Air pollution is responsible for 6.5 million deaths worldwide each year according to a World Health Organization report made public in 2016. An important measure of air pollution is the concentration of airborne particles. As part of the Department of Environment, Food and Rural Affairs (DEFRA) network, the National Physical Laboratory (NPL) calibrates and maintains three particle counters. We visited NPL during their Open House earlier this month and spoke to Jordan Tompkins, higher research scientist in the gas and particle metrology group at NPL, about how they “count” individual nanoparticles in Amperes.
“What you would like to do is pass a particle through a laser [beam], it breaks the laser [beam], you count the particle,” says Tompkins. “Nanoparticles are too small to do that, so you have to grow them.”
He explains how the counters heat up the alcohol butanol and then cool it through a condenser containing the nanoparticles. “The nanoparticles act as nucleation sites, so now you have a blob of butanol where the nanoparticle was, and this is big enough to break the laser,” adds Tompkins.
The information collected by the counters is limited to the size distribution and the number concentration. To find out what the particles are made of would require catching them in a filter over a period of time for further analysis, which would miss transient events such as a surge in concentration as a lorry passes.
“The particles can be anything, but you do see elevated concentrations at roadsides,” says Tompkins. “For example, there is a monitoring site at Marylebone Road and you see higher concentrations there than you would see at a background urban site or a rural site.”
Linking to SI units
The work on these particle counters takes place in one of the ampere labs at NPL. The counters actually charge each particle and with a known flow of particles, it is then possible to convert the concentrations of the number of particles into a current in amperes.
Why not just leave the concentrations as measured in particles? “The SI units are really well defined so if you are linked to one of those your measurement is also very well defined,” explains Tompkins. “If you’re not linked to one of those you end up with higher uncertainties in your measurement.”
In November the General Conference on Weights and Measures (CGPM) will vote to redefine four of the SI units – including the ampere – in terms of fundamental constants. A lot of the work to enable these redefinitions also stems from experiments at NPL.