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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.

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.

Highlights 15• Clear seasonal variation of Lung Deposited Surface Area (LDSA) concentrations is 16 observed with higher values in winter. 17• Calculated LDSA seems to be a good surrogate to equivalent black carbon mass 18 concentration. 19• Traffic emissions appear to be the main source of LDSA in Leicester. 20• LDSA concentrations are nearly doubled during new particle formation episodes. Lung Deposited Surface Area (LDSA) has been identified as a potential metric for the 2 correlation of a physical aerosol particle properties with health outcomes. Currently, there is 3 little urban LDSA data. As a case study, we investigated measurements of LDSA (alveolar) 4 concentrations in a mid-size European city. LDSA and associated measurements were carried 5 out over 1.5 years at an urban background site in Leicester, UK. Average LDSA concentrations 6 in the cold (November-April) and warm (May-October) seasons of UK were 37 and 23 µm 2 7 cm -3 , respectively. LDSA correlates well (R 2 =0.65-0.7, r=0.77-0.8) with traffic related 8 pollutants, such as equivalent black carbon (eBC) and NOX. We also report for the first time in 9 the UK the correlation between an empirically derived LDSA and eBC. Furthermore, the effect 10 of wind speed and direction on the LDSA was explored. Higher LDSA concentrations are 11 observed at low wind speeds (1-2 m s -1 ), owing to local traffic emissions. In addition, the diurnal 12 variation of LDSA showed a second peak in the afternoon under warm and relatively clean 13 atmospheric conditions, which can be attributed to photochemical new particle formation (NPF) 14 and growth into the Aitken mode range. These NPF events increased the average background 15 LDSA concentrations from 15.5 to 35.5 µm 2 cm -3 , although they might not be health-relevant. 16Overall, the results support the notion that local traffic emissions are a major contributor to 17 observed LDSA concentrations with a clear seasonal pattern with higher values during winter.