Airborne particles less than ten micrometres in diameter (called PM₁₀’s) found in vehicle exhaust emissions have been linked to higher rates of respiratory and other illnesses. 

But relatively little is known about levels of nanoparticles (less than 100 nanometres in diameter) found in urban areas.

Now a team of UK researchers have measured nanoparticles in the air at an urban roadside in the morning and evening rush hours. 

Justin Lingard and colleagues from the University of Leeds measured the hourly number concentration (number of particles per cubic centimetre) and size distribution of particles of six nanometres to ten micrometres diameter at a roadside site in Leeds during July 2003.

Nanoparticles are of potentially greater concern than other PM₁₀’s because they can penetrate deep into the lung to the site of air–blood interaction and may cross into the bloodstream.

UK air quality standards for particulates are based on mass concentrations of PM₁₀’s.  ‘However,’ said Lingard, ‘vehicle engine emissions are dominated by nanoparticles, which account for only one to 20 percent of the mass emitted, but 90 percent of the particle numbers. This questions the use of mass only measurements.’

Lingard’s team collected particles and then divided them into four size ranges or modes. The researchers carried out statistical analyses on each mode in order to get a better understanding of the sources of the particles, the way they are formed and their fate in the atmosphere.

The team noticed the particle size distribution changed significantly through the day, reflecting the contribution of different formation processes. For example, in the early afternoon, when radiation from the sun is at a maximum, a large proportion of sub-eleven nanometre particles were present. This suggests that they are formed by photolysis. During rush hour periods, however, larger particles characteristic of direct vehicle emissions were dominant.

‘Existing studies provide some information on the size distribution of urban nanoparticles.  However, more complete information on the non-linear evolution of number concentrations and modal parameters is required,’ said Lingard.