Abstract. The hygroscopic properties of submicron aerosol particles were determined at a suburban site (Wuqing) in the North China Plain among a cluster of cities during the period 17 July to 12 August, 2009. A High Humidity Tandem Differential Mobility Analyser (HH-TDMA) instrument was applied to measure the hygroscopic growth factor (GF) at 90%, 95% and 98.5% relative humidity (RH) for particles with dry diameters between 50 and 250 nm. The probability distribution of GF (GF-PDF) averaged over the period shows a distinct bimodal pattern, namely, a dominant more-hygroscopic (MH) group and a smaller nearly-hydrophobic (NH) group. The MH group particles were highly hygroscopic, and their GF was relatively constant during the period with average values of 1.54 ± 0.02, 1.81 ± 0.04 and 2.45 ± 0.07 at 90%, 95% and 98.5% RH (D 0 = 100 nm), respectively. The NH group particles grew very slightly when exposed to high RH, with GF values of 1.08 ± 0.02, 1.13 ± 0.06 and 1.24 ± 0.13 respectively at 90%, 95% and 98.5% RH (D 0 = 100 nm). The hygroscopic growth behaviours at different RHs were well represented by a single-parameter Köhler model. Thus, the calculation of GF as a function of RH and dry diameter could be facilitated by an empirical parameterization of κ as function of dry diameter. A strong diurnal pattern in number fraction of different hygroscopic groups was observed. The average number fraction of NH particles during the day was about 8%, while during the nighttime fractions up to 20% were reached. Correspondingly, the state of mixing in terms of water uptake varied significantly during a day. Simulations using a particle-resolved aerosol box model (PartMC-MOSAIC) suggest that the diurnal variations of aerosol hygroscopicity and mixing state were mainly caused by the evolution of the atmospheric mixing layer. The shallow nocturnal boundary layer during the night facilitated the accumuCorrespondence to: C. S. Zhao (zcs@pku.edu.cn) lation of freshly emitted carbonaceous particles (mainly hydrophobic) near the surface while in the morning turbulence entrained the more aged and more hygroscopic particles from aloft and diluted the NH particles near the surface resulting in a decrease in the fraction of NH particles.

Abstract. Visibility degradation is a pervasive and urgent environmental problem in China. The occurrence of low visibility events is frequent in the North China Plain, where the aerosol loading is quite high and aerosols are strongly hygroscopic. A parameterization of light extinction (K ex ) for low visibilities on hazy days is proposed in this paper, based on visibility, relative humidity (RH), aerosol hygroscopic growth factors and particle number size distributions measured during the Haze in China (HaChi) Project. Observational results show that a high aerosol volume concentration is responsible for low visibility at RH <90 %; while for RH >90 %, decrease of visibility is mainly influenced by the increase of RH. The parameterization of K ex is developed on the basis of aerosol volume concentrations and RH, taking into accounts the sensitivity of visibility to the two factors and the availability of corresponding data. The extinction coefficients calculated with the parameterization schemes agree well with the directly measured values.

Abstract. The largest uncertainty in the estimation of climate forcing stems from atmospheric aerosols. In early spring and summer of 2009, two periods of in-situ measurements on aerosol physical and chemical properties were conducted within the HaChi (Haze in China) project at Wuqing, a town between Beijing and Tianjin in the North China Plain (NCP). Aerosol optical properties, including the scattering coefficient (σ sp ), the hemispheric back scattering coefficient (σ bsp ), the absorption coefficient (σ ap ), as well as the single scattering albedo (ω), are presented. The diurnal and seasonal variations are analyzed together with meteorology and satellite data. The mean values of σ sp, 550nm of the dry aerosol in spring and summer are 280±253 and 379±251 Mm −1 , respectively. The average σ ap for the two periods is respectively 47±38 and 43±27 Mm −1 . The mean values of ω at the wavelength of 637 nm are 0.82±0.05 and 0.86±0.05 for spring and summer, respectively. The relative high levels of σ sp and σ bsp are representative of the regional aerosol pollution in the NCP. Pronounced diurnal cycle of σ sp , σ ap and ω are found, mainly influenced by the evolution of boundary layer and the accumulation of local emissions during nighttime. The pollutants transported from the southwest of the NCP are more significant than that from the two megacities, Beijing and Tianjin, in both spring and summer. An optical closure experiment is conducted to better under-

