Abstract. Effects of a new aerosol particle formation (NPF) and particle diameter growth process as a single source of atmospheric particle number concentrations were evaluated and quantified on the basis of experimental data sets obtained from particle number size distribution measurements in the city centre and near-city background of Budapest for 5 years. Nucleation strength factors for a nucleation day (NSF NUC ) and for a general day (NSF GEN ) were derived separately for seasons and full years. The former characteristic represents the concentration increment of ultrafine (UF) particles specifically on nucleation days with respect to accumulationmode (regional background) concentrations (particles with equivalent diameters of 100-1000 nm; N 100−1000 ) due solely to the nucleation process. The latter factor expresses the contribution of nucleation to particle numbers on general days; thus, it represents a longer time interval such as season or year. The nucleation source had the largest effect on the concentrations around noon and early afternoon, as expected. During this time interval, it became the major source of particles in the near-city background. Nucleation increased the daily mean concentrations on nucleation days by mean factors of 2.3 and 1.58 in the near-city background and city centre, respectively. Its effect was largest in winter, which was explained by the substantially lower N 100−1000 levels on nucleation days than those on non-nucleation days. On an annual timescale, 37 % of the UF particles were generated by nucleation in the near-city background, while NPF produced 13 % of UF particles in the city centre. The differences among the annual mean values, and among the corresponding seasonal mean values, were likely caused by the variability in controlling factors from year to year. The values obtained represent the lower limits of the contributions. The shares determined imply that NPF is a non-negligible or substantial source of particles in near-city background environments and even in city centres, where the vehicular road emissions usually prevail. Atmospheric residence time of nucleationmode particles was assessed by a decay curve analysis, and a mean of 02:30 was obtained. The present study suggests that the health-related consequences of the atmospheric NPF and growth process in cities should also be considered in addition to its urban climate implications.

Abstract. Multiple atmospheric properties were measured semi-continuously in the Budapest platform for Aerosol Research and Training laboratory, which represents the urban background for the time interval of 2008–2018. Data of 6 full measurement years during a decennial time interval were subjected to statistical time trend analyses by an advanced dynamic linear model and a generalized linear mixed model. The main interest in the analysed data set was on particle number concentrations in the diameter ranges from 6 to 1000 nm (N6−1000), from 6 to 100 nm (N6−100, ultrafine particles), from 25 to 100 nm (N25−100) and from 100 to 1000 nm (N100−1000). These data were supported by concentrations of SO2, CO, NO, NOx, O3, PM10 mass, as well as air temperature, relative humidity, wind speed, atmospheric pressure, global solar radiation, condensation sink, gas-phase H2SO4 proxy, classes of new aerosol particle formation (NPF), and growth events and meteorological macro-circulation patterns. The trend of the particle number concentrations derived as a change in the statistical properties of background state of the data set decreased in all size fractions over the years. Most particle number concentrations showed decreasing decennial statistical trends. The estimated annual mean decline of N6−1000 was (4–5) % during the 10-year measurement interval, which corresponds to a mean absolute change of −590 cm−3 in a year. This was interpreted as a consequence of the decreased anthropogenic emissions at least partly from road traffic alongside household heating and industry. Similar trends were not observed for the air pollutant gases. Diurnal statistical patterns of particle number concentrations showed tendentious variations, which were associated with a typical diurnal activity–time pattern of inhabitants in cities, particularly of vehicular road traffic. The trend patterns for NPF event days contained a huge peak from late morning to late afternoon, which is unambiguously caused by NPF and growth processes. These peaks were rather similar to each other in the position, shape and area on workdays and holidays, which implies that the dynamic and timing properties of NPF events are not substantially influenced by anthropogenic activities in central Budapest. The diurnal pattern for N25−100 exhibited the largest relative changes, which were related to particle emissions from high-temperature sources. The diurnal pattern for N100−1000 – which represents chemically and physically aged particles of larger spatial scale – were different from the diurnal patterns for the other size fractions.