Submicrometer aerosol particles

We will illustrate the necessity of including solute from CCN by a simple calculation, recalling that pH = 5.6 is the supposed equilibrium value for water in contact with 300 ppm of CO2. (That calculation will appear later.) In clean, marine air, the concentration of submicrometer aerosol particles (by far the most numerous) is small, say 0.25 pg m . It is known from measurements that the molecular form is often NH4HSO4, and we assume it is all dissolved in 0.125 g/m of liquid water in a cloud - which is typical for fair-weather marine clouds. Thus the average concentration of sulfate ion [SO4 ], mol/L, is... 

Cloud Water and Precipitation Collectors. Several methods have been developed for collecting cloud water samples (24-26). Probably the device most commonly used in warm clouds is the slotted rod collector developed by the Atmospheric Science Research Center at the State University of New York (SUNY) at Albany. Commonly known as the ASRC collector (25), this collector consists of an array of rods constructed from Delrin (a form of nylon). Each rod is hollow and has a slot located at its forward stagnation line. The rod radius determines the collection efficiency as a function of particle size, the rods are sized to collect cloud droplets but not submicrometer aerosol particles, and the 50% cutoff is calculated to be at about 3 xm. 

Submicrometer aerosol particles, 0.001m s + a factor of three... 

Identification of organic components of ambient aerosols and estimation of the contributions of various pathways are of immediate interest for control strat es and could by achieved by using the existing monitoring networks so as to provide more information on aerosol chemical composition. In view of the adverse effects (e.g., on health and visibility) associated with submicrometer aerosols, an air quality standard for submicrometer particles might be more adequate than the present standard for total suspended particles. 

Calhoun, J. A., T. S. Bates, and R. J. Charlson, Sulfur Isotope Measurements of Submicrometer Sulfate Aerosol Particles over the Pacific Ocean, Geophys. Res. Lett, 18, 1877-1880 (1991). Capaldo, K. P., and S. N. Pandis, Dimethylsulfide Chemistry in the Remote Marine Atmosphere Evaluation and Sensitivity Analysis of Available Mechanisms, J. Geophys. Res., 102, 23251-23267 (1997). 

High efficiency denuders that concentrate atmospheric S02 were coupled to an ion chromatograph to yield detection limits on the order of 0.5 ppt (106). A newer approach has been introduced for the quantitative collection of aerosol particles to the submicrometer size (107). When interfaced to an inexpensive ion chromatograph for downstream analysis, the detection limit of the overall system for particulate sulfate, nitrite, and nitrate are 2.2,0.6, and 5.1 ng/m3, respectively, for an 8-min sample. A two-stage membrane sampling system coupled with an ion trap spectrometer has been utilized for the direct analysis of volatile compounds in air, with quantitation limits to low ppt levels (108). Toluene, carbon tetrachloride, tricholoroethane, and benzene were used in these studies. The measurement of nitrogen dioxide at ppb level in a liquid film droplet has been described (109) (see Air pollution). A number of elements in environmental samples have been determined by thermal ionization ms (Table 6). The detection limit for Pu was as low as 4 fg. 

For practical and fundamental reasons, there was a need to learn about the interactions of bodies much larger than the atoms and small molecules in gases. What interested people were systems we now call mesoscopic, with particles whose finite size Wilhelm Ostwald famously termed "the neglected dimension" 100-nm to 1 ()()-//m colloids suspended in solutions, submicrometer aerosols sprayed into air, surfaces and interfaces between condensed phases, films of nanometer to millimeter thickness. What to do ... 

Calhoun, J.A., Bates, T.S. Charlson, R.J. (1991) Sulfur isotope measurements of submicrometer sulfate aerosol particles over the Pacific Ocean. Geophysical Research Letters 18, 1877-1880. 

Figure 7-11 compares number density size distributions for soil and aerosol particles at several locations in North Africa to show the dominance of submicrometer particles in all populations studied. The maximum number density was found near 0.1 jtm radius in all samples, and particles as small as 0.02 (xm were detected. 

Table 8-6 presents an overview on the concentrations of the major ions in rainwater observed at various locations. Table 8-7 provides some information on cloud and fog waters. In maritime regions seasalt is an important source of cloud condensation nuclei, and it undergoes effective below-cloud scavenging as well. Sodium chloride accordingly contributes the largest fraction of all ions in rainwater. Some of the other ions usually are somewhat enriched in comparison with their relative abundances in seasalt. The enrichment of potassium and calcium is due to the admixture of aerosol from continental sources, and that of sulfate arises from the oxidation of gaseous precursors such as dimethyl sulfide of S02- This excess sulfate is associated almost exclusively with submicrometer-sized particles (see Section 7.5.1). 

An aerosol distribution can be described by the number concentrations of particles of various sizes as a function of time. Let us define Nk(t) as the number concentration (cm-3) of particles containing k monomers, where a monomer can be considered as a single molecule of the species representing the particle. Physically, the discrete distribution is appealing since it is based on the fundamental nature of the particles. However, a particle of size 1 pm contains on the order of 1010 monomers, and description of the submicrometer aerosol distribution requires a vector (N2, N-j,..., N10io) containing 1010 numbers. This makes the use of the discrete distribution impractical for most atmospheric aerosol applications. We will use it in the subsequent sections for instructional purposes and as an intermediate step toward development of the continuous general dynamic equation. 

Aerosol mass spectrometry (AMS) is used to monitor the chemical composition of particulate matter in the atmosphere. Commercial AMS instruments can provide size and chemical mass loading data on aerosol particles in real time [176]. Such instruments integrate sampling and MS analysis sub-systems. They can be installed permanently or used as components of mobile laboratories [176]. Both quadrupole and TOF AMS devices can provide quantitative data on the chemical composition of volatile/semi-volatile submicrometer aerosols [177]. Importantly, AMS can provide non-refractory aerosol mass... 

Because of the manifold partiele formation processes, the chemical composition of submicrometer eontinental aerosol particles is rather complex. The major constituents of these partieles are usually sulfates, nitrates, soot, organic compounds, and soil dust. Sulfates are derived mainly from anthropogenic SO2, ni-... 

Among the aerosols that gave performance cui ves with y > 1, the only obvious common characteristic was that a large fraction of each was composed of submicrometer particles. 

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