Spatial and temporal distribution of atmospheric aerosols in the lowermost troposphere over the Amazonian tropical rainforest
We present measurements of aerosol physicochemical properties below 5 km altitude over the tropical rain forest and the marine boundary layer (MBL) obtained during the LBA-CLAIRE 1998 project. The MBL aerosol size distribution some 50-100 km of the coast of French Guyana and Suriname showed a bi-modal shape typical of aged and cloud processed aerosol. The average particle number density in the MBL was 383 cm(-3). The daytime mixed layer height over the rain forest for undisturbed conditions was estimated to be between 1200-1500 m. During the morning hours the height of the mixed layer increased by 144-180 m h(-1). The median daytime aerosol number density in the mixed layer increased from 450 cm(-3) in the morning to almost 800 cm(-3) in the late afternoon. The evolution of the aerosol size distribution in the daytime mixed layer over the rain forest showed two distinct patterns. Between dawn and midday, the Aitken mode particle concentrations increased, whereas later during the day, a sharp increase of the accumulation mode aerosol number densities was observed, resulting in a doubling of the morning accumulation mode concentrations from 150 cm(-3) to 300 cm(-3). Potential sources of the Aitken mode particles are discussed here including the rapid growth of ultrafine aerosol particles formed aloft and subsequently entrained into the mixed layer, as well as the contribution of emissions from the tropical vegetation to Aitken mode number densities. The observed increase of the accumulation mode aerosol number densities is attributed to the combined effect of: the direct emissions of primary biogenic particles from the rain forest and aerosol in-cloud processing by shallow convective clouds. Based on the similarities among the number densities, the size distributions and the composition of the aerosol in the MBL and the nocturnal residual layer we propose that the air originating in the MBL is transported above the nocturnal mixed layer up to 300-400 km inland over the rain forest by night without significant processing.
Surface tension effects of humic-like substances in the aqueous extract of tropospheric fine aerosol
Organic aerosol constituents can influence the surface tension of nucleating cloud droplets and thereby modify the critical supersaturation necessary to activate aerosol particles. Model calculations, based on experiments carried out with different surrogates, led to contradictory conclusions on the effect of organic components on activation, indicating that the results depend very much on the surrogate selected. In order to reduce this uncertainty surface tension measurements were performed on real atmospheric aerosol components. Humic-like substances (HULIS) that accounted for 60% of the water-soluble organic carbon present in rural aerosol were isolated from 32 samples covering different seasons. The isolated organic matter present in a concentration of about 1 g L-1, decreased the surface tension of the aqueous solutions by 25 - 42% as compared to pure water. This effect was further enhanced when humic-like substances were mixed with ammonium sulphate. In order to support model calculations Szyskowski functions were fitted to the data to formulate the surface tension effect as a function of concentration. Finally, natural humic substances ( fulvic and humic acids) were investigated under the same conditions. The experiments revealed that the surface tension decreasing effect of atmospheric humic-like substances differed from that of the studied terrestrial and aquatic humic substances: the latter substances decreased the surface tension of the aqueous solution to a lesser extent ( 7 - 23%) than atmospheric HULIS. This deviation can be explained with the different composition ( e. g. the ratio of aromatic to aliphatic moieties) of the substances investigated.
Aerosol characteristics of air masses in northern Europe: Influences of location, transport, sinks, and sources
Synoptic-scale air masses at different stations were classified following a definition based on Berliner Wetterkarte. This air mass classification has been related to 1 year of aerosol number size distributions measurements performed at four different stations extending from Aspvreten in Sweden (58.8 degrees N) to Pallas in northern Finland (68 degrees N). The air mass classification describes both class of air mass, based on the origin of the air mass, and character of air in terms of marine, mixed, and continental air masses. The aerosol size distribution properties were evaluated in relation to the air masses. Emphasis was put on the differences between marine, mixed, and continental character air masses. It is shown that continental air masses exceed marine and mixed character air masses both in number and mass concentration. Different classes of air masses were further associated with different aerosol size distribution properties. It is also shown that although serving as a somewhat good qualifier for the aerosol at individual stations, the air mass classification cannot be used to estimate the aerosol burden over large geographical areas. Instead, a sharp gradient was shown to exist between different stations, although aerosol properties were observed in equal air masses according to the definition by Berliner Wetterkarte. This gradient manifests as a south-northerly decrease in aerosol total number and volume, indicating that the aerosol properties including the aerosol size distribution are less conservative than the thermodynamic properties (e.g., pseudo-potential temperature and humidity profiles) that characterize the different air masses. Further, using a pseudo-Lagrangian approach, the aerosol turnover time was estimated for different sized aerosols in air moving from south to north (i.e., depletion of aerosols in air arriving from the continent). Turnover time of Aitken particles was found to be in the range of 1-2 days, while accumulation mode turnover time was estimated to be in the order of 2-3 days.
