This　study　investigated　the　＂roadside-to-ambient＂　evolution　of　particle　physicochemical　and　optical　properties　in　typical　urban　atmospheres　of　Hong　Kong　through　collection　of　chemically-resolved　PM2.5　data　and　PM2.5　size　distribution　at　a　roadside　and　an　ambient　site.　Roadside　particle　size　distribution　showed　typical　peaks　in　the　nuclei　mode　(30-40　nm)　while　ambient　measurements　peaked　in　the　Aitken　mode　(50-70　nm),　revealing　possible　condensation　and　coagulation　growth　of　freshly　emitted　particles　during　aging　processes.　Much　higher　levels　of　anthropogenic　chemical　components,　i.e.　nitrate,　sulfate,　ammonium,　organic　carbon　(OC)　and　elemental　carbon　(EC),　but　lower　levels　of　OC/EC　and　secondary　inorganic　aerosols　(SIA)/EC　ratios　appeared　in　roadside　than　ambient　particles.　The　high　OC/EC　and　SIN　EC　ratios　in　ambient　particles　implied　high　contributions　from　secondary　aerosols.　Black　carbon　(BC),　a　strong　light　absorbing　material,　showed　large　variations　in　optical　properties　when　mixed　with　other　inorganic　and　organic　components.,　Particle-bound　polycyclic　aromatic　hydrocarbons　(p-PAHs),　an　indicator　of　brown　carbon　(BrC),　showed　significant　UV-absorbing　ability.　The　average　BC　and　p-PAHs　concentrations　were　3.8　and　87.6　ng　m(-3),　respectively,　at　the　roadside,　but　were　only　1.5　and　18.1　ng　m(-3)　at　the　ambient　site,　suggesting　BC　and　p-PAHs　concentrations　heavily　driven　by　traffic　emissions.　In　contrast,　PM2.5　UV　light　absorption　coefficients　(b(abs-BrC,370nm))　at　the　ambient　site　(4.2　Mm(-1))　and　at　the　roadside　site　(4.1　Mm(-1))　were　similar,　emphasizing　that　particle　aging　processes　enhanced　UV　light　absorbing　properties,　a　conclusion　that　was　also　supported　by　the　finding　that　the　Absorption　Angstrom　coefficient　(AAC)　value　at　UV　wavelengths　(AAC　(uv　band))　at　the　ambient　site　were　similar　to　1.7　times　higher　than　that　at　the　roadside.　Both　aqueous　reaction　and　photochemically　produced　secondary　organic　aerosol　(SOA)　for　ambient　aerosols　contributed　to　the　peak　values　of　b(abs-BrC,370nm)　in　ambient　particles　at　midnight　and　around　noon,　highlighting　that　secondary　BrC　had　different　sources　and　particle　aging　in　the　atmosphere　affected　BrC　and　BC　properties　and　related　aerosol　light　absorption.　(c)　2017　Elsevier　Ltd.　All　rights　reserved.