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- ItemLong-term measurements of particle number size distributions and the relationships with air mass history and source apportionment in the summer of Beijing(München : European Geopyhsical Union, 2013) Wang, Z.B.; Hu, M.; Wu, Z.J.; Yue, D.L.; He, L.Y.; Huang, X.F.; Liu, X.G.; Wiedensohler, A.A series of long-term and temporary measurements were conducted to study the improvement of air quality in Beijing during the Olympic Games period (8–24 August 2008). To evaluate actions taken to improve the air quality, comparisons of particle number and volume size distributions of August 2008 and 2004–2007 were performed. The total particle number and volume concentrations were 14 000 cm−3 and 37 μm−3 cm−3 in August of 2008, respectively. These were reductions of 41% and 35% compared with mean values of August 2004–2007. A cluster analysis on air mass history and source apportionment were performed, exploring reasons for the reduction of particle concentrations. Back trajectories were classified into five major clusters. Air masses from the south direction are always associated with pollution events during the summertime in Beijing. In August 2008, the frequency of air mass arriving from the south was 1.3 times higher compared to the average of the previous years, which however did not result in elevated particle volume concentrations in Beijing. Therefore, the reduced particle number and volume concentrations during the 2008 Beijing Olympic Games cannot be only explained by meteorological conditions. Four factors were found influencing particle concentrations using a positive matrix factorization (PMF) model. They were identified as local and remote traffic emissions, combustion sources as well as secondary transformation. The reductions of the four sources were calculated to 47%, 44%, 43% and 30%, respectively. The significant reductions of particle number and volume concentrations may attribute to actions taken, focusing on primary emissions, especially related to the traffic and combustion sources.
- ItemSize-resolved measurement of the mixing state of soot in the megacity Beijing, China: Diurnal cycle, aging and parameterization(München : European Geopyhsical Union, 2012) Cheng, Y.F.; Su, H.; Rose, D.; Gunthe, S.S.; Berghof, M.; Wehner, B.; Achtert, P.; Nowak, A.; Takegawa, N.; Kondo, Y.; Shiraiwa, M.; Gong, Y.G.; Shao, M.; Hu, M.; Zhu, T.; Zhang, Y.H.; Carmichael, G.R.; Wiedensohler, A.; Andreae, M.O.; Pöschl, U.Soot particles are the most efficient light absorbing aerosol species in the atmosphere, playing an important role as a driver of global warming. Their climate effects strongly depend on their mixing state, which significantly changes their light absorbing capability and cloud condensation nuclei (CCN) activity. Therefore, knowledge about the mixing state of soot and its aging mechanism becomes an important topic in the atmospheric sciences. The size-resolved (30–320 nm diameter) mixing state of soot particles in polluted megacity air was measured at a suburban site (Yufa) during the CAREBeijing 2006 campaign in Beijing, using a volatility tandem differential mobility analyzer (VTDMA). Particles in this size range with non-volatile residuals at 300 °C were considered to be soot particles. On average, the number fraction of internally mixed soot in total soot particles (Fin), decreased from 0.80 to 0.57 when initial Dp increased from 30 to 320 nm. Further analysis reveals that: (1) Fin was well correlated with the aerosol hygroscopic mixing state measured by a CCN counter. More externally mixed soot particles were observed when particles showed more heterogeneous features with regard to hygroscopicity. (2) Fin had pronounced diurnal cycles. For particles in the accumulation mode (Dp at 100–320 nm), largest Fin were observed at noon time, with "apparent" turnover rates (kex → in) up to 7.8% h−1. (3) Fin was subject to competing effects of both aging and emissions. While aging increases Fin by converting externally mixed soot particles into internally mixed ones, emissions tend to reduce Fin by emitting more fresh and externally mixed soot particles. Similar competing effects were also found with air mass age indicators. (4) Under the estimated emission intensities, actual turnover rates of soot (kex → in) up to 20% h−1 were derived, which showed a pronounced diurnal cycle peaking around noon time. This result confirms that (soot) particles are undergoing fast aging/coating with the existing high levels of condensable vapors in the megacity Beijing. (5) Diurnal cycles of Fin were different between Aitken and accumulation mode particles, which could be explained by the faster growth of smaller Aitken mode particles into larger size bins. To improve the Fin prediction in regional/global models, we suggest parameterizing Fin by an air mass aging indicator, i.e., Fin = a + bx, where a and b are empirical coefficients determined from observations, and x is the value of an air mass age indicator. At the Yufa site in the North China Plain, fitted coefficients (a, b) were determined as (0.57, 0.21), (0.47, 0.21), and (0.52, 0.0088) for x (indicators) as [NOz]/[NOy], [E]/[X] ([ethylbenzene]/[m,p-xylene]) and ([IM] + [OM])/[EC] ([inorganic + organic matter]/[elemental carbon]), respectively. Such a parameterization consumes little additional computing time, but yields a more realistic description of Fin compared with the simple treatment of soot mixing state in regional/global models.