The largest uncertainty in the estimation of radiative forcings on climate stems from atmospheric aerosols. In winter and summer of 2009, two periods of in-situ measurements on aerosol physical and chemical properties were conducted within the HaChi project at Wuqing, a town between Beijing and Tianjin in the North China Plain (NCP). Aerosol optical properties including scattering coefficient (&sigma;_sp), hemispheric back scattering coefficient (&sigma;_bsp), absorption coefficient (&sigma;_ap, as well as single scattering albedo (&omega;) are presented. The characteristics of diurnal and seasonal variations are analyzed together with the meteorological and satellite data. The mean values of &sigma;_{sp, 550 nm} of the dry aerosol in winter and summer are 280 &plusmn; 253 and 379 &plusmn; 251 Mm⁻¹, respectively. The average &sigma;_ap for the two periods are respectively 47 &plusmn; 38 and 43 &plusmn; 27 Mm⁻¹. The mean values of &omega; are 0.83 &plusmn; 0.05 and 0.87 &plusmn; 0.05 for winter and summer, respectively. The relative high levels of &sigma;_sp and &sigma;_bsp are representative of the regional polluted aerosol of the North China Plain. Pronounced diurnal cycle of &sigma;_sp, &sigma;_ap and &omega; are found, mainly influenced by the evolution of boundary layer and accumulation of local emissions during night-time. Regional transport of pollutants from southwest in the NCP is significant both in winter and summer, while high values of &sigma;_sp and &sigma;_ap correlate with calm winds in winter, which indicating the significant contribution of local emissions. An optical closure experiment is conducted to better understand uncertainties of the measurements. Good correlations (<i>R</i>>0.98) are found between values measured by nephelometer and values calculated with a modified Mie model. Monte Carlo simulations show an uncertainty of about 30% for the calculations. Considering all possible uncertainties of measurements, calculated &sigma;_sp and &sigma;_bsp agree well with measured values, indicating a stable performance of instruments and thus a reliable aerosol optical data

An unusual peak in atmospheric aerosol particle number and mass concentration occurred on September 20 and 21, 2006 at the Schneefernerhaus (SFH) Observatory, Germany. A source-receptor analysis using the Lagrangian transport model FLEXPART suggested that polluted air from source regions in the Western and Central U.S. travelled within 4-8 days over the Atlantic to the European Alpine region. A warm conveyor belt over the northern Great Plains was identified as the essential process lifting the boundary layer air to the high altitudes required for a rapid transatlantic transit. The layer arriving at SFH had an aerosol mass concentration of about 30 µg m −3 , and a particle number size distribution showing an aged accumulation mode aerosol with mode diameter around 0.3 µm. A combination of in-situ humidity, 214 Po and carbon monoxide measurements as well as upper air observations (radiosoundings) suggested that the layer had no previous contact with the local/European boundary layer. A screening of three years of FLEXPART simulations (2005-2007) yielded this case study as the only event of transatlantic anthropogenic aerosol to SFH where mixing with the boundary layer could safely be excluded. The event therefore represents rare surface-based evidence of transatlantic transport of fine and ultrafine aerosols. Zusammenfassung Die Verteilung atmosphärischer Spurenstoffe sowie ihre Auswirkungen auf die Luftqualität kann nicht ohne die Betrachtung weitreichender Transportvorgänge verstanden werden. Die Konzentrationen atmosphärischer Aerosolpartikel werden in Bodennähe normalerweise stark durch die Mischungsvorgänge in der Grenzschicht geprägt, weshalb interkontinentaler Ferntransport dort in der Regel nur verwischt wahrgenommen werden kann. Diese Arbeit beschreibt einen Fall bodengestützter Beobachtungen an der Zugspitze (Schneefernerhaus) vom 20. September 2006, bei dem eine Aerosolschicht auf Quellen innerhalb Nordamerikas zurückgeführt werden konnte. Als Transportmechanismus wurde ein warmes Förderband (warm conveyor belt)über den dortigen Great Plains erkannt, welches Grenzschichtluft in die für einen schnellen Transatlantiktransport erforderlichen Höhen brachte. Das ankommende Aerosol war durch ein gealtertes Größenspektrum an Akkumulationsmodepartikeln (Modaldurchmesser 0,3 µm) geprägt und Lagrangeschen Modellrechnungen zufolge zwischen 4 und 8 Tagen aus Nordamerika unterwegs. Betrachtungen der Zerfallsaktivität von 214 Po und der Aerosol-, Kohlenmonoxid-und Feuchtemesswerte an der Zugspitze und am tiefergelegenen Hohenpeißenberg sowie Radiosondenprofile zeigten, dass das an der Zugspitze ankommende Aerosol nicht durch Einflüsse aus der lokalen Grenzschicht gestört war. Der Fall verdeutlicht, dass Verschmutzungsaerosol aus Nordamerika in Europa direkt bis an den Boden gelangen kann-wenngleich eine Häufigkeitsanalyse ergab, dass derartiger Direkttransport in die Alpenregion eher selten auftritt (ca. 1/Jahr). Der gezeigte Fall war im Dreijahreszeitraum 2005-2007 der einzige transatlantische Ferntransport, bei dem...