PM2.5 refers to fine particles (2.5 micrometres or smaller in diameter ), which are produced by combustion (氧化），including motor vehicles , power plants , forest fires, and some industrial processes .

The WHO tracks air quality ( how much PM 2.5 is in the air ) at 1622 locations in 92 countries -- but all are urban areas , while Pakistan , Egypt and Mongolia are among the most polluted countries according to the report , this only refers to pollution in its cities . Air quality in the Karakoram mountain range or the Gobi Desert will, of course , be fresh .Similarly ,Russia appears to be among the worst performing countries -- but its ranking is based only on air quality in Moscow . Of the 92 countries to feature , Australia has the least polluted urban areas , followed by Brunei and New Zealand. Estonia is Europe’s top performing nation ,followed by Finland and Iceland .The UK just misses out on the top 20, coming 21st. .

London’s annual PM2.5 mean is 12μg/m3. That puts it below the WHO target and behind a lot of world capitals. Of those countries for which the WHO had data ,Madrid , Oslo , Dublin, Montevideo, Helsinki, Ottawa , Washington DC and Canberra perform better than London .Top of the table are Stockholm and Tallinn, with an annual mean of 5μg/m3, followed by Edinburgh ,with 6. London exceeds Paris and Berlin , however ( both 16), and is the same as Lisbon (also 12).

Stockholm was the first city to be crowned European Green Capital in 2010---and it clearly hasn’t rested on its great honors . Since taking the accolade (荣誉），the Swedish capital has continued to keep on with eco-initiatives and has successfully decreased carbon emissions by 25 per cent since the Nineties. The city aims to be fossil-fuel free by 2050 , a target it hopes to achieve by improving public transport ,decreasing waste and increasing biodiversity, among other things . The city also has a strong culture of cycling , which has kept many residents out of cars .

【小题1】What is the WHO air quality report probably about ?

A．PM2.5 in the air.

B．Quantity of polluted air.

C．PM2.5 in the dust.

D．Quantity of combustion air.

【小题2】Which country is the least polluted for the urban areas of 92 surveyed countries ?

A．Russia	B．Australia
C．Finland	D．New Zealand

【小题3】What can we conclude from Stockholm was crowned European Green Capital ?

A．The trees are the most in Stockholm.

B．There is no air pollution in Stockholm.

C．PM2.5 in Stockholm is the lowest in the world.

D．Eco-environment in Stockholm is better protected .

【小题4】What does the passage mainly talk about?

A．The definition of PM2.5.

B．The findings of air quality study.

C．The importance of protecting nature.

D．The WHO air quality report in 92 countries.