Air masses and aerosols chemical components in the free troposphere at the subtropical northeast atlantic region

The dynamics and the aerosol chemistry of the air masses reaching the free troposphere of the subtropical Northeast Atlantic region during the period 1995–98 have been studied. Seven days backward trajectories were calculated daily with HYSPLIT-4 model for Izaña Global Atmospheric Watch (GAW) Observatory (28.3°N 16.5°W, 2367 m a.s.l.). These back-trajectories were classified by means of a k-means clustering strategy. The daily air masses have been coded using 16 variables to detect the aerosol load of each one of them. Four clusters were found: Cluster 1, representative of Atlantic oceanic middle troposphere air masses, (OMT), has an average frequency of occurrence of 50.6%. Cluster 2, which includes air masses originated in the African continent (AfD), has been recorded in a 19.8% of time. Cluster 3 represents a mixture at least of two of the next sources: Europe, Africa and Ocean, (EAM), with a frequency of 12.7%. Finally, Cluster 4 includes air masses with a high load of maritime aerosols, (MaA), and it has been detected in a 16.9%. An analysis of four aerosol components: NO3−, NH4+, non-sea-salt-SO42−, and mineral dust and its relation with the origin and transport of the air masses have been done. The highest quantities of mineral dust and nss-SO42− are linked with African air masses with a mean value of 86.5 and 1.9 μg/m3 respectively. Whereas the highest levels of NO3−, 1.0 μg/m3, and NH4+, 0.4 μg/m3, were obtained for AfD and EAM. The lowest levels were associated with OMT and MaA air masses types: 12.7, 0.6, 0.2, and 0.5 μg/m3 for dust, NO3−, NH4+, and nss-SO42− in average for the four studied years. However, it is remarkable that the values of the median for dust are 2.2 and 3.5 μg/m3 in clusters MaA and OMT respectively. Using non-parametric statistical tests the distributions of concentrations in each cluster by year have been compared in order to detect similarities. The results show that the aerosol loads of OMT and MaA air masses are quite similar and the same occurs for AfD and EAM air masses. However, the correlation analysis between the levels of anions and ammonium evidenced important differences among the air mass types. In AfD air masses is clear a low correlation between levels of nss-SO42− and NH4+ (r2 = 0.08) suggesting that the sulfate speciation was dominated by sulfate species others than ammonium sulfate, such as calcium sulfate. CaSO4⋅2H2O (gypsum) is mainly present in the coarse mode, where the radiative effects of sulfate are less important that in the accumulative mode. For OMT air masses is noticeable an important increasing on the correlation between the levels of anions and those of NH4+ for the two last years of the study period (1997–1998, r2 = 0.61 –0.85%) with respect to the first ones (1995–1996, r2 = 0.25–0.49%), coinciding with the second strongest ENSO (El Niño Southern Oscillation) event recorded. This behavior indicates a change in the speciation of the aerosol component.

[1]  M. Willison,et al.  Seasonal variation in atmospheric aerosol concentration and composition at urban and rural sites in northern England , 1985 .

[2]  Itsushi Uno,et al.  Recent trends in precursor concentrations and oxidant distributions in the Tokyo and Osaka areas , 1996 .

[3]  J. Merrill.  Isentropic airflow probability analysis , 1994 .

[4]  J. Kiehl,et al.  The Relative Roles of Sulfate Aerosols and Greenhouse Gases in Climate Forcing , 1993, Science.

[5]  S. H. Melfi,et al.  Validation of the Saharan dust plume conceptual model using lidar, meteosat, and ECMWF Data , 1999 .

[6]  X. Querol,et al.  Seasonal evolution of suspended particles around a large coal-fired power station : Chemical characterization , 1998 .

[7]  H. Maring,et al.  Anthropogenic influences on the chemical and physical properties of aerosols in the Atlantic subtropical region during July 1994 and July 1995 , 1999 .

[8]  P. V. Velthoven,et al.  Observations of aerosols in the free troposphere and marine boundary layer of the subtropical Northeast Atlantic: Discussion of processes determining their size distribution , 1997 .

