Abstract
Abstract. 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 the temporal variations of the mass loading, chemical composition, and size distribution of submicron particulate matter (PM1) in Lanzhou, northwest China, during 11 July–7 August 2012. The average (PM1 mass concentration including non-refractory (PM1 (NR-(PM1) measured by HR-ToF-AMS and black carbon (BC) measured by MAAP during this study was 24.5 μg m−3 (ranging from 0.86 to 105 μg m−3), 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. Positive matrix factorization (PMF) of the high-resolution organic mass spectra identified four distinct factors which represent, respectively, two primary OA (POA) emission sources (traffic and food cooking) and two secondary OA (SOA) types – a fresher, semi-volatile oxygenated OA (SV-OOA) and a more aged, low-volatility oxygenated OA (LV-OOA). Traffic-related hydrocarbon-like OA (HOA) and BC displayed distinct diurnal patterns, both with peak at ~ 07:00–11:00 (BJT: UTC +8), corresponding to the morning rush hours, while cooking-emission related OA (COA) peaked during three meal periods. The diurnal profiles of sulfate and LV-OOA displayed a broad peak between ~ 07:00 and 15:00, while those of nitrate, ammonium, and SV-OOA showed a narrower peak between ~ 08:00–13:00. The later morning and early afternoon maximum in the diurnal profiles of secondary aerosol species was likely caused by downward mixing of pollutants aloft, which were likely produced in the residual layer decoupled from the boundary layer during nighttime. The mass spectrum of SV-OOA was similar to that of coal combustion aerosol and likely influenced by coal combustion activities in Lanzhou during summer. The sources of BC were estimated by a linear decomposition algorithm that uses the time series of the NR-PM1 components. Our results indicate that a main source of BC was local traffic (47%) and that transport of regionally processed air masses also contributed significantly to BC observed in Lanzhou. Finally, the concentration and source of polycyclic aromatic hydrocarbons (PAHs) were evaluated.
Highlights
Air pollution has become a serious issue in China due to rapid urbanization and industrialization in recent years (Chan and Yao, 2008, and references therein)
The average mass concentration of PM1 was 24.5 (±15.2) μg m−3 for the entire study with organics accounting for 47 %, following by sulfate (16 %), black carbon (BC) (12 %), ammonium (11 %), nitrate (10 %), and chloride (4 %)
Four Organic aerosol (OA) factors were identified by Positive matrix factorization (PMF) analysis of the high-resolution mass spectra, including hydrocarbon-like OA (HOA) (16 % of total organics), cooking-emission related OA (COA) (24 %), semi-volatile oxygenated OA (SV-OOA) (27 %), and low-volatility oxygenated OA (LV-OOA) (32 %)
Summary
Air pollution has become a serious issue in China due to rapid urbanization and industrialization in recent years (Chan and Yao, 2008, and references therein). Poor air quality in Lanzhou is attributed to several important factors: (1) the valley’s trough-shaped topography, which often results in an inversion during winter and spring and traps pollutants at the ground level (Hu and Zhang, 1999); (2) coal consumption for industrial and residential heating during wintertime, which can lead to emissions of large amounts of particulate and gaseous pollutants (Zhang et al, 2000); (3) large amounts of dust particles, especially during spring, due to the frequent dust storm events originated from the Hexi Corridor region in northwestern China (Wang et al, 2005); and (4) rapidly increasing vehicle usage, which was estimated to contribute an increasingly large fraction of the air pollutants in recent years (Zhou, 1997)
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