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A field measurement campaign was carried out during the late winter and early spring of 2015 in Budapest,the capital of Hungary.The size distribution (SD) and optical absorption of carbonaceous particulate matter (CPM) was measured online using a Scanning Mobility Particle Sizer (SMPS),a 7λ-aethalometer and an inhouse developed 4λ-Photoacoustic Spectrometer.Based on the SD data,the measurement period could be classified into days with and without new particle formation events (normal days and nucleation days),although particular nucleation-like events were observed on normal days as well.Three characteristic size modes were observed with CMDs of circa 15,25 and 110 nm that corresponded to the nucleation,traffic and heating modes.Based on the temporal behavior of these modes both types of days were divided into distinctive daily periods (heating hours,traffic hours and nucleation hours).The optical absorption spectra (OAC and ARE) also displayed the same part of day behavior to that of SD.That way this paper is among the first to assess the optical response of urban nucleation events.Due to the simultaneous measurement of OAC by the 7λ-aethalometer and a 4λ-Photoacoustic Spectrometer,OAC was measured overall at 11 wavelengths.That way aethalometer correction factors (fand C) were determined at all aethalometer wavelengths using in situ reference photoacoustic measurements.Correction factors were found to have both wavelength and time of the day variation.In the case off,no clear trend could be observed,however,Crefvalues increased both as a function of wavelength.