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Meteorology Tags > Tag based links for Aerosol

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CALIPSO lidar observations of the optical properties of Saharan dust: A case study of long-range transport: Journal of Geophysical Research, Vol. 113 (15 April 2008), D07207.Zhaoyan Liu, Ali Omar, Mark Vaughan, Johnathan Hair, Chieko Kittaka, Yongxiang Hu, Kathleen Powell, Charles Trepte, David Winker, Chris Hostetler, Richard Ferrare, Robert Pierce

Source: Journal of Geophysical Research, Vol. 113 (15 April 2008), D07207.
A height resolved global view of dust aerosols from the first year CALIPSO lidar measurements: Journal of Geophysical Research, Vol. 113 (30 August 2008), D16214.Based on the first year of CALIPSO lidar measurements under cloud-free conditions, a height-resolve d global distribution of dust aerosols is presented for the first time. Results indicate that spring is the most active dust season, during which ?20% and ?12% of areas between 0 and 60°N are influenced by dust at least 10% and 50% of the time, respectively. In summer within 3?6 km, ?8.3% of area between 0 and 60°N is impacted by dust at least 50% of the time. Strong seasonal cycles of dust layer vertical extent are observed in major source regions, which are similar to the seasonal variation of the thermally driven boundary layer depth. The arid and semiarid areas in North Africa and the Arabian Peninsula are the most persistent and prolific dust sources. African dust is transported across the Atlantic all yearlong with strong seasonal variation in the transport pathways mainly in the free troposphere in summer and at the low altitudes in winter. However, the trans-Atlantic dust is transported at the low altitudes is important for all seasons, especially transported further cross the ocean. The crossing Atlantic dusty zones are shifted southward from summer to winter, which is accompanied by a similar southward shift of dust-generatin g areas over North Africa. The Taklimakan and Gobi deserts are two major dust sources in East Asia with long-range transport mainly occurring in spring. The large horizontal and vertical coverage of dust aerosols indicate their importance in the climate system through both direct and indirect aerosol effects.Dong Liu, Zhien Wang, Zhaoyan Liu, Dave Winker, Charles Trepte

Source: Journal of Geophysical Research, Vol. 113 (30 August 2008), D16214.
Simultaneous three-waveleng th depolarization measurements of clouds and aerosols using a coherent white light continuum: Journal of Applied Physics, Vol. 103, No. 4. (2008)Toshihir o Somekawa, Chihiro Yamanaka, Masayuki Fujita, Maria Galvez

Source: Journal of Applied Physics, Vol. 103, No. 4. (2008)
An intercompariso n of lidar-derived aerosol optical properties with airborne measurements near Tokyo during ACE-Asia: Journal of Geophysical Research, Vol. 108, No. D23. (29 August 2003), 8651.During the ACE-Asia intensive observation period (IOP), an intercompariso n experiment with ground-based lidars and aircraft observations was conducted near Tokyo. On 23 April 2001, four Mie backscatter lidars were simultaneously operated in the Tokyo region, while the National Center for Atmospheric Research C-130 aircraft flew a stepped-ascent profile between the surface and 6 km over Sagami Bay southwest of Tokyo. The C-130 observation package included a tracking Sun photometer and in situ packages measuring aerosol optical properties, aerosol size distribution, aerosol ionic composition, and SO2 concentration. The three polarization lidars suggested that the observed modest concentrations of Asian dust in the free troposphere extended up to an altitude of 8 km. We found a good agreement in the backscattering coefficient at 532 nm among lidars and in situ 180° backscatter nephelometer observations. The intercompariso n indicated that the aerosol layer between 1.6 and 3.5 km was a remarkably stable and homogenous in mesoscale. We also found reasonable agreement between the aerosol extinction coefficients (?a ? 0.03 km?1) derived from the airborne tracking Sun photometer, in situ optical instruments, and those estimated from the lidars above the planetary boundary layer (PBL). We also found considerable vertical variation of the aerosol depolarization ratio (?a) and a negative correlation between ?a and the backscattering coefficient (?a) below 3.5 km. Airborne measurements of size-dependent optical parameters (e.g., the fine mode fraction of scattering) and of aerosol ionic compositions suggests that the mixing ratio of the accumulation-m ode and coarse-mode (dust) aerosols was primarily responsible for the observed variation of ?a. Aerosol observations during the intercompariso n period captured the following three types of layers in the atmosphere: a PBL (surface to 1.2?1.5 km) where fine (mainly sulfate) particles with a low ?a (

