6.3 Photochemical smog

Question 1

source of tropospheric ozone

Answer 1

when fossil fuels are burned, two of the pollutants emitted one of these being nitrogen monoxide (NO) (this type of ozone is formed as the result of VOCs (volatile organic compounds)

Nitrogen DIOxide interacts with sunlight, reforming nitrogen MONOxide
Oxygen radical combines with o2 to make ozone

the main source of NOx is road transport

Question 2

what is tropospheric ozone

Answer 2

in the troposphere, ozone is considered a pollutant

Question 3

the effects of tropospheric ozone

Answer 3

• ozone is a toxic gas and an oxidising agent
• it damages crops and forests
• irritates eyes
• can cause breathing difficulties
• ground level ozone reduces plant photosynthesis and can therefore reduce crop yields significantly
• highly reactive and can attack fabrics and rubber materials

Question 4

how are secondary pollutants formed?

Answer 4

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.

Question 5

how many primary + secondary pollutants are found in photochemical smog

Answer 5

100

Question 6

why is tropospheric/ground-level ozone a secondary pollutant

Answer 6

bc it is formed by reactions involving oxides of nitrogen (NOx)

Question 7

why does ozone drift to surrounding areas

Answer 7

because it can take a number of hours, hence the polluted air may have drifted into suburban+ surrounding areas

Question 8

thermal inversions

Answer 8

trap smog in valleys + basins i.e. LA, Santiago, Mexico City, Rio de Janero

the air is unable to disperse bc cold air that is trapped beneath a layer of less dense, warm air from the surrounding mountains and hills prevents the warm air from rising

cold air is denser than warm air, so it traps the warm air below and causes concentrations of air pollutants to build up near the ground instead of being dissipated by ‘normal’ air movements.

Question 9

what about urban areas?

Answer 9

urban microclimates generally have less vegetation than surrounding rural areas, and the concentration of buildings, industries and offices generates much heat.

Question 10

how does deforestation or burning produce smog? case study

Answer 10

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

Question 11

why are places with high air pressure more prone to smog?

Answer 11

winds in high pressure systems are usually weak meaning pollutants remain in the area and are not dispersed

poor air quality -> persists for few days bc stable high pressure conditions generally prevail

notably Mediterranean climates have conditions of high air pressure all season, meaning poor air quality can remain for months

Question 12

elements of areas prone to smog

Answer 12

local topography - low lying areas
climate - high air pressure areas
population density - number of vehicles

Question 13

economic losses caused by urban air pollution

Answer 13

• cost of clean up strategies
• loss of tourism
• decreased worker productivity
• increased cost of health care
• decreased crop productivity
• cost of replacing materials

Question 14

case study economic losses caused by urban air pollution

Answer 14

world bank indicates that the cost of air pollution in China is worth approx. 4% of its GDP anually

Question 15

pollution management strategies in which photochemical smog can be managed

Answer 15

• reduction in the burning of fossil fuels -> effective way of limiting the release of pollutants
• increased use of public transport -> reduces total emmissions of fossil fuels
• promotion of clean technology/hybrid cars
• provision of park-and-ride schemes -> limit the number of cars entering urban areas
• preventing cars in parts of the city (green zone in FFM)
• reducing fossil fuel combustion through urban design (i.e. south facing windows, cavity and loft insolation)
• relocating industries and power stations away from the centres of population
• ensuring industries and power stations have tall chimneys to help disperse pollutants
• filtering and capturing pollutants
• wearing masks to help reduce inhalation of pollutants
• designing cities so that there are more open spaces and water courses to help reduce the temp and allow evaporative cooling (e.g. the restoration of the Cheong-Gye-Cheon river in Seoul, South Korea)
• fitting cars with catalytic convertors to reduce NOx emissions

Question 16

limitations of pollution management strategies for tackling urban air pollution

Answer 16

• 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

Question 17

how does photochemical smog get produced?

Answer 17

Photochemical smog is formed when nitrogen oxides (NOx) and volatile organic hydrocarbons react when activated by sunlight.

The steps for the formation of this smog are:
1) Nitrogen oxide (NO) is formed at high temperatures when fossil fuel combustion occurs.

2) NO is then converted to the yellow-brown gas, nitrogen dioxide (NO2) when released into the atmosphere. This is nick-named brown-air smog.

3) When exposed to UV radiation from the sun, some NO2 reacts with hydrocarbons to form photochemical oxidants such as ozone, nitric acid, aldehydes and other pollutants.