In 2011, concentrations reached a new high of 1813 parts per billion (ppb), 259% higher than pre-industrial levels.

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And in the prohibition zone, up to 2, 799 ppb is allowed.

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Emissions of nitrous oxide -- 40% of which are estimated to come from human activity -- reached 324.2 ppb, up one ppb on the 2010 figure and 120% higher than pre-industrial times.

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That level has been raised to 85 ppb.

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