[9]  Nitrate in the accumulation mode: Data from measurement campaigns in eastern germany , 1996 .

[10]  D. Savoie.  Nitrate And Non-Sea-Salt Sulfate Aerosols Over Major Regions Of The World Ocean: Concentrations, Sources, And Fluxes , 1984 .

[11]  P. Formenti,et al.  Radiative properties and direct radiative effect of Saharan dust measured by the C-130 aircraft during SHADE: 1. Solar spectrum , 2003 .

[12]  K. E. Taylor,et al.  Response of the climate system to atmospheric aerosols and greenhouse gases , 1994, Nature.

[13]  M. Mcphaden,et al.  Genesis and evolution of the 1997-98 El Nino , 1999, Science.

[14]  Peter J. Rousseeuw,et al.  Finding Groups in Data: An Introduction to Cluster Analysis , 1990 .

[15]  A. Pszenny,et al.  Direct comparison of cellulose and quartz fiber filters for sampling submicrometer aerosols in the marine boundary layer , 1993 .

[16]  J. Coakley,et al.  Climate Forcing by Anthropogenic Aerosols , 1992, Science.

[17]  K. Shine.  Radiative Forcing of Climate Change , 2000 .

[18]  Sonia M. Kreidenweis,et al.  African dust aerosols as atmospheric ice nuclei , 2003 .

[19]  Roy M. Harrison,et al.  Size-differentiated composition of inorganic atmospheric aerosols of both marine and polluted continental origin , 1983 .

[20]  J. P. Díaz,et al.  Radiative properties of aerosols in Saharan dust outbreaks using ground‐based and satellite data: Applications to radiative forcing , 2001 .

[21]  I. Tegen,et al.  Relative importance of climate and land use in determining present and future global soil dust emission , 2004 .

[22]  Peter J. Lamb,et al.  African Droughts and Dust Transport to the Caribbean: Climate Change Implications , 2003, Science.

[23]  J. Penner,et al.  Introduction to special section: Outstanding problems in quantifying the radiative impacts of mineral dust , 2001 .

[24]  Frank McGovern,et al.  The 2nd Aerosol Characterization Experiment (ACE-2): general overview and main results , 2000 .

[25]  E. Mészáros,et al.  Concentration and dry deposition of atmospheric sulfur and nitrogen compounds in Hungary , 1984 .

[26]  Yinon Rudich,et al.  Desert dust suppressing precipitation: A possible desertification feedback loop , 2001, Proceedings of the National Academy of Sciences of the United States of America.

[27]  Paul J. DeMott,et al.  Saharan dust storms and indirect aerosol effects on clouds: CRYSTAL‐FACE results , 2003 .

[28]  S. Warren,et al.  Oceanic phytoplankton, atmospheric sulphur, cloud albedo and climate , 1987, Nature.

[29]  Jonathan P. Taylor,et al.  Radiative properties and direct effect of Saharan dust measured by the C‐130 aircraft during Saharan Dust Experiment (SHADE): 2. Terrestrial spectrum , 2003 .

[30]  Mar Viana,et al.  Influence of African dust on the levels of atmospheric particulates in the Canary Islands air quality network , 2002 .

[31]  C. Pio,et al.  Chlorine loss from marine aerosol in a coastal atmosphere , 1998 .

[32]  Xavier Querol,et al.  Characterisation of TSP and PM2.5 at Izaña and Sta. Cruz de Tenerife (Canary Islands, Spain) during a Saharan Dust Episode (July 2002) , 2005 .

[33]  A. Diaz,et al.  A five-year climatology of back-trajectories from the Izaña baseline station, Tenerife, Canary Islands , 1992 .

[34]  P. Chylek,et al.  Effect of absorbing aerosols on global radiation budget , 1995 .

[35]  I. Uno,et al.  Seasonal variation in atmospheric aerosols concentration covering northern Kyushu, Japan and Seoul, Korea , 1996 .

[36]  R. Harrison,et al.  Field intercomparison of filter pack and denuder sampling methods for reactive gaseous and particulate pollutants , 1990 .