Particulate matter(PM)pollution has led to great scientific,governmental andpublic concerns due to its adverse effects on air quality and human health.Among all theconstituents in particle matter,organics are least understood.One of the most challengingquestions is the formation and evolution of secondary organic aerosol(SOA).We deployed anAerodyne high-resolution time of-flight aerosol mass spectrometer(HR-ToF-AMS)tocharacterize particulate matter at a suburban coastal site in Hong Kong in May,Sep,Nov 2011 andFeb 2012.A local photochemical event of NR-PM1 was captured in a period of land-sea-breeze effectsover this region with high solar irradiance.A clear diurnal cycle,which was overall consistentwith solar irradiance,was observed for most NR-PM1 species.Moreover,there were notablechanges of organic aerosol(OA)components,with 1)higher primary organic aerosol(POA)concentrations in the early morning,2)rapidly increasing semi-volatile oxygenated organicaerosol(SVOOA)concentration from the early morning till noon,and 3)an increase oflow-volatility oxygenated organic aerosol(LVOOA)concentrations in the afternoon.The sizedistribution of OA shifted to larger particles from early morning to late afternoon.These resultssuggest that conversion of fresh to aged OA may have occurred.The increase of sulfur oxidationratio(SOR=Nso4/(Nso4+Nso2)); n refers to the molar concentration)and degree of oxygenation(O Sc≈2×O:C-H:C)of organics coincided with the increase of photochemical age,estimated from the ratio of ambient benzene to toluene concentrations.As photochemical age increased,theSVOOA/ΔCO ratio substantially increased near noontime but slightly decreased in the lateafternoon at even higher photochemical activity.There are also strong positive correlations ofLVOOA/ΔCO,SO4/ΔCO and methanesulfonic acid(MSA)/ΔCO ratios with photochemical age.This observations indicate that significant amounts of OOA,SO4 and MSA were formed in theatmosphere on a timescale of~8 h and a transition from fresher SVOOA to aged LVOOA haveoccurred.