Atmospheric Aerosols: Size Distributions and Composition

Transcription

1 2.1 Atmospheric Aerosols: Size Distributions and Composition The Table below (from Seinfeld and Pandis) lists typical properties of different types of atmospheric model aerosols. Note that the characteristics are expressed in terms of three modes. The Aitken and accumulation modes define the fine mode. Sometimes there are three modes in the fine fraction: very small particles (diam < 0.01 m) may be present. These are freshly formed and thus sometimes called the ultrafine or nucleation mode. Coarse particles are those generally larger than about 1 micron (often, the cut is at an aerodynamic diameter of 2.5 m). Sometimes they are called giant particles, or sometimes that term is reserved for particles in yet a larger coarse mode. Notice that Mode I in the Table does not necessarily correspond to Aitken particles, and similarly for the other modes. FINE COARSE

2 2.2 Table 7.5 shows total properties of several key aerosol types. Marine number concentrations tend to be the lowest, and these aerosols associated with cleanest conditions. Urban aerosols are of course the most polluted with the highest number concentrations. Remote continental aerosols have the lowest mass concentrations. Annually-averaged PM10 mass concentrations for IMPROVE sites (rural and remote continental). Notice in the mountain West, values range from ~5 9 g m -3, whereas in rural National Parks in the eastern U.S., the annual values range ~ g m -3. These compare well with the values in Table 7.5.

3 2.3 The sources of fine and coarse particles can be quite different, and thus their composition tends to be quite different as well, as exemplified in the Table 7.7 from Seinfeld and Pandis: Can also have significant number concentrations of fine-mode sea salt Sea salt Let s look at size distributions for various broad aerosol types in a little more detail.

4 2.4 Marine Aerosols Marine aerosols contain not only sea salt, but significant organic content as well. Organic matter comes from marine-derived sterols, fatty alcohols, and fatty acids. Further, some ocean regions are a source of dimethylsulfide (DMS, produced by phytoplankton). This DMS is oxidized in part to sulfur dioxide (SO 2 ), which is further oxidized in the gas or aqueous phase to sulfate, SO Sulfate is exclusively in the particulate phase (i.e., it is too nonvolatile to be in the gas phase). Sulfate measured in marine aerosols can be classified into that present in sea water already, and non-sea-salt sulfate that comes from oxidation of S gases. Sea water is 55% by weight Cl, 30.6% Na, 7.68% sulfate, 3.69% Mg, 1.16% Ca and 1.1% K. There are many other trace elements present as well. Sea salt aerosols are produced by bubble-bursting processes, which can produce significant number concentrations of fine particles, although most sea salt mass is contained in coarse particles (see Figs 7.15 and 7.16 from Seinfeld and Pandis). number volume

5 2.5 Notice the minimum in the aerosol number distribution, circled in red in Fig The minimum occurs at diameters ~ 0.1 m, and is frequently seen in clean marine boundary-layer air masses, especially those that have been cloud-topped (e.g., in regions with persistent stratus or stratocumulus decks). The minimum (sometimes called the Hoppel minimum after William Hoppel who first described it thoroughly) is thought to arise from cloud processing of the aerosol. The panel below, from the paper by Jensen et al., JGR, 1996, shows shipboard-measured aerosol size distributions during a transect between polluted and clean marine air in the eastern Atlantic Ocean off the Azores (a region characterized by persistent stratus, often nonprecipitating or drizzling slowly). Notice the polluted distributions are unimodal with higher number concentrations, while the characteristic minimum appears for the cleaner conditions. The idea is that as the air mass enters cloud, particles larger than about 0.08 m are activated to cloud droplets. The droplets take up SO 2 from the gas phase, along with oxidants such as ozone and hydrogen peroxide, and quickly oxidize the SO 2 to sulfate. When the drop evaporates, this sulfate is left behind with the original material that was in the particle the drop formed on, thereby adding to the mass 9and size) of that particle. After repeated cloud cycles, enough mass has been added to grow the CCN-active particles above 0.1 m, and create a hole near 0.09 m. Small particles may be replenished by nucleation of fresh particles from the gas phase. This probably involves formation of tiny new sulfuric acid-water droplets, and growth to Aitken-mode sizes by condensation of sulfate, ammonia, and/or organic vapors. Nucleation can only happen when most of the existing particle surface area has been removed

6 2.6 The accumulation mode is drizzled out when CCN-active particles are in a cloud that precipitates. Similar features as were seen in clean marine air in the Atlantic were observed in the Southern Ocean by Brechtel et al. (JGR, 1998): Nucleation events were periodically observed in this study, for very clean air masses. Also, nucleation was observed aloft by aircraft downwind of penguin colonies on Macquarie Island. The hypothesis is that ammonia emissions from the colonies enhanced the possibility of nucleation and/or growth of sulfuric acid particles.

