Free Tropospheric Aerosols

Typical remote continental aerosol number, surface, and volume distributions. 

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Transport on Free Tropospheric Aerosols over the Central USA in Springtime, Geophys. Res. Lett., 25, 1367-1370 (1998). 

Raes, F., Entrainment of Free Tropospheric Aerosols as a Regulating Mechanism for Cloud Condensation Nuclei in the Remote Marine Boundary Layer, J. Geophys. Res., 100, 2893-2903 (1995). 

Raes, F., Entrainment of free tropospheric aerosols as a regulating mechanism for cloud condensation... 

The first measurements of Pb isotopes in Greenland snow were reported in 1993 (45). The samples were taken from a 10.7 m long, 10.5 cm diameter, snow core drilled at Summit, central Greenland, in 1989 (72°35 N, 37°38 W, mean annual accumulation rate 21.5 g cm year ). Cores were drilled with a polycarbonate auger to minimise the Pb contamination. The core contained snow deposited between the years 1967 and 1988. The 3.23 km elevation of the site provided representative samples of free tropospheric aerosols. An expanded data set and a more complete description and interpretation of these data were later reported by Rosman et al. (46). The latter included samples from the upper part of a 70 m snow core including snow deposited between 1960 and 1974. Data on all four Pb isotopes were given for these samples ( Pb/ Pb, ° Pb/ ° Pb and Pb/ °" Pb). Aliquots of these samples were also analysed for heavy metals by Boutron et al. (47) who showed there was a reduction in the Pb concentration in Greenland snow after 1970, which they attributed mainly to the reduction in the use of alkyl-leaded petrol. 

Background free tropospheric aerosol is found in the mid- and upper troposphere above the clouds. The modes in the number distribution correspond to mean diameters of 0.01 and 0.25 (Jaenicke 1993) (Figure 8.19). The middle troposphere spectra typically indicate... 

Kent G. S., McCormick, M. S., and Shaffner, S. C. (1991) Global optical climatology of the free tropospheric aerosol from 1.0 pm satellite occultation measurements, J. Geophys. Res., 96, 5249-5267. 

Froyd KD, Murphy SM, Murphy DM, de (jouw JA, Eddingsaas NC, Wennberg PO (2010) Contribution of isoprene-derived organosulfates to free tropospheric aerosol mass. Proc Natl Acad Sci USA 107 21360-21365... 

Aerosol surface area is likely to be variable even within a remote marine air mass. Previous MBL aerosol studies describe changes in aerosol concentration and composition due to entrainment from the free troposphere (Bates et al., 1998, 2001 Covert et al., 1998). Raes et al. (1997) found an observable link between vertical transport patterns and aerosol variability in the MBL specifically in the Aitken mode (<0.2/u.m). Hence entrainment of aerosol from the free troposphere appears to occur frequently, even in remote MBL air masses. In addition, aerosols have the capacity to travel great distances in the free troposphere, before being entrained into the MBL. 

Raes, F., VanDingenen, R., Cuevas, E., VanVelthoven, P. F. J., and Prospero, J. Observations of aerosols in the free troposphere and marine boundary layer of the subtropical Northeast Atlantic Discussion of processes determining their size distribution, J. Geophys. Res.-A., 102, 21 315-21 328,1997. 

Figure 3.25 shows the results of one set of calculations of the effects of aerosol particles whose properties were judged to be characteristic of continental or urban situations, respectively, on the transmission of UV and visible radiation to the earth s surface (Erlick and Frederick, 1998). The ratio of the transmission with particles to that without is plotted in two wavelength regions, one in the UV and one in the visible. Two different relative humidity scenarios are shown. The average summer relative humidity was 70% RH in the boundary layer and 20% RH in the free troposphere. The high relative humidity case assumes 90% RH in the boundary layer and 30% in the free troposphere. (The RH in the stratosphere was taken to be 0% in both cases see Chapter 12.)... 

Krotkov, N. A., P. K. Bhartia, J. R. Herman, V. Fioletov, and J. Kerr, Satellite Estimation of Spectral Surface UV Irradiance in the Presence of Tropospheric Aerosols. 1. Cloud-Free Case, J. Geophys. Res., 103, 8779-8793 (1998). 

Raes, F., R. Van Dingenen, E. Cuevas, P. F. J. Van Velthoven, and J. M. Prospero, Observations of Aerosols in the Free Troposphere and Marine Boundary Layer of the Subtropical Northeast Atlantic Discussion of Processes Determining Their Size Distribution, J. Geophys. Res., 102, 21315-21328 (1997). 

Krotkov, N.A., Bhartia P.K, Herman J.R., Fioletov V. And Kerr J., Satellite estimation of spectral surface UV irradiance in the presence of tropospheric aerosols 1. Cloud free case, J. Geopys. Res., 103,8779-... 