Comparative health impact assessment of local and regional particulate air pollutants in Scandinavia
The ongoing program Clean Air for Europe (CAFE) is an initiative from the EU Commission to establish a coordinated effort to reach better air quality in the EU. The focus is on particulate matter as it has been shown to have large impact on human health. CAFE requested that WHO make a review of the latest findings on air pollutants and health to facilitate assessments of the different air pollutants and their health effects. The WHO review project on health aspects of air pollution in Europe confirmed that exposure to particulate matter (PM), despite the lower levels we face today, still poses a significant risk to human health. Using the recommended uniform risk coefficients for health impact assessment of PM, regardless of sources, premature mortality related to long-range transported anthropogenic particles has been estimated to be about 3500 deaths per year for the Swedish population, corresponding to a reduction in life expectancy of up to about seven months. The influence of local sources is more difficult to estimate due to large uncertainties when linking available risk coefficients to exposure data, but the estimates indicate about 1800 deaths brought forward each year with a life expectancy reduction of about 2-3 months. However, some sectors of the population are exposed to quite high locally induced concentrations and are likely to suffer excessive reductions in life expectancy. Since the literature increasingly supports assumptions that combustion related particles are associated with higher relative risks, further studies may shift the focus for abatement strategies. CAFE sets out to establish a general cost effective abatement strategy for atmospheric particles. Our results, based on studies of background exposure, show that long-range transported sulfate rich particles dominate the health effects of PM in Sweden. The same results would be found for the whole of Scandinavia and many countries influenced by transboundary air pollution. However, several health studies, including epidemiological studies with a finer spatial resolution, indicate that engine exhaust particles are more damaging to health than other particles. These contradictory findings must be understood and source specific risk estimates have to be established by expert bodies, otherwise it will not be possible to find the most cost effective abatement strategy for Europe. We are not happy with today's situation where every strategy to reduce PM concentrations is estimated to have the same impact per unit change in the mass concentration. Obviously there is a striking need to introduce more specific exposure variables and a higher geographical resolution in epidemiology as well as in health impact assessments.
An investigation of processes controlling the evolution of the boundary layer aerosol size distribution properties at the Swedish background station Aspvreten
Aerosol size distributions have been measured at the Swedish background station Aspvreten (58.8degrees N, 17.4degrees E). Different states of the aerosol were determined using a novel application of cluster analysis. The analysis resulted in eight different clusters capturing different stages of the aerosol lifecycle. The atmospheric aerosol size distributions were interpreted as belonging to fresh, intermediate and aged types of size distribution. With aid of back trajectory analysis we present statistics concerning the relation of source area and different meteorological parameters using a non-Lagrangian approach. Source area is argued to be important although not sufficient to describe the observed aerosol properties. Especially processing by clouds and precipitation is shown to be crucial for the evolution of the aerosol size distribution. As much as 60% of the observed size distributions present features that are likely to be related to cloud processes or wet deposition. The lifetime properties of different sized aerosols are discussed by means of measured variability of the aerosol size distribution. Processing by clouds and precipitation is shown to be especially crucial in the size range 100 nm and larger. This indicates an approximate limit for activation in clouds to 100 nm in this type of environment. The aerosol lifecycle is discussed. Size distributions indicating signs of recent new particle formation ( similar to 30% of the observed size distributions) represent the first stage in the lifecycle. Aging of the aerosol size distribution may follow two branches: either growth by condensation and coagulation or processing by non-precipitating clouds. In both cases mass is accumulated. Wet removal is the main process capable of removing aerosol mass. Wet deposition is argued to be an important mechanism in reaching a state where nucleation may occur (i.e. sufficiently low aerosol surface area) in environments similar to the one studied.
Speciation and origin of PM10 and PM2.5 in selected European cities
PM characteristics of seven selected regions within the European Union (EU) were analysed and compared. Results of levels and speciation studies of PM10 and PM2.5 (with at least one year of data coverage from 1998 to 2002) at regional, urban background and kerbside sites were assessed. Based on the examples selected, PM10 levels (annual mean) ranged from 19 to 24 mug m(-3) at regional background sites, from 28 to 42 mug m(-3) at urban background, and from 37 to 53 mug m(-3) at kerbside sites. PM2.5 levels varied from 8 to 20 mug m(-3) at regional background sites, 20 to 30 mug m(-3) at urban background and 25 to 40 mug m(-3) at kerbside sites. The ratio PM2.5/PM10 is highly dependent on the type of site and varied widely between different EU regions. Source apportionment results showed that, on an annual average, the natural contribution (mineral and marine) at EU regional sites was in the range of 4-8 mug m(-3) in PM10 decreasing in PM2.5, but contributions up to 19 mug m(-3) were reported for specific locations. At urban sites, carbonaceous aerosols and secondary inorganic compounds accounted for a major fraction of PM10, and especially of the PM2.5 mass. Quantitative data on the contributions of the regional background, city background and local traffic to the mean annual levels of PM10, PM2.5 and major components were supplied. Climatic differences, long-range transport processes and winter traffic peculiarities (the latter in northern countries) contributed to the increase of PM10 and PM2.5 masses. At kerbside sites, an important dust contribution to PM2.5 is highlighted. (C) 2004 Elsevier Ltd. All rights reserved.
