Environmental chemistry

E. 10 smog

smog State the source of primary pollutants

and the conditions necessary for the formation of photochemical smog.

Outline the formation of secondary pollutants in photochemical smog.

smog a chemical ‘soup’ many different chemicals primary pollutants secondary pollutants that have been

formed from the primary pollutants by free radical reactions brought about by UV from the Sun.

photochemical smog: conditions (1)photochemical smog: conditions (1)

sunshine: sunlight converts primary pollutants into secondary pollutants; visible sunlight is enough to break bonds in NO2 as

there is delocalisation which weakens a bond;

windless day, hot, dry

thermal inversion (cold air below warm air so the air cannot rise)

photochemical smog: conditions (2)photochemical smog: conditions (2)

relief: city in a valley surrounded by hills

primary air pollutants: nitrogen oxides and volatile hydrocarbons – VOCs

source: internal combustion engines

time: morning ‘rush hour

formation of formation of secondary pollutantssecondary pollutants

NO + ½ O2 NO2

NO2 + hv NO + O

O + O2 O3

H2O + O3 O2 + 2OH 

OH + NO2 HNO3

RCH2 + OH H2O + RCHRCH + O2 RCHOORC(O)OO + NO2 RC(O)OONO2 (PAN)

smogsmog

primary

pollutants volatile

hydrocarbons - VOC

nitrogen oxides

secondary pollutants ketones, aldehydes, PANs, peroxides, ozone,

photochemical smog photochemical smog (http://www.eco.pref.mie.jp/english/data-syu/data-sokuhou/img/R06-01_e.gif)

smog:smog: variation in day

thermal inversion Normally, in the troposphere, temperature decreases as you go

up; this makes warm and less dense air rise taking pollutants with it. The warmer air is replaced by cooler air which is again warmed up continuing the process;

Thermal inversion occurs when cooler air slips underneath warmer air so that a layer of warm air is trapped between 2 layers of cool air. The bottom cool layer of air, which is denser than the warm layer of air above it, remains close to the surface. Any pollutants released in that layer of cooler air stay close to the Earth’s surface; this allows the primary pollutants to react and produce secondary pollutants and smog.

smog: effectssmog: effects PAN’s cause eyes to water and respiratory

problems, also catalyses formation of sulphuric acid,

ozone in the troposphere is: an eye irritant and also irritates respiratory system causes deterioration of rubber as it breaks –C=C- and bleaches

dyes affects leaves and therefore growth/damages crops

reduced visibility secondary pollutants are toxic to plants corrosion of building materials by acids and

ozone.