Air Quality Guidelines Global Update 2005 - World Health ...

SOURCES OF AIR POLLUTION
Fig. 6. Results of receptor modelling of PM2.5 in three Indian cities
Concentration (μg/m 3 )
350
300
250
200
150
100
50
0
Delhi, spring
Source: World Bank (16).
The oxides of nitrogen/ozone system
As mentioned above, emissions of oxides of nitrogen occur predominantly in
the form of nitric oxide, which typically comprises around 95% of nitrogen oxides
from a combustion source. The pollutant of far greater concern in relation
to human health is nitrogen dioxide (NO2). The main pathway of conversion is
via reaction with atmospheric ozone (O3), which is present in the background
atmosphere from a range of sources, including atmospheric transport from the
stratosphere. There is also a pathway by which this chemistry can be reversed,
with nitrogen dioxide breaking down as a result of absorption of sunlight to form
nitric oxide (NO) and an oxygen atom (O), which reacts with an oxygen molecule
(O2) to re-form ozone. The three reactions are as follows:
NO + O3 → NO2 + O2
NO2 + hυ (sunlight) → NO + O (2)
O + O2 → O3
Delhi, summer
Delhi, autumn
Delhi, winter
Calcutta, winter
Unidentified
Secondary ammonium
Secondary nitrates
Secondary sulfates
Biomass
Coal
Road dust
Petrol
Diesel
Since each of these reactions is relatively fast, an equilibrium (the photostationary
state), containing amounts of all three components, is quite rapidly established.
Since reaction 2 depends on sunshine it becomes ineffective at night, and if there
is sufficient ozone present reaction 1 can convert all nitric oxide to nitrogen dioxide,
which often occurs in rural areas. In a highly polluted environment there is
Mumbai, spring
Mumbai, autumn
Mumbai, winter
(1)
(3)
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