How important is nucleation in regional/global modelling?
Modelling nucleation and subsequent growth is a difficult task especially in global and regional models. Therefore it is important to know under which conditions the freshly nucleated particles are able to survive the coagulation barrier and grow to the Aitken mode sizes and further to cloud condensation nuclei thus affecting climate. By using a sectional aerosol dynamics model AEROFOR at various conditions, the regions of parameter space in which nucleation plays an important role are identified and the contour plots are presented. Sulphuric acid is assumed to participate in both nucleation and condensation processes, additionally there is present some other condensable vapour. Our simulations show that nucleation is almost always an important process in the atmosphere excluding the cases when condensable vapour concentration is not high enough so that the nucleated particles have time to coagulate away before reaching the Aitken mode sizes. Thus it is well established to include nucleation in regional/global models.
A European aerosol phenomenology-1: physical characteristics of particulate matter at kerbside, urban, rural and background sites in Europe
This paper synthesizes data on aerosol (particulate matter, PM) physical characteristics, which were obtained in European aerosol research activities at free- troposphere, natural, rural, near-city, urban, and kerbside sites over the past decade. It covers only two sites in the semi-arid Mediterranean area, and lacks data from Eastern Europe. The data include PM10 and/or PM2.5 mass concentrations, and aerosol particle size distributions. Such data sets are more comprehensive than those currently provided by air quality monitoring networks (e.g. EMEP, EUROAIRNET). Data available from 31 sites in Europe (called "The Network") were reviewed. They were processed and plotted to allow comparisons in spite of differences in the sampling and analytical techniques used in various studies. A number of conclusions are drawn as follows: Background annual average PM10 and PM2.5 mass concentrations for continental Europe are 7.0+/-4.1 and 4.8+/-2.4 mug m(-3), respectively. The EU 2005 annual average PM10 standard of 40 mug m(-3) is exceeded at a few sites in The Network. At all near city, urban and kerbside sites, the EU 2010 annual average PM10 standard of 20 mug m(-3), as well as the US-EPA annual average PM2.5 standard of 15 mug m(-3) are exceeded. In certain regions, PM10 and PM2.5 in cities are strongly affected by the regional aerosol background. There is no "universal" (i.e. valid for all sites) ratio between PM2.5 and PM10 mass concentrations, although fairly constant ratios do exist at individual sites. There is no universal correlation between PM mass concentration on the one hand, and total particle number concentration on the other hand, although a 'baseline' ratio between number and mass is found for sites not affected by local emissions. This paper is the first part of two companion papers of which the second part describes chemical characteristics. (C) 2004 Elsevier Ltd. All rights reserved.
Spatial and temporal distribution of the atmospheric aerosols in the lowermost troposphere over the Amazonian tropical rain forest: Do the primary biogenic aerosol emissions control the CCN population?
One year boundary layer aerosol size distribution data from five nordic background stations
Size distribution measurements performed at five different stations have been investigated during a one-year period between 01 June 2000 and 31 May 2001 with focus on diurnal, seasonal and geographical differences of size distribution properties. The stations involved cover a large geographical area ranging from the Finnish Lapland (67degrees N) down to southern Sweden (56degrees N) in the order Varrio, Pallas, Hyytiala, Aspvreten and Vavihill. The shape of the size distribution is typically bimodal during winter with a larger fraction of accumulation mode particles compared to the other seasons. Highest Aitken mode concentration is found during summer and spring during the year of study. The maximum of nucleation events occur during the spring months at all stations. Nucleation events occur during other months as well, although not as frequently. Large differences were found between different categories of stations. Northerly located stations such as Pallas and Varrio presented well-separated Aitken and accumulation modes, while the two modes often overlap significantly at the two southernmost stations Vavihill and Aspvreten. A method to cluster trajectories was used to analyse the impact of long-range transport on the observed aerosol properties. Clusters of trajectories arriving from the continent were clearly associated with size distributions shifted towards the accumulation mode. This feature was more pronounced the further south the station was located. Marine- or Arctic-type clusters were associated with large variability in the nuclei size ranges. A quasi-lagrangian approach was used to investigate transport related changes in the aerosol properties. Typically, an increase in especially Aitken mode concentrations was observed when advection from the north occurs, i.e. allowing more continental influence on the aerosol when comparing the different measurement sites. When trajectory clusters arrive to the stations from SW, a gradual decrease in number concentration is experienced in all modes as latitude of measurement site increases.