Chemical composition, sources, and processes of urban aerosols during summertime in northwest China: insights from high-resolution aerosol mass spectrometry

摘要

An Aerodyne high-resolution time-of-flight aerosol mass spectrometer(HR-ToF-AMS) was deployed along with a scanning mobility particle sizer(SMPS) and a multi-angle absorption photometer (MAAP) to measure thetemporal variations of the mass loading, chemical composition, and sizedistribution of submicron particulate matter (PM) in Lanzhou,northwest China, during 11 July–7 August 2012. The average(PM mass concentration including non-refractory (PM (NR-(PM)measured by HR-ToF-AMS and black carbon (BC) measured by MAAP during thisstudy was 24.5 μg m (ranging from 0.86 to105 μg m), with a mean composition consisting of 47% organics, 16%sulfate, 12% BC, 11% ammonium, 10% nitrate, and 4% chloride.Organic aerosol (OA) on average consisted of 70% carbon, 21% oxygen,8% hydrogen, and 1% nitrogen, with the average oxygen-to-carbon ratio(O / C) of 0.33 and organic mass-to-carbon ratio (OM / OC) of 1.58. Positivematrix factorization (PMF) of the high-resolution organic mass spectraidentified four distinct factors which represent, respectively, two primaryOA (POA) emission sources (traffic and food cooking) and two secondary OA(SOA) types – a fresher, semi-volatile oxygenated OA (SV-OOA) and a moreaged, low-volatility oxygenated OA (LV-OOA). Traffic-relatedhydrocarbon-like OA (HOA) and BC displayed distinct diurnal patterns, bothwith peak at ~ 07:00–11:00 (BJT: UTC +8), corresponding tothe morning rush hours, while cooking-emission related OA (COA) peaked during three mealperiods. The diurnal profiles of sulfate and LV-OOA displayed a broad peakbetween ~ 07:00 and 15:00, while those of nitrate, ammonium, andSV-OOA showed a narrower peak between ~ 08:00–13:00. The latermorning and early afternoon maximum in the diurnal profiles of secondaryaerosol species was likely caused by downward mixing of pollutants aloft,which were likely produced in the residual layer decoupled from the boundarylayer during nighttime. The mass spectrum of SV-OOA was similar to that ofcoal combustion aerosol and likely influenced by coal combustion activitiesin Lanzhou during summer. The sources of BC were estimated by a lineardecomposition algorithm that uses the time series of the NR-PM components. Our results indicate that a main source of BC was local traffic(47%) and that transport of regionally processed air masses alsocontributed significantly to BC observed in Lanzhou. Finally, theconcentration and source of polycyclic aromatic hydrocarbons (PAHs) wereevaluated.