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22To gain a better understanding on the spatiotemporal variation of ultrafine particles (UFPs) in urban 23 environments, this study reports on the first results of a long-term UFP monitoring network, set up in were still obtained in terms of particle numbers (20-38% for total particle numbers and up to 49% for 38 size-resolved particle numbers), confirming the importance of local source contributions and the need

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• Particle total number concentration (TNC) does not always reflect variations in traffic 14 emissions 15• Primary and secondary sources contribute in a seasonally variant and quantifiable way 16 to particle number concentrations in Leicester. 17• New particle formation was a significant contributor around midday to TNC in the 18Leicester urban atmosphere. 19• (2014 and 2015). A derived chemical climatology 36for the pollutants showed maximum concentrations for all pollutants during the cold period 37 except O3 which peaked during spring. Quantification of primary and secondary sources of 38 ultrafine particles (UFPs) was undertaken using eBC as a tracer for the primary particle number 39 concentration in the Leicester urban area. At the urban background site, which is influenced by 40 fresh vehicle exhaust emissions, TNC was segregated into two components, TNC = N1 + N2. 41The component N1 represents components directly emitted as particles and compounds which 42 nucleate immediately after emission. The component N2 represents the particles formed during 43 the dilution and cooling of vehicle exhaust emissions and by in situ new particle formation 44 (NPF). The values of highest N1 (49%) were recorded during the morning rush hours (07:00-45 09:00 h), correlating with NOx, while the maximum contribution of N2 to TNC was found at 46 midday (11:00-14:00 h), at around 62%, correlated with O3. Generally, the percentage of N2 47 (57%) was greater than the percentage of N1 (43%) for all days at the AURN site over the 48 period of the study. For the first time the impact of wind speed and direction on N1 and N2 49 was explored. The overall data analysis shows that there are two major sources contributing to 50 TNC in Leicester: primary sources (traffic emissions) and secondary sources, with the majority 51 of particles of secondary origin.

A series of experiments was undertaken on an intercity train carriage aimed at providing a “proof of concept” for three methods in improving our understanding of airflow behaviour and the accompanied dispersion of exhaled droplets. The methods used included the following: measuring CO2 concentrations as a proxy for exhaled breath, measuring the concentrations of different size fractions of aerosol particles released from a nebuliser, and visualising the flow patterns at cross-sections of the carriage by using a fog machine and lasers. Each experiment succeeded in providing practical insights into the risk of airborne transmission. For example, it was shown that the carriage is not well mixed over its length, however, it is likely to be well mixed along its height and width. A discussion of the suitability of the fresh air supply rates on UK train carriages is also provided, drawing on the CO2 concentrations measured during these experiments.

Highlights • Total particle number concentrations were dominated by nucleation and Aitken modes. • School holiday has impact on particle number size distribution (PNSD) during Easter. • The frequency of new particle formation events (NPF) was higher than previous studies in the urban UK.