe Atmospheric Deposition Study

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Air Chemistry Sampler

• Atmospheric Reactions of NOX and VOCs
• NOX and VOCs Sunlight ? O3 PM
• Hydroxyl radical (OH) and ozone from
  sunlight-initiated reactions of NOX and VOCs
• NO2 OH ? HNO3 (nitric acid, 5-30 per hour)
• HNO3 NH3 ? NH4NO3 (ammonium nitrate)
• at low sulfuric acid, low temperatures, and wet
  conditions
• Other N species PAN, HONO, NO3, N2O5

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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,
  

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Toxic Air Contaminants

• Formal Identification
• Risk Assessment
• Long-Term Health Effects
• e.g., cancer, birth defects
• Control
• Reduction of Exposures

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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

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Why Regulate Particles?

• HEALTH
• Health effects are significant
• Premature death and cardiorespiratory disease
• Body of evidence is substantial
• WELFARE
• Reduced visibility

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HOW SMALL IS PM?
Hair cross section (60 mm)
Human Hair (60 mm diameter)
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PM10 AND PM2.5 SIZE VS. COMPOSITION
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Size Distributions of Several Particulate Source
Emissions
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Chemicals From Different Particle Emissions
Sources

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California Emission Trends
CO2
NOx
VOC
SOx
CO
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Ozone and NOX Trends
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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

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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

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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

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Three-Step Deposition Model

• Resistance Analogy
• Aerodynamic Resistance
• Laminar Layer Resistance
• Surface Resistance

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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 O3 and NO2
• Leaf area, open stomata

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Leaf Area in California
October
July
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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

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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

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Lake Tahoe AtmosphericDeposition Study
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• Objectives
• Methods and Equipment
• Special Studies
• Calculations

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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

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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)
• (0.3-0.5, 0.5-1.0, 1.0-2.5, 2.5-5, 5-10, and
  10-25 ?m)
• Spatial Diurnal Patterns

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LTADS Meteorological Measurements

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

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Spatial and Temporal Variations
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Spatial and Temporal Variations
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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

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