The Environment Agency provides information about air quality trends with useful links:
It is also the case that some aspects of this type of air pollution are natural. San Pedro's Bay (LA) was named 'The Bay of Smokes' in 1542. The blue haze of The Blue Ridge Mountains of Virginia and The Blue Mountains in Australia is due to chemistry similar to that giving rise to Photochemical Smog.
Banjo players in the Blue Ridge Mountains
Photochemical Smog
Photochemical Smog is sometimes known as Los Angeles Smog, as this was where it was first identified. It has now been identified in many cities across the globe. What occurs during the day in a major city is illustrated in the figure.
What happens?
As the morning rush hour starts, NO begins to build up from exhaust emissions. At dawn, NO is converted to NO2 and as the day progresses, oxidants — mainly O3 — are generated. Peroxyacetyl nitrate (PAN), a powerful lchrymator is formed and the air beomes hazy as particulates are generated. At the same time oxidised compounds such as formaldehyde are formed and nitric and sulphuric acid concentrations increase.
How does it happen?
Consider the oxidation of CH4, a process that occurs in the natural atmosphere.
OH + CH4 → CH3 + H2O (1)
CH3 + O2 + M → CH3O2 + M (2)
CH3O2 + NO → CH3O + NO2 (3)
CH3O + O2 → HO2 + H2CO (4)
A similar sequence of reactions can be written that takes the oxidation all the way to CO2.
The oxidation can only start proper once OH is generated in the photolytic process:
O3 + hν → O2 + O(1D) (5)
O(1D) + H2O → OH + OH (6)
Once the oxidation has started, NO can be converted to NO2 in step (3). Note that NO+NO+O2 is too slow for this reaction to be important, and NO+O3 requires ozone to be present, which itself requires NO2 to be present.
Once NO2 has been formed, O3 is generated through the photolysis of NO2.
NO2 + hν → O + NO (7)
2(O2 + O + M → O3 + M) (8)
OH + CH4 → CH3 + H2O (1)
CH3 + O2 + M → CH3O2 + M (2)
CH3O2 + NO → CH3O + NO2 (3)
CH3O + O2 → HO2 + H2CO (4)
NO + HO2 → NO2 + OH (9)
CH4 + 4O2 + hν → H2CO + H2O + 2O3
Note that O3 is generated catalytically without NOx destruction for as long as there is radiation and some kind of fuel (CH4 for illustration).
The oxidation of longer chain hydrocarbons leads to the formation of PAN, a lachrymator.
OH + CH3CHO → CH3CO + H2O (10)
CH3CO + O2 + M → CH3C(O)O2 + M (11)
CH3C(O)O2 + NO2 → CH3C(O)O2NO2 (12)
PAN
Particulate matter is formed from a variety of reactions, including secondary organic aerosol from the ozonolysis of large hydrocarbons.
The details of SOA formation are unclear, but pinic acid has been suggested as a possible nucleating species (Christoffersen et al., Atmos. Environ., 32 1657-1661 (1998)) as have diacyl peroxides (P.J. Ziemann, J. Phys. Chem. A, 106, 4390-4402 (2002).)
Aldehydes are generated in processes such as reaction (4) and have an impact on the overall process because they can be photolysed and are an additional source of radicals.
H2CO + hν → H + HCO (13)
H + O2 + M → HO2 + M (14)
HCO + O2 → HO2 + CO (15)
The whole photochemical process kicks in more rapidly in the presence of aldehydes, as illustrated in these smog chamber results.
Other OH radical sources in smog episodes are HONO photolysis and ozonolysis.
In high NOx conditions, the heterogeneous formation of HONO occurs.
NO + NO2 H2O → 2HONO (16)
This process is not properly understood quantitatively, but is thought to occur on soot particles. The HONO can then be phoptolysed to give OH radicals.
HONO + hν → OH + NO (17)
The ozonolysis of alkenes may be a significant source of OH radicals in the rural nighttime environment and is also a source in polluted urban environments(Paulson and Orlando, Geophys. Res. Lett. 23 3727-3730 (1996)).
Nitric acid is formed simply from the reaction of NO2 with OH radicals.
OH + NO2 + M → HONO2 + M (18)
Sulphuric acid formation seems to be accelerated in smog conditions. This may be due to accelerated SO2 oxidation on particulates, but may be related to intermediates formed in the ozonolysis of alkenes.
Impact of Photochemical Smog
Particulates: reduce visibility, may be carcinogenic, containing heavy metals and polycyclic aromatic hydrocarbons.
Ozone: causes irritation of the air passages, provoking asthma attacks in the susceptible, damages vegetation.
PAN: lachrymator, phytotoxic, irritates air passages.
NO2: affects visibility, irritates air passages.
Control Strategies
Control strategies involve reducing hydrocarbon and NOx levels through the use of catalytic converters, or changing one hydrocarbon for another according to its photochemical ozone creation potential (POCP). However, there are complications. Legislation in California in 1966 required the use of lean-burn to reduce hydrocarbon emissions, but led to increased NOx. This led to reduced O3 in downtown LA, partly because NO destroys O3 in a direct reaction. However, as the air travelled downwind, the NO2 is photolysed causing increased O3 concentrations in these areas. A problem with reducing NOx is illustrated in the figure below. If you are on the LHS of the figure, reducing NOx lowers O3, but on the LHS, it increases NOx. Three-way catalytic converters convert NOx, CO and hydrocarbon emissions to N2, CO2 and CO2+H2O respectively. These cost money, of course.