7 2.7 The following figures, from a modeling study of aerosol dynamics in the marine boundary layer by Fitzgerald et al. (JGR, 1998), shows the effects of individual processes on the size distribution. settling Nucleation and sea salt sources Condensation of sulfuric acid coagulation

8 2.8 Urban aerosols Urban aerosols are mixtures of primary (direct) emissions from industries, transportation, power generation, and natural sources, and secondary material formed by gas-to-particle conversion processes. Thus the distribution is highly variable, depending on distance from sources. Most of the number concentration is in particles smaller than 0.1 µm. The figure (from Seinfeld and Pandis) shows a schematic distribution based on the parameters in Table 7.3. Free tropospheric aerosols (mid- to upper-troposphere) Some longer-term measurements have been made from elevated sites; otherwise data come from aircraft. Nucleation events have been observed near cloud edges, above clouds, and in very clean and cold air masses. It was earlier thought that free-tropospheric aerosols were primarily sulfate, because SO 2 would be lifted by convection and slowly convert to sulfate in the free troposphere. While sulfate is certainly found in the aerosol, at least one observational study (TARFOX, off the East Coast of the US) found the ratio of organic carbon to sulfate in aerosols increased with altitude (although absolute mass concentrations decreased). A recent study (Heald et al., GRL, 2005) showed that the mass of organic carbon aerosol in the free troposphere over the Pacific was much larger than could be explained by current models that include primary organic particle emissions as well as conversion of gaseous organic emissions to particulate matter.

9 2.9 Finally, convective storms in arid regions lift huge amounts of dust into the middle troposphere, where it can undergo long-range transport. Our group has observed Saharan dust in Big Bend National Park, Texas, and a few years ago hazy skies over CO could be traced to transported Asian dust. Dust from the Gobi desert and loess areas moving over the Pacific Ocean

10 2.10 Glen Canyon National Recreational Area April 16, On February 26th,2000, SeaWIFS returned this dramatic close-up view of a vast, developing cloud of Saharan desert dust blowing from northwest Africa (lower right) a thousand miles or more out over the Atlantic Ocean.

11 2.11 Rural and remote aerosols These particle types are characterized by primary particles of natural origin (sea salt, dust, pollens, plant waxes) along with variable influence from anthropogenic sources (compare eastern and western US in map below). Both types have sources from secondary aerosol formation (oxidation of precursor gases: SO 2, and organic emissions that may be from smoke, from anthropogenic sources, or from vegetation). The relative mass concentrations in the fine and coarse modes reflect the nature of the sources dominant at a particular site. The Figure below, from Seinfeld and Pandis, shows a representative remote continental aerosol. In the Figure below, notice how important carbon is to all urban aerosols. Also notice that sulfate is a much bigger fraction of the PM2.5 mass in the eastern U.S., because most strong SO 2 sources (coal-burning power plants) are located there. Crustal material is an important mass component in rural Texas and the CO plateau, and nitrate is common but not a major fraction except in California.

12 2.12 Big Bend FINE ONLY; MISSING SEA -SALT; CARBON & SOIL MASSES UNCERTAIN

13 2.13 Aerosols in Big Bend, Texas We made measurements of particle composition and size distributions in July-October 1999 at Big Bend National Park, Texas. The data below are form Hand et al. (Atmos. Env, 2002): High accumulation mode mass Larger geometric volume mean diameter ACCUMULATION MODE STATISTICS Smaller standard deviation

14 2.14 Notice all of these data are for DRY distributions. This is because particles can contain quite a bit of water but it is variable, depending on RH and on particle composition. Larger geometric volume mean diameter with time (increasing local influence) COARSE MODE STATISTICS Standard deviation more variable, larger for larger mass conc

15 2.15 How are aerosol size distributions shaped? In the preceding we already saw examples of how particular processes shape the atmospheric aerosol size distribution. Hinds has summarized these in the figure below: In-cloud conversion Notice that many of these processes (e.g., coagulation) MIX TOGETHER particles of different types that is, they serve to put different chemical species together into the same particle. Indeed, recent single-particle measurements show that very few atmospheric particles are pure - that is, consist of a single species such as ammonium sulfate. Aged particles especially tend to be mixtures. Also, notice the gases and vapors that are listed for gas-to-particle conversion. SO 2 is mostly from burning of S-containing fuel (coal), although it also has some natural sources. Ammonia is released from car exhaust, fertilizer, feedlots. NOx has combustion sources (transportation and power production). HC cover a huge range of compounds, from anthropogenic to biogenic. In-cloud conversion in the past has meant primarily sulfate production from SO 2. New work points to the role of clouds in potentially oxidizing HCs to non-volatile species. Very recent observations show oxalic acid is produced in cloud from a varierty of gaseous precursors and then can remain in the particle phase when the cloud evaporates. How fast are each of these processes? How do they change not only size, but the geometric standard deviation of the size distribution?

