Week 4 Flashcards
(28 cards)
What two things doe ozone production require?
Hydrocarbons (RH) and NOx which act as precursors in a sunlight driven cycle
3 main sources of NMHCs/VOCs
Anthropogenic
Biogenic
Biomass Burning
Why is tropospheric ozone important?
- Its a precursor to OH- playing a role in maintaining the oxidising capacity of the troposphere
Production of O3 by CO
- CO+OH–> CO2+HO2
- NO+HO2 –> NO2+OH
- NO2+hv–> NO+O3
Production of O3 by CH4
- CH4+OH–> CH3O2 + H2O
- NO + CH3O2 –> NO2 + CH3O
- NO2 + hv –> NO + O3
Production of O3 from NMVOCs
- RH + OH –> RO2 + H2O
- NO +RO2 –> NO2 +RO
- NO2 + hv –> NO + O3
For CO, CH4 and NMVOC, what is the first step in O3 production?
Reaction with OH
For CO, CH4 and NMVOC, what is the second step in O3 production?
Reaction with NO
For CO, CH4 and NMVOC, what is the final step in O3 production?
Photolysis of NO2
Why does reactivity increase with the size of the hydrocarbon?
Because of the larger number of C-H bonds available for H abstraction by OH
Importance of troposheric ozone
Precursor to OH- plates a role in maintaining oxidising capacity of the troposphere
Impacts of tropspheric ozone on a surface level
At elevated concentrations, O3 is a GHG. It can also react with NO to produce NO2 which is involved in the production of nitric acid which may lead to acid deposition. O3 can also lead to irritation/swelling of human airways and lead toe exacerbated respiratory conditions e.g. asthma
What is the NOₓ-limited regime for O₃ production?
The NOₓ-limited regime occurs when NOₓ (NO + NO₂) concentrations are low relative to VOCs, typically in rural or remote areas. O₃ production is limited by NOₓ availability, as peroxy radicals (RO₂/HO₂) cannot efficiently convert to O₃ without sufficient NO. O₃ increases linearly with NOₓ but is insensitive to VOC changes
What is the VOC-limited regime for O₃ production?
The VOC-limited regime occurs when NOₓ concentrations are high relative to VOCs, common in urban areas. O₃ production is limited by VOC availability, as high NO titrates O₃ (NO + O₃ → NO₂ + O₂). O₃ production is sensitive to VOCs but less responsive to additional NOₓ.
How does NOₓ affect O₃ in a NOₓ-limited environment?
In a NOₓ-limited environment, adding NOₓ significantly increases O₃ production because NOₓ facilitates the conversion of peroxy radicals (RO₂/HO₂) to NO₂, which photolyses to form O₃ (NO₂ + hν → NO + O; O + O₂ → O₃).
Why might reducing NOₓ increase O₃ in a VOC-limited environment?
In a VOC-limited environment, reducing NOₓ decreases NO, which normally titrates O₃ (NO + O₃ → NO₂ + O₂). Less titration can initially increase O₃ levels before further NOₓ reductions lower O₃ production, showing a nonlinear response.
What role do VOCs play in O₃ production?
VOCs react with OH radicals to form peroxy radicals (RO₂/HO₂), which convert NO to NO₂. NO₂ photolysis produces O₃. In VOC-limited regimes, VOC availability drives O₃ production, while in NOₓ-limited regimes, VOCs are abundant and NOₓ is the limiting factor.
Where is the NOₓ-limited regime commonly found?
The NOₓ-limited regime is common in rural or remote areas (e.g., forests, oceans) with low anthropogenic NOₓ emissions and relatively high VOCs from natural sources like vegetation.
Where is the VOC-limited regime commonly found?
The VOC-limited regime is common in urban or industrial areas with high NOₓ emissions from traffic, combustion, or industry, and relatively lower VOC concentrations.
What is the titration effect in O₃ chemistry?
The titration effect occurs when high NO concentrations react with O₃ (NO + O₃ → NO₂ + O₂), reducing O₃ levels. This is significant in VOC-limited regimes with abundant NOₓ.
How should air quality management differ between NOₓ-limited and VOC-limited regimes?
In NOₓ-limited regimes, control NOₓ emissions to reduce O₃. In VOC-limited regimes, prioritize VOC emission reductions, but manage NOₓ carefully to avoid short-term O₃ increases due to reduced titration.
What factors determine whether O₃ production is NOₓ-limited or VOC-limited?
The NOₓ/VOC ratio, sunlight intensity, OH radical availability, and sinks (e.g., HNO₃ or PAN formation) determine the regime. Low NOₓ/VOC ratios favor NOₓ-limited conditions; high ratios favor VOC-limited conditions.
What caused the Great Smog of London in 1952?
The Great Smog was caused by cold weather trapping coal smoke (SO₂, soot) under an inversion layer. High sulfur emissions from coal burning formed sulfuric acid aerosols, creating dense, toxic London-type (sulfurous) smog.
How does Los Angeles smog differ from the 1952 London smog?
LA smog is photochemical, driven by sunlight, high NOₓ (from vehicles), and VOCs, forming ozone and secondary aerosols. London’s 1952 smog was sulfurous, from coal-derived SO₂ and primary particulates, with minimal ozone.