- ItemCloud condensation nuclei (CCN) from fresh and aged air pollution in the megacity region of Beijing(München : European Geopyhsical Union, 2011) Gunthe, S.S.; Rose, D.; Su, H.; Garland, R.M.; Achtert, P.; Nowak, A.; Wiedensohler, A.; Kuwata, M.; Takegawa, N.; Kondo, Y.; Hu, M.; Shao, M.; Zhu, T.; Andreae, M.O.; Pöschl, U.Atmospheric aerosol particles serving as cloud condensation nuclei (CCN) are key elements of the hydrological cycle and climate. CCN properties were measured and characterized during the CAREBeijing-2006 campaign at a regional site south of the megacity of Beijing, China. Size-resolved CCN efficiency spectra recorded for a supersaturation range of S=0.07% to 0.86% yielded average activation diameters in the range of 190 nm to 45 nm. The corresponding effective hygroscopicity parameters (κ) exhibited a strong size dependence ranging from ~0.25 in the Aitken size range to ~0.45 in the accumulation size range. The campaign average value (κ =0.3 ± 0.1) was similar to the values observed and modeled for other populated continental regions. The hygroscopicity parameters derived from the CCN measurements were consistent with chemical composition data recorded by an aerosol mass spectrometer (AMS) and thermo-optical measurements of apparent elemental and organic carbon (EC and OC). The CCN hygroscopicity and its size dependence could be parameterized as a function of only AMS based organic and inorganic mass fractions (forg, finorg) using the simple mixing rule κp ≈ 0.1 · forg + 0.7 · finorg. When the measured air masses originated from the north and passed rapidly over the center of Beijing (fresh city pollution), the average particle hygroscopicity was reduced (κ = 0.2 ± 0.1), which is consistent with enhanced mass fractions of organic compounds (~50%) and EC (~30%) in the fine particulate matter (PM1). Moreover, substantial fractions of externally mixed weakly CCN-active particles were observed at low supersaturation (S=0.07%), which can be explained by the presence of freshly emitted soot particles with very low hygroscopicity (κ < 0.1). Particles in stagnant air from the industrialized region south of Beijing (aged regional pollution) were on average larger and more hygroscopic, which is consistent with enhanced mass fractions (~60%) of soluble inorganic ions (mostly sulfate, ammonium, and nitrate). Accordingly, the number concentration of CCN in aged air from the megacity region was higher than in fresh city outflow ((2.5–9.9) × 103 cm−3 vs. (0.4–8.3) × 103 cm−3 for S=0.07–0.86%) although the total aerosol particle number concentration was lower (1.2 × 104 cm−3 vs. 2.3 × 104 cm−3). A comparison with related studies suggests that the fresh outflow from Chinese urban centers generally may contain more, but smaller and less hygroscopic aerosol particles and thus fewer CCN than the aged outflow from megacity regions.