Australian Science Illustrated — Issue 54 2017

38 | SCIENCE ILLUSTRATED

O

n a Friday afternoon in January

2017, toxic smog veils Beijing,

just like it has many times before,

causing a red alert. Nursery

schools and schools close, old

polluting cars are not allowed on the streets,

and heavy industry such as steel rolling mills

are asked to close down. The huge city

comes to a halt. During the next few days,

people try to stay indoors, and wear masks if

they venture outside. The citizens of Beijing

know very well that this thick, acrid smog

should not be underestimated.

In 2014, China’s prime minister, Li Keqiang,

officially declared war against air pollution,

promising the population to “make the sky

blue again”. Now, the citizens of Beijing are

beginning to see the results.

LETHAL BREATHING

In a lifetime, about 250 million litres of air

pass through our lungs. That's a similar

volume of gas as the amount of hydrogen in

the Hindenberg. So If the air is even slightly

polluted, the total quantity of toxins ending

up in the body is tremendous.

For decades, scientists have known that

smog causes diseases such as asthma,

bronchitis, lung cancer, and other breathing

difficulties. In recent years, other complications

have been added to the list – including

dementia, obesity, and arteriosclerosis.

Annually, some 7.2 million people die an early

TRANSPORT

In the past 50 years, the number of motorists in Asia has grown

almost thousandfold. Electric cars are the logical step towards

cleaner roads, but billions of petrol cars must first be replaced.

Exhaust Controls Air Pollution

Even if still charged by coal-fired electricity, at least an EV isn't belching exhaust fumes all

over your kids as you drop them off. Car exhaust is toxic, and as we wait for battery tech to im-

prove, we must rely on ever-more sophisticated filters to "clean" car emissions.

1 Exhaust gas passes through a

particle filter made of

the mineral cordierite.

The filter captures

soot and ash particles.

4 The exhaust smoke still

contains a large

quantity of toxic

NOx gases, which

cannot be

captured by the

particle filter.

Diluted ammonia

is injected into

the exhaust pipe,

mixing with the

hot smoke.

5 In a reaction chamber,

the ammonia

(NH 3 ) combines

with the NOx gases

(NO and NO 2 ). The

chemical reaction

is triggered by a

catalyst. The idea

is to neutralise the

harmful gas.

6 The result of the reaction

is molecular

nitrogen (N 2 ),

which is already

the most common

gas in the

atmosphere, and

water vapour

(H 2 0). Only abut 10

% of the particle

mass and NOx

gases escape.

2 The filter has an extremely high

surface area, which

traps and binds

particles, but allows

the gas itself to pass.

3 The filter loses its effect, it

clogs up. So, the filter

is heated to 600

degrees to burn the

particles into ash.

Exhaust from unfiltered diesel engines is very polluting. It soot and ash, plus toxic NOx gas.

This is why diesel engines must be equipped with sophisticated particulate filters.

Toxic exhaust converted into water drops

Now

SMOG REDUCTION:

90 %

CLAUS LUNAU

ENGINE

PARTICLE FILTER LIQUID INJECTED. REACTION CHAMBER

TECHNOLOGY SMOG