6.3 Photochemical smog Flashcards
source of tropospheric ozone
the reaction of nitrogen oxides (NOₓ) and volatile organic compounds (VOCs) in the presence of sunlight. These precursor gases mainly come from vehicle emissions, industrial processes, and burning of fossil fuels. Ozone forms as a secondary pollutant during these reactions in the lower atmosphere.
what is tropospheric ozone
in the troposphere, ozone is considered a pollutant
effects of tropospheric ozone on forests / crops
- interferes with ability of sensitive plants to produce and store food
- damages leaves of trees and other plants -> harms appearance of vegetation in urban areas, national parks, recreation areas
- dieback of German forests (previously linked to acid rain)
how are secondary pollutants formed?
in the presence of sunlight, when primary pollutants undergo a variety of reactions with other chemicals already present in the atmosphere. tropospheric ozone is an example of a secondary pollutant.
how many primary + secondary pollutants are found in photochemical smog
100
why is tropospheric/ground-level ozone a secondary pollutant
bc it is formed by reactions involving oxides of nitrogen (NOx)
why does ozone drift to surrounding areas
because it can take a number of hours, hence the polluted air may have drifted into suburban+ surrounding areas
thermal inversions
trap smog in valleys + basins i.e. LA, Santiago, Mexico City, Rio de Janero
During a thermal inversion, a layer of warm air sits above cooler air near the ground, which prevents the normal vertical mixing of air. Since cold air is denser, it stays trapped underneath the warm layer, along with pollutants like NOₓ and VOCs. This causes air pollution to build up, often leading to photochemical smog, especially in cities surrounded by mountains where the air is already more stagnant.
- more common in winter: more homes heated, vehicles take longer to warm / are less efficient
what about urban areas?
urban microclimates generally have less vegetation than surrounding rural areas, and the concentration of buildings, industries and offices generates much heat.
how does deforestation or burning produce smog? case study
forest fires of Southeast Asia in the late 1990s and in 2012-2014 produced the Asian ‘brow haze’, throughout much of Malaysia, Indonesia and Singapore
elements of areas prone to smog
- high air pressure = weak winds = no dispersal of pollutants
- low-lying / depressions = pollutants collect / build up
- large populations = more vehicles = more nitrogen oxides released
- greater reliance on fossil fuels = more pollutants
- sunlight = catalyse reactions
economic losses caused by urban air pollution
- cost of clean up strategies
- loss of tourism
- decreased worker productivity
- increased cost of health care
- decreased crop productivity
- cost of replacing materials
case study economic losses caused by urban air pollution
world bank indicates that the cost of air pollution in China is worth approx. 4% of its GDP anually
pollution management strategies in which photochemical smog can be managed
- changing human activity:
- wearing masks to help reduce inhalation of pollutants
- more use of bikes / walking
- more use of public transport instead of private cars
- more use of energy efficient products like hybrid cars
- carpooling schemes - reduce the amount of pollutants released into the environment:
- fitting cars with catalytic convertors to reduce NOx emissions
- greater enforcement of emission standards (green zone in FFM) /legislation / taxation
- public info regarding air quality
clean up:
- reforistation
- re-greening
- conservation areas (eg. La Rambla in Barcelona)
limitations of pollution management strategies for reducing photochemical smog
- most urban air pollution comes from cars, especially old cars
- vehicles using diesel produce emissions of particulate matter
- catalytic convertors reduce fuel efficiency and increase co2 emissions
- public transport can be expensive + may be inconvenient
- sustainable urban design is expensive
how does photochemical smog get produced?
Photochemical smog forms when primary pollutants like nitrogen oxides (NOₓ) and volatile organic compounds (VOCs) are released from vehicles and industrial processes. UV breaks apart nitrogen dioxide (NO₂), producing nitrogen oxide (NO) and oxygen atoms, which combine with oxygen molecules to form ozone (O₃). VOCs then react with NOₓ and ozone to create secondary pollutants like peroxyacyl nitrates (PANs). This process, intensified by sunlight and stagnant air, leads to the accumulation of smog
what primary pollutants from the combustion of fossil fuels -> tropospheric ozone
- carbon monoxide
- carbon dioxide
- black carbon / soot
- unburned hydrocarbons
- oxides of nitrogen
- oxides of sulfur
= primary pollutants
what are the main sources of VOCs and nitrogen oxides?
- road transport
- solvent release (drying paints, glues, inks)
- petrol handling
how is nitrogen dioxide created?
exhaust gases leave engine, nitric oxide is oxidised into nitrogen dioxide
what is the role of VOCs in making tropospheric ozone?
react with NO and turn it back into NO2 which can be split up to make more ozone
effects of tropospheric ozone on humans
- harm lung tissue
- increase respiratory tract infections
- aggravate asthma, bronchitis, pneumonia
- long-term: premature ageing of lungs
- children born/raised in areas with high levels of ozone -> 15% reduction of lung capacity
effects of tropospheric ozone on materials
damages fabric and rubber
how do forest fires cause smog?
Release NOₓ and VOCs, which react in sunlight to form tropospheric ozone, the main component of photochemical smog. They also release particulates and can worsen conditions through thermal inversions.
