Ozone and particles

The white haze in this photo is a complex mixture of chemicals of which the key ones are ground-level ozone and nitrogen dioxide.
Image courtesy of Simone Cottrell, © Royal Botanic Garden

We’ve all woken up to beautiful sunny mornings only to see smog hanging over the city. This white haze is a complex mixture of chemicals of which the key ones are ground-level ozone and nitrogen dioxide.

Ozone is formed when pollutants from cars, industry and other sources react chemically in strong sunlight. These pollutants are commonly referred to as NOx (oxides of nitrogen) and VOCs (volatile organic compounds).

NOx is mainly made up of nitric oxide and nitrogen dioxide. These are primary pollutants emitted when fossil fuels are burnt, i.e. fuel in motor vehicles, coal in industry.

VOCs mainly come from unburnt petrol and diesel from motor vehicle exhaust, fuel vapours from petrol stations and petrochemical industries, vapours from paints, aerosols and solvents used in commercial and domestic activities, as well as natural emissions from vegetation and fires.

Ground-level ozone is the "bad" ozone. Ozone is a good thing in the stratosphere, around 10–50 km above the earth’s surface, where the ozone layer offers some protection against damaging radiation from the sun.

Ozone is a secondary pollutant. It is not directly emitted from a source. For it to form in the lower atmosphere takes three ingredients:

• oxides of nitrogen (NOx – made up of both nitrogen oxide and nitrogen dioxide)
• volatile organic compounds (VOCs – mainly hydrocarbons)
• warm, sunny conditions

NOx is predominantly a primary pollutant as it’s mostly emitted when fossil fuels are burnt:

• oil in motor vehicles – petrol, diesel, etc
• coal in industry – power companies, etc.

VOCs mainly come from:

• unburnt petrol and diesel from motor vehicle tailpipes
• fuel vapours from motor vehicles and petrochemical industries
• solvents used in commercial and domestic activities
• natural emissions from vegetation.

Because ozone is created when NOx reacts with VOCs in warm, sunny conditions, it’s mainly a problem in the summer months. However as temperatures continue to increase as a result of climate change, the high-ozone season is likely to become longer in the future leading to greater exposure to high ozone levels.

Annual sources of NO_x

Annual sources of VOCs

Annual sources of VOCs and NO_x – based on 2003 man-made sources in the NSW Greater Metropolitan Region
Source: Current and Projected Air quality in NSW technical paper, 2007

The more built-up the area, the bigger the problem. The goals set by the National Environment Protection Measure for Ambient Air Quality (Air NEPM) have been exceeded by Sydney every year since 1995. In contrast, an exceedence has only been recorded twice in the lower Hunter region.

(An exceedence is when a goal or standard for a particular pollutant is not met.)

Ozone exceedences in Sydney

This graph shows the number of days each year where the 4-hour ozone standard has been exceeded in Sydney.

Source: NSW Cleaner Vehicles and Fuels Strategy, 2008

The brown haze in this photo is particle pollution

We've all noticed a brown haze in metropolitan areas, usually during the cooler months of the year, in the late afternoon or the early morning when there’s little or no wind.

This is particle pollution. It occurs when tiny airborne particles are formed in the atmosphere and, without any wind to scatter them, they reach high concentrations and affect visibility.

These particles are generally less than 50 micrometers (µm) in diameter - one micrometer is equal to one thousandth of a millimetre. To put it into context, the average human hair is about 70µm in diameter.

You’d think these microscopic particles were too small to do much damage. Unfortunately, size is their strength. Fine particles are light and tend to stay floating in the air for days or weeks until they’re removed by rain. During this time they can travel remarkable distances from their source and affect air quality right across a region.

Larger, heavier particles stay in the air for a shorter time before being deposited locally, sometimes soiling or damaging materials, but certainly adding to the grimy look of some built-up areas.

Together, these particles are referred to as total suspended particulates (TSP).

Particles come in many forms. They can be directly emitted or formed in the atmosphere and include solids and liquid droplets. Sources include:

• domestic solid fuel heaters (particularly wood burners)
• motor vehicles (particularly diesels)
• industrial processes
• bushfires
• open burning, including hazard reduction burns
• pollens
• fungi
• sea spray

The main human-made sources of particles in the NSW Greater Metropolitan Region (Sydney, the Lower Hunter and Illawarra) are industry (i.e. coal mining), mobile sources (both on- and off-road) and domestic sources (mostly solid fuel heaters). See below.

While calm conditions during winter can allow fine particles to build up to high levels, extreme levels in summer are usually due to bushfires. They could also be a by-product of the chemical reaction that creates ozone since concentrations of PM₁₀ can be high in metropolitan Sydney even when there are no bushfires and dust storms. Agricultural burning and emissions from solid-fuel heaters are often the culprits in rural and regional areas.

Annual sources of PM₁₀

Based on 2003 man-made sources in the NSW Greater Metropolitan Region
Source: Current and Projected Air Quality in NSW technical paper, 2007

Detailed information on ozone and particulates can be found in the Chapter 4.1 of NSW State of the Environment 2009 report.

Read the Report of the NSW Chief Health Officer on this topic.

The NSW Cleaner Vehicles and Fuels Strategy (PDF, 1.1MB) builds on the State Government's actions to date to reduce pollution from vehicles.

For a more detailed examination of diesel particles, visit the California EPA Air Resources Board (CARB) website.