16 2.16 Coagulation In Brownian (thermal) coagulation, the Brownian (diffusive) metion of the particles brings them into contact. The mass concentration of the aerosol is unchanged, but number concentration is reduced and the size distribution narrows. For simple monodisperse coagulation, dn dt = KN where K is the coagulation coefficient. Because the rate is proportional to N 2, it is fast at high N and then slows as coagulation reduces N. The coagulation coefficient is smallest for like-sized particles, and for this case is generally not larger (at standard conditions) than about 5 x cm 3 /s. This value was used in the lifetime estimates shown in Fig 12.3 (Hinds). 2 (Seinfeld and Pandis)

17 2.17 For any chosen maximum number concentration, this estimate tells us the longest that particle population could have existed. For example, if a number concentration of 10 6 were observed, those particles must have been formed an hour ago or less, since if they had aged longer their number concentrations would have been reduced to a lower value by simple monodisperse coagulation. Table 12.4 gives average coagulation coefficients for polydisperse (lognormal) size distributions. Notice the highest values are for distributions with large geometric standard deviation (GSD) and the smallest median diameters (see Fig 12.5). If the distribution is initially broad, coagulation makes it narrower. However, if it s initially monodisperse, coagulation makes it polydisperse. Which wins out? Friedlander (1977) showed that eventually a stable size distribution forms, with GSD ~1.5 (the selfpreserving size distribution).

18 2.18 (Hinds) Gravitational settling Gravitational settling tends to be important only for particles larger than ~10 microns. For a 100-m (10 4 cm) thick marine boundary layer, settling could occur on time scales of about an hour (see figure below, from Seinfeld and Pandis). The process is not important for accumulation-mode aerosols.

19 2.19 Condensation For now, consider only relatively large particles, for which we can write the simplified growth law: dd dt p = 4D M ( p p ) v RT D p p s If the particle surface is a perfect sink for vapor, p s =0. The equation says that the rate of growth of the diameter is inversely proportional to particle diameter. This means that small particles change their size very rapidly, but large particles do not seem to grow much at all in diameter space. The figure below, from Seinfeld and Pandis, shows the change in aerosol mass distribution with condensation. How fast is this process? It is a strong function of how much condensing vapor there is. Generally, for growth in the free troposphere, the rate is probably small. A recent publication gives an estimate of nm per hour for growth of freshly nucleated particles; the rate is much smaller for larger particles. For secondary organic aerosol forming from highly reactive biogenic precursors, a lot of particle growth can occur on short time scales. Observations often show a particle burst in the morning, with the mode grown into the Aitken size range during the day. In-cloud oxidation is very fast, so a fog can grow particles substantially in a few hours (the above rate expression does not apply).

20 2.20

21 2.21 Nucleation and growth Kulmala et al. (2005; see citation below) point out that Nucleation, the formation of ultrafine particles detected at a few nm, and subsequent growth to 100 nm in 1 2 days, has been observed frequently in the continental boundary layer. Such observations span from northern-most sub-arctic Lapland (Vehkam aki et al., 2004), over the remote boreal forest (M akel a et al., 1997; Kulmala et al., 1998, 2001b) and suburban Helsinki (V akev a et al., 2000), to industrialised agricultural regions in Germany (Birmili et al., 2001) and also to coastal environments around Europe (O Dowd et al., 1999). The figure below shows the characteristic banana in the time-dependent size distributions, where a morning nucleation event grows into the background size distribution on the time scale of about a day. Growth rates were estimated to be nm h -1 from the data sets they examined. On the growth of nucleation mode particles: source rates of condensable vapor in polluted and clean environments, M. Kulmala, T. Pet aj a, P. M onkk onen, I. K. Koponen, M. Dal Maso, P. P. Aalto, K. E. J. Lehtinen, and V.-M. Kerminen, Atmos. Chem. Phys., 5, , 2005.