Wintertime organic and inorganic aerosols in Lanzhou, China: sources, processes, and comparison with the results during summer

Abstract

Abstract. Lanzhou, which is located in a steep alpine valley in western China, is one of the most polluted cities in China during the wintertime. In this study, an Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS), a seven-wavelength aethalometer, and a scanning mobility particle sizer (SMPS) were deployed during 10 January to 4 February 2014 to study the mass concentrations, chemical processes, and sources of submicrometer particulate matter (PM1). The average PM1 concentration during this study was 57.3 µg m−3 (ranging from 2.1 to 229.7 µg m−3 for hourly averages), with organic aerosol (OA) accounting for 51.2 %, followed by nitrate (16.5 %), sulfate (12.5 %), ammonium (10.3 %), black carbon (BC, 6.4 %), and chloride (3.0 %). The mass concentration of PM1 during winter was more than twice the average value observed at the same site in summer 2012 (24.5 µg m−3), but the mass fraction of OA was similar in the two seasons. Nitrate contributed a significantly higher fraction to the PM1 mass in winter than summer (16.5 % vs. 10 %), largely due to more favored partitioning to the particle phase at low air temperature. The mass fractions of both OA and nitrate increased by ∼ 5 % (47 to 52 for OA and 13 to 18 % for nitrate) with the increase of the total PM1 mass loading, while the average sulfate fraction decreased by 6 % (17 to 11 %), indicating the importance of OA and nitrate for the heavy air pollution events in Lanzhou. The size distributions of OA, nitrate, sulfate, ammonium, and chloride all peaked at ∼ 500 nm, with OA being slightly broader, suggesting that aerosol particles were internally mixed during winter, likely due to frequently calm and stagnant air conditions during wintertime in Lanzhou (average wind speed: 0.82 m s−1).The average mass spectrum of OA showed a medium oxidation degree (average O ∕ C ratio of 0.28), which was lower than that during summer 2012 (O ∕ C = 0.33). This is consistent with weaker photochemical processing during winter. Positive matrix factorization (PMF) with the multi-linear engine (ME-2) solver identified six OA sources, i.e., a hydrocarbon-like OA (HOA), a biomass burning OA (BBOA), a cooking-emitted OA (COA), a coal combustion OA (CCOA), and two oxygenated OA (OOA) factors. One of the OOAs was less oxidized (LO-OOA), and the other one more oxidized (MO-OOA). LO-OOA was the most abundant OA component (22.3 % of OA mass), followed by CCOA (22.0 %), COA (20.2 %), MO-OOA (14.9 %), BBOA (10.8 %), and HOA (9.8 %). The mass fraction of primary OA ( = HOA + BBOA + COA + CCOA) increased during high PM pollution periods, indicating that local primary emissions were a main reason for the formation of air pollution events in Lanzhou during winter. Radiocarbon (14C) measurement was conducted on four PM2.5 filter samples from this study, which allowed for a quantitative source apportionment of organic carbon (OC). The non-fossil sources on average accounted for 55 ± 3 % of OC, which could be mainly from biomass burning and cooking activities, suggesting the importance of non-fossil sources for the PM pollution in Lanzhou. Together with the PMF results, we also found that a large fraction (66 ± 10 %) of the secondary OC was from non-fossil OC.

Aerosol chemistry and particle growth events at an urban downwind site in the North China Plain

“…4h) was also similar to that observed in megacities Crippa et al, 2013;210 Elser et al, 2015;Zhang et al, 2015b;Hu et al, 2016;Xu et al, 2016) and was characterized by two peaks around the mealtime hours, reflecting the influence of cooking emissions. However, the COA concentration in this study was much lower than that reported in megacities (e.g., peak concentration: 2.5 μg m -3 versus 14 μg m -3 in Beijing; Hu et al, 2016), suggesting much less cooking emissions at the rural site.…”

Chemical characterization of long-range transport biomass burning emissions to the Himalayas: insights from high-resolution aerosol mass spectrometry

“…Overall, the PM 1 mass is closely correlated (R 2 = 0.97) with that of SMPS particle volume during the entire campaign, with a linear regression slope of 2.86. This slope is significantly higher than the estimated average PM 1 density of 1.44 g cm −3 , which is calculated based on the measured particle compositions in this study and the assumed particle densities of 1.2 for organics, 1.78 for (NH 4 ) 2 SO 4 , 1.72 for NH 4 NO 3 , 1.52 for NH 4 Cl, and 1.8 g cm −3 for BC (Zhang et al, 2005b;Xu et al, 2016). This discrepancy is likely introduced by various factors, including different transmission sizes between HR-ToF-AMS and SMPS (up to ∼ 1.0 µm in D va for AMS vs. limited size range of 14.6-661.2 nm in D m for SMPS), rough calculation of PM 1 density using assumed composition densities and spherical shape without considering the particle porosity, as well as the use of the empirical and constant CE value of 0.5 in this study.…”

“…The O / C ratio (0.85) of BBOA in this study was quite a bit higher than BBOA factors identified at other urban/rural sites in previous studies Huang et al, 2011;Mohr et al, 2012;Sun et al, 2016;Xu et al, 2016), suggesting its long-range transport feature. This aged BBOA feature was similar with those obtained at other remote sites worldwide, such as a remote forest site in Finland (Raatikainen et al, 2010), a remote background site in Greece (Bougiatioti et al, 2014), and a national air quality background sites in southern China , where OA were generally highly oxidized.…”

Volatility measurement of atmospheric submicron aerosols in an urban atmosphere in southern China

“…The average O / C value in this campaign is within the typical O / C range of 0.28-0.56 previously observed under polluted urban environments in China (Huang et al, , 2012He et al, 2011;Xu et al, 2016b;Hu et al, 2016;Lee et al, 2013). The diurnal variation in O / C plotted in Fig.…”