More stringent engine standards were already set to take effect three years hence, and the EPA was now requiring at least a 90% reduction in the amount of nitrogen oxides and soot emanating from diesel engines.

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The combined effect of charging and improved vehicle technology, notably cleaner-burning diesel engines that are being used in more buses and taxis, has led to an overall reduction of 13% in nitrogen oxides and 12% in PM10 via vehicle tailpipe emissions in the charging zone.

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The result, when it is burned, is the near-complete abolition of soot, and a reduction of up to 80% in nitrogen-oxide emissions.

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In Europe the trucking industry has settled on a technology called selective catalytic reduction, in which urea is periodically injected into the exhaust system to break down the NOx, converting nitrogen oxides into (somewhat) harmless ammonia.

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