Source: Journal of Geophysical Research, Vol. 108, No. D23. (29 August 2003), 8651.
Application of lidar depolarization measurement in the atmospheric boundary layer: Effects of dust and sea-salt particles: Journal of Geophysical Research, Vol. 104, No. D24. (1999), pp. 31781-31792.We intensively observed the atmospheric boundary layer with a polarization lidar, a Sun photometer, and a high-volume sampler at a coastal area of Tokyo Bay. The purpose of the observation is to investigate a phenomenon discovered in the past summer: relatively high depolarization ratio events (?10% at peak) in the lower atmosphere associated with sea breeze. From the chemical analyses of the simultaneously sampled aerosols, we found that the depolarization ratio might be related to crystallized sea salt and dust particles. A boundary structure was clearly revealed by the depolarization ratio in the lower atmosphere, which might correspond to the mixed layer (the internal boundary layer) or the sea breeze in which crystallized sea salt and/or dust particles were diffused. We also presented the first numerical calculation on the depolarization ratio of the cubic particles to apply crystallized sea-salt (NaCl) particles by the dipole discrete approximation (DDA) method: the calculation yields 8?22% of depolarization ratio for the effective size larger than 0.8 ?m at the investigated wavelength (532 nm).Toshiyuki Murayama, Hajime Okamoto, Naoki Kaneyasu, Hiroki Kamataki, Kazuhiko Miura

Source: Journal of Geophysical Research, Vol. 104, No. D24. (1999), pp. 31781-31792.
Optical Properties of Atmospheric Aerosol in Maritime Environments: Journal of the Atmospheric Sciences, Vol. 59, No. 3. (1 February 2002), pp. 501-523.System atic characterizati on of aerosol over the oceans is needed to understand the aerosol effect on climate and on transport of pollutants between continents. Reported are the results of a comprehensive optical and physical characterizati on of ambient aerosol in five key island locations of the Aerosol Robotic Network (AERONET) of sun and sky radiometers, spanning over 2?5 yr. The results are compared with aerosol optical depths and size distributions reported in the literature over the last 30 yr. Aerosol found over the tropical Pacific Ocean (at three sites between 20°S and 20°N) still resembles mostly clean background conditions dominated by maritime aerosol. The optical thickness is remarkably stable with mean value of ?a(500 nm) = 0.07, mode value at ?am = 0.06, and standard deviation of 0.02?0.05. The average Ĺngström exponent range, from 0.3 to 0.7, characterizes the wavelength dependence of the optical thickness. Over the tropical to subtropical Atlantic (two stations at 7°S and 32°N) the optical thickness is significantly higher: ?a(500 nm) = 0.14 and ?am = 0.10 due to the frequent presence of dust, smoke, and urban?industri al aerosol. For both oceans the atmospheric column aerosol is characterized by a bimodal lognormal size distribution with a fine mode at effective radius Reff = 0.11 ± 0.01 ?m and coarse mode at Reff = 2.1 ± 0.3 ?m. A review of the published 150 historical ship measurements from the last three decades shows that ?am was around 0.07 to 0.12 in general agreement with the present finding. The information should be useful as a test bed for aerosol global models and aerosol representation in global climate models. With global human population expansion and industrializat ion, these measurements can serve in the twenty-first century as a basis to assess decadal changes in the aerosol concentration, properties, and radiative forcing of climate.Alexan der Smirnov, Brent Holben, Yoram Kaufman, Oleg Dubovik, Thomas Eck, Ilya Slutsker, Christophe Pietras, Rangasayi Halthore

Source: Journal of the Atmospheric Sciences, Vol. 59, No. 3. (1 February 2002), pp. 501-523.
A study of internal and external mixing scenarios and its effect on aerosol optical properties and direct radiative forcing: Journal of Geophysical Research, Vol. 107, No. D10. (24 May 2002), 4094.The optical properties and hence the radiative forcing of atmospheric aerosols are determined, in part, by the way in which the various constituents are externally or internally mixed. The mixing state must be known to compute the effective refractive index, water activity, and size distribution of the aerosols. In this study we found that the percentage difference in the optical properties, including extinction, single scattering albedo, and asymmetry parameter, between an internal mixture and external mixture of black carbon and ammonium sulfate can be over 25% for the dry case and over 50% for the wet case for typical mass mixing ratios. The differences are a result of a complicated combination of nonlinear Mie theory on the refractive index, assumptions about the coagulated particle sizes for internal mixtures, and the role of water uptake and deliquescence as a function of relative humidity. The computed optical properties are used to estimate the globally average clear-sky direct radiative forcing for different mixing assumptions. The results are displayed as a function of relative humidity to conveniently see the mixing effects for dry aerosols at less than the crystallizatio n point, for dry internal and wet external mixtures between the crystallizatio n and deliquescence points, and for fully wet mixtures above the deliquescence point. For a 9:1 ammonium sulfate to black carbon mass ratio, nearly all the cooling effect predicted for an external mixture is lost for the internally mixed assumption, especially for relative humidities less than the deliquescence point.Glen Lesins, Petr Chylek, Ulrike Lohmann

