Lecture 5 - LEC347

Lecture 5 - LEC347

1. Major air pollutants

   • Primary
     • CO2
     • SO2
     • NO
     • Particles
     • Hydrocarbons
   • Secondary
     • NO2
     • Ozone
     • Acid rain
     • Particles

2. Tropospheric ozone

   • Tropospheric ozone concentrations
     • Episodes of high surface ozone - metropolitan areas [ozone] >200-400 ppb
       • Dehli, India
       • Beijing, China
       • Karamay, China
       • Mexico City, Mexico
     • Background [ozone] has increased from ~10 ppb since the industrial revolution
       • Northern hemisphere - background monthly mean 35-50 ppb
     • [ozone] in the troposphere comes from...
       • Stratosphere-troposphere exchange (~10%)
       • Photochemical reactions with volatile organic compounds (VOCs, incl. CH4) and NOx
         • Ozone production is sensitive to NOx and VOC levels (limited by both)
           • Ozone production increases with [NOx] e.g. rural areas of industrial countries with moderate [NOx]
           • Ozone production increases with [VOC] e.g. in urban locations with v high [VOC]
       • Sensitive to climate-related factors
         • [Ozone] highest in summer due to higher temps., increased sunlight and stagnating high-pressure systems
         • [Ozone] reduced in winter (cool wet summers)
   • Plant responses to ozone
     • Ozone deposition - expressed as a rate (deposition velocity)
       • Stomata - 30-90% ozone uptake
       • Non-stomatal sinks = plant cuticles, soil and ozone decomposition
         • Non-stomatal removal processes are not as harmful to plants as stomatal uptake (however can cause some damage to cuticles and soil biota)
     • Defences against affecting net primary productivity
       • The rate of ozone penetration into the leaf can be reduced
       • Can increase capacity to tolerate ROS generated from ozone
     • Stomatal responses to ozone
       • Acute ozone exposure = short-term exposure, high [ozone] i.e. >150 ppb
         • Causes a transient reduction in stomatal conductance --> dose-dependent
           • >80 ppb ozone = no response; 80-434 ppb = 'response'; >434 ppb = 'saturation'
           • 3 minute pulse of ozone gives max. response
             • Makes guard cells temp. desensitized to ozone
             • Stomatal responsiveness to ozone reappears when stomatal conductance has fully recovered
         • Induces a burst of ROS
           • This activates guard cell signalling cascade causing stomatal closure
       • Chronic ozone exposure = long-term exposure, low [ozone]
         • Causes a sustained reduction in stomatal conductance (dose-dependent)
         • Water stress ameliorates the impact of chronic ozone exposure as water stress causes stomata to close so less ozone gets in
           • Chronic ozone, however, inhibits drought-induced stomatal closure in Beech
             • Reduces the sensitivity of stomata to ABA
             • Soybean and maize yield is more sensitive to ozone under drought conditions
           • Percentage frequency of visual symptoms of ozone damage are lower in droughted Norway Spruce compared to watered ones

3. Atmospheric ozone

   • Statospheric ozone
     • 90% atmospheric O3
     • Acts as a primary UV radiation shield
     • Current issues: long-term global downward trends and springtime Antarctic ozone hole
   • Tropospheric ozone
     • 10% atmospheric O3
     • Toxic on humans
       • Very high [ozone] (10ppm) is lethal
       • 500M people globally exposed to harmful ozone levels
       • ~0.7 respiratory mortalities/year anthropogenic ozone
       • Cardiopulmonary and lung cancer mortality
       • ~6.3M years of life lost due to anthropogenic ozone
       • 75% Asian continent affected
       • Ozone-effected bronchus appears 'sunburnt'
     • Toxic on plants
       • Community effects
         • Different species respond differently to ozone - can affect ecosystem composition
       • Whole-plant effects
       • Leaf effects
       • Cellular effects
       • Visual symptoms of ozone damage
         • 1. Water-soaked appearance
           • Reversible, recoverable effect
         • 2. Chlorotic flecking
         • 3. Stipple of dead or pigmented cells
         • 4. Growth suppression, early abscisson
       • EXAMPLE: Effect of different concentrations of elevated ozone on soybean
         • General trend of decrease in physiological parameters with increasing [ozone]

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