Based on the use of the NARCM regional model of climate and formation of the field of concentration and size distribution of aerosol, Munoz-Alpizar et al. (2003) calculated the transport, diffusion, and deposition of sulfate aerosol using an approximate model of the processes of sulfur oxidation that does not take the chemical processes in urban air into account. However, the 3-D evolution of microphysical and optical characteristics of aerosol was discussed in detail. The results of numerical modeling were compared with observational data near the surface and in the free troposphere carried out on March 2, 4, and 14, 1997. Analysis of the time series of observations at the airport in Mexico City revealed low values of visibility in the morning due to the small thickness of the ABL, and the subsequent improvement of visibility as ABL thickness increased. Estimates of visibility revealed its strong dependence on wind direction and aerosol size distribution. Calculations have shown that increased detail in size distribution presentation promotes a more reliable simulation of the coagulation processes and a more realistic size distribution characterized by the presence of the accumulation mode of aerosol with the size of particles 0.3 pm. In this case, the results of visibility calculations become more reliable, too. 

All sulfur compounds showed relatively constant profiles throughout the lower fright levels which is consistent with the neutral conditions in the mixed layer. As compared to the data obtained during the ship cruise (Tables I and II) the concentrations in the lowest flight level (30 m) were about 2-3 times lower for DMS, nearly the same for SO2, and about 3 times higher for MSA and nss-SC>42 The fraction of nss-SC>42 to total sulfate at this level was 18%, similar to the results from the ship cruise. The relatively higher concentrations observed for MSA and nss-SC>42 indicate a significant accumulation of aerosol particles in the mixed layer due to the postfrontal inversion between the mixed layer and the free troposphere. 

The night-time replenishment of ozone is caused by entrainment of ozone from the free troposphere into the boundary layer. The overnight loss of peroxide is due to deposition over the sea surface (and heterogeneous loss to the aerosol surface), as peroxide has a significant physical loss rate, in contrast to ozone which does not. Therefore, the daytime anti-correlation of ozone and peroxide is indicative of the net photochemical destruction of ozone. 

Removal of free ions occurs by two mechanisms ion-ion recombination (essentially saturated ternary ion-ion recombination effective binary coefficient a = 2 10 cm s ) and ion-attachment to aerosol particles. The latter process leads to so-called large ions which are, in fact, electrically-charged aerosols rather than ions in a strictly physical sense [60]. Usually, ion-attachment is the most important sink for free ions throughout the troposphere as the tropospheric aerosol content is relatively large (Fig. 2). In this respect, the tropospheric ionization-deionization balance differs greatly from the stratospheric one. 

Upon drying, sea salt particles remain in a metastable highly concentrated solution state below their deliquescence relative humidity of —75%. Only when they reach their crystallization (or effluescence) point, which is —45% relative humidity for NaCl, will they assume the crystalline form. This hysteresis effect is well documented by laboratory experiments (e.g., Shaw and Rood, 1990 Tang, 1997 Pmppacher and Klett, 1997 Lee and Hsu, 2000) and implies that, in the MBL, sea salt aerosol will usually be present in an aqueous form. Only in very dry marine regions and in the free troposphere, where the relative humidity is less than 45%, these particles can be expected to be dry. Even then a semiliquid layer can be present on the surface which makes sruface reactions easier. 

Aneja VP, Murthy AB, Battye W, et al. 1998. Analysis of ammonia and aerosol concentrations and deposition near the free troposphere at Mt. Mtichell, NC, USA. Atmos Environ 32(3) 353-358. 

Galasyn, J. F.. Tschudy, K. L., and Huebert, B. J., Seasonal and diurnal variability of nitric acid vapor and ionic aerosol species in the remote free troposphere at Mauna Loa, Hawaii. J. Geophys. Res. 92, 3105 (1987). 

Fig. 7-1. Left Idealized particle size distributions for the rural continental and the maritime aerosols. The distribution of sea-salt particles that contribute to the maritime aerosol is shown separately. The transition from the rural to the urban aerosol is indicated. Right Determination of remote tropospheric aerosol size distribution by a combination of instrumental techniques. [ Single-stage and free-wing impactors, O—O set of five double-stage impactors singleparticle optical scattering analyzer these data were obtained at the observatory lzana, Tenerife,...

Table 10-12 includes concentrations of particulate sulfate in the free troposphere at elevations above 4 km. Although the data come from various parts of the world, the concentrations are fairly uniform, ranging from 20 to 130 ng S/m3 STP. Thus, sulfate is an important constituent of the tropospheric background aerosol. If, as in Section 7.6, we assume a mixing ratio for the background aerosol of 1 xg/m3 STP, sulfate is found to contribute roughly 25% by mass. 

Fig. 10-9. Flux diagram for sulfur in the unperturbed marine atmosphere. Fluxes are given in units of p.gS/m2day. Numbers in boxes indicate column densities in units of p.gS/m2. DMS, Dimethyl sulfide MSA, methane sulfonic acid (associated with the aerosol). The mixing ratio of S02 is 60 ng S/m3, independent of altitude. The mixing ratio of SOis 280 ng S/m3 in the boundary layer and 80 ng S/m3 in the free troposphere. Contrary to the model of Kritz (1982), the fluxes are confined to the boundary layer. There exists no significant net flux into or out of the free troposphere. The dry deposition velocity for S02 is 5mm/s.

These vertical profiles are rough representations of long-term averages. Significant variability is observed in aerosol concentrations in anthropogenic plumes, areas influenced by local sources, or during nucleation events in the free troposphere. 

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