Source: Journal of Geophysical Research, Vol. 107, No. D10. (24 May 2002), 4094.
Can Anthropogenic Aerosols Decrease the Snowfall Rate?: Journal of the Atmospheric Sciences, Vol. 61, No. 20. (2004), pp. 2457-2468.Obse rvations by Borys, Lowenthal, Cohn, and Brown in midlatitude orographic clouds show that for a given supercooled liquid water content, both the riming and the snowfall rates are smaller if the supercooled cloud has more cloud droplets as, for example, caused by anthropogenic aerosols. The climatic implication of this effect was studied in global climate model simulations by replacing the constant riming efficiency with a size-dependent one appropriate for planar crystals and aggregates, respectively. In the model simulations that use a size-dependent riming collection efficiency, the pollution-indu ced decrease in cloud droplet size causes a decrease in the riming rate in stratiform clouds despite larger liquid water contents in polluted clouds. Contrary to the above-mentione d observations, in all model simulations the snowfall rate increases because of feedbacks in the climate system. Anthropogenic aerosol particles increase the aerosol and cloud optical thickness, which reduces the solar radiation at the top of the atmosphere and the surface. This in turn causes a cooling in Northern Hemisphere midlatitudes that favors precipitation formation via the ice phase.U Lohmann

Source: Journal of the Atmospheric Sciences, Vol. 61, No. 20. (2004), pp. 2457-2468.
Anthropogenic aerosol effects on convective cloud microphysical properties in southern Sweden: Tellus B, Vol. 0, No. 0. (0), pp. ???-???.ABSTRA CT In this study, we look for anthropogenic aerosol effects in southern Scandinavia's clouds under the influence of moderate levels of pollution and relatively weak dynamic forcing. This was done by comparing surface aerosol measurements with convective cloud microphysical profiles produced from satellite image analyses. The results show that the clouds associated with the anthropogenic- affected air with high PM0.5, had to acquire a vertical development of ~3.5 km before forming precipitation- sized particles, compared to less than 1 km for the clouds associated with low PM0.5 air-masses. Additionally, a comparison of profiles with precipitation was done with regard to different potentially important parameters. For precipitating clouds the variability of the cloud thickness needed to produce the precipitation (Deltah14) is directly related to PM0.5 concentrations , even without considering atmospheric stability, the specific aerosol size distribution or the aerosols' chemical composition. Each additional 1 mug m-3 of PM0.5 was found to increase Deltah14 by ~200-250 m. Our conclusion is that it is indeed possible to detect the effects of anthropogenic aerosol on the convective clouds in southern Scandinavia despite modest aerosol masses. It also emphasizes the importance of including aerosol processes in climate-radiat ion models and in numerical weather prediction models.E Freud, J Strom, D Rosenfeld, P Tunved, E Swietlicki

Source: Tellus B, Vol. 0, No. 0. (0), pp. ???-???.
Aerosol anthropogenic component estimated from satellite data: Geophysical Research Letters, Vol. 32 (3 September 2005), L17804.Satelli te instruments do not measure the aerosol chemical composition needed to discriminate anthropogenic from natural aerosol components. However the ability of new satellite instruments to distinguish fine (submicron) from coarse (supermicron) aerosols over the oceans, serves as a signature of the anthropogenic component and can be used to estimate the fraction of anthropogenic aerosols with an uncertainty of ±30%. Application to two years of global MODIS data shows that 21 ± 7% of the aerosol optical thickness over the oceans has an anthropogenic origin. We found that three chemical transport models, used for global estimates of the aerosol forcing of climate, calculate a global average anthropogenic optical thickness over the ocean between 0.030 and 0.036, in line with the present MODIS assessment of 0.033. This increases our confidence in model assessments of the aerosol direct forcing of climate. The MODIS estimated aerosol forcing over cloud free oceans is therefore ?1.4 ± 0.4 W/m2.YJ Kaufman, O Boucher, D Tanré, M Chin, LA Remer, T Takemura

Source: Geophysical Research Letters, Vol. 32 (3 September 2005), L17804.

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