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. Particle number size distributions have been measured simultaneously byscanning mobility particle sizers (SMPSs) at five sites in central London fora 1 month campaign in January–February 2017. These measurements wereaccompanied by condensation particle counters (CPCs) to measure total particlenumber count at four of the sites and Aethalometers measuring black carbon(BC) at five sites. The spatial distribution and inter-relationships of theparticle size distribution and SMPS total number counts with CPC total numbercounts and black carbon measurements have been analysed in detail as well asvariations in the size distributions. One site (Marylebone Road) was in astreet canyon with heavy traffic, one site (Westminster University) was on arooftop adjacent to the Marylebone Road sampler, and a further sampler waslocated at Regent's University within a major park to the north of MaryleboneRoad. A fourth sampler was located nearby at 160 m above ground level on theBT tower and a fifth sampler was located 4 km to the west of the mainsampling region at North Kensington. Consistent with earlier studies it wasfound that the mode in the size distribution had shifted to smaller sizes atthe Regent's University (park) site, the mean particle shrinkage rate being0.04 nm s −1 with slightly lower values at low wind speeds and somelarger values at higher wind speeds. There was evidence of completeevaporation of the semi-volatile nucleation mode under certain conditions atthe elevated BT Tower site. While the SMPS total count and black carbon showedtypical traffic-dominated diurnal profiles, the CPC count data typicallypeaked during night-time as did CPC∕SMPS and CPC∕BC ratios. This is thought tobe due to the presence of high concentrations of small particles (2.5–15 nmdiameter) probably arising from condensational growth from traffic emissionsduring the cooler night-time conditions. Such behaviour was most marked at theRegent's University and Westminster University sites and less so atMarylebone Road, while at the elevated BT Tower site the ratio of particlenumber (CPC) to black carbon peaked during the morning rush hour and not atnight-time, unlike the other sites. An elevation in nucleation mode particlesassociated with winds from the west and WSW sector was concluded to resultfrom emissions from London Heathrow Airport, despite a distance of 22 kmfrom the central London sites.

Interpretation of particle number size distributions measured across an urban area during the FASTER campaign