Aerosol Growth

Growth of newly formed, still very small aerosol particles is important for at least three reasons. Only sufficiently grown particles become long lived, scatter sunlight efficiently, and eventually act as CCN. Particle growth proceeds by Y condensation and coagulation. Growth by Y condensation may involve various types of different trace gases Yi. Our gaseous H2SO4 measurements made in ground level air (Fiedler et al., 2005) indicate that gaseous H2SO4 contributes on average only about 5% to the growth of particles with diameters larger than 3 nm. Probably growth is mostly due to so far unidentified condensable organic trace gases. These condensable organics must be formed by chemical reactions involving relatively short lived organic precursor gases.

However, in the upper troposphere the situation may be different. Here the short lived organic precursor gases may be less important since they probably become destroyed already during transport from the surface to the upper troposphere. Also the atmospheric trace gas ammonia which promotes sulphuric acid nucleation in continental ground level air may not be sufficiently abundant in the upper troposphere. Due to its large solubility in liquid water ammonia should experience very efficient wet removal and therefore should not be efficiently transported to the upper troposphere. In contrast SO2, the precursor of gaseous H2SO4 is less soluble in liquid water. Its photochemical lifetime is about 10-20 days (see above) and therefore in the upper troposphere gaseous H2SO4 formation can continue for about 10-20 days.

Days

Figure 8. Results of a 10-day model simulation of atmospheric gaseous H2SO4, and aerosol particle number concentrations at 10 km altitude. The air mass under consideration was assumed to ascend to 10 km altitude and reside there for a period of 10 days in cloud-free conditions. The initial SO2 mole fraction was assumed to be 200 pptV. See text for details. From Arnold et al. (2006).

Days

Figure 8. Results of a 10-day model simulation of atmospheric gaseous H2SO4, and aerosol particle number concentrations at 10 km altitude. The air mass under consideration was assumed to ascend to 10 km altitude and reside there for a period of 10 days in cloud-free conditions. The initial SO2 mole fraction was assumed to be 200 pptV. See text for details. From Arnold et al. (2006).

Hence new particles generated in the upper troposphere by INU or HONU can grow at least by gaseous H2SO4 condensation. Note that gaseous H2SO4 condensation does not require particularly low temperatures (see Figure 6) and therefore can take place also in the middle and lower troposphere. This implies that particles formed in the upper troposphere may still continue to grow by gaseous H2SO4 condensation after downward transport and can act as CCN at lower altitudes.

Figure 8 shows as an example a 10 day model simulation (Arnold et al., 2006) considering an air mass which has ascended to 10 km and resided there for 10 days at clear-sky conditions. After arrival at 10 km the initial SO2 concentration was 200pptV. The diurnal maximum OH concentration is 1 x 106 cm-3 which implies an e-folding lifetime for SO2 of about 23 days.

On day 1, as the sun is rising, OH increases, leading to H2SO4 production. As a consequence H2SO4 rises to about 2 x 107cm-3s-1 leading to HONU which in turn leads to a rise of the total aerosol particle number concentration Ntot to nearly 1 x 105 cm-3. Also the number concentrations N 4 and N 6 of particles with diameters larger than 4 and 6 nm rise steeply maximizing on day 1. In the afternoon as the solar elevation decreases OH, H2SO4 decrease steeply again. Also Ntot and to a lesser extent also N4 and N6 decrease due to coagulation. On the following days, the diurnal H2SO4 maximum decreases to about 4 x 106 cm-3 which is due to the decrease of OH and the increase of the aerosol surface leading to a decrease of the condensational H2SO4 lifetime. After day 1 the diurnal Ntot maxima nearly vanish, reflecting decreased HONU and increased scavenging of new particles via coagulation. The N12, N20, and N30 curves maximize on days 2, 4, and 6.

Note that the model results of particle formation and growth depend very critically on temperature, relative humidity, SO2, OH, and the initial aerosol surface. For example, when increasing the initial aerosol abundance, an increasing fraction of the photochemically formed H2SO4 will condense on pre-existing particles rather than nucleate.

In the atmosphere water vapour super-saturations are usually less than 1% and almost never exceed 2%. The corresponding minimum diameters of H2SO4/H2O aerosol particles acting as CCN are about 30 nm and 20 nm respectively. An inspection of Figure 8 reveals that within about 6 days the concentration N30 of CCN sized particles increases very substantially from 60 cm-3 to nearly 1000 cm-3.

Particle growth is not controlled by temperature but is controlled by the rate of gaseous sulphuric acid formation which in turn is proportional to [SO2] x [OH]. The concentration of SO2 is controlled by emissions of SO2 and SO2-precursors at the Earth surface, by atmospheric transport of these species, and by SO2 removal by clouds.

Also, OH concentrations may be higher than considered by the above model case. This is particularly true for summer and for low latitudes. Higher OH will reduce the SO2 lifetime and increase the rate of gaseous H2SO4 formation. In turn this will increase new particle formation and growth.

Variations of the CR ionization rate Q induce a variation of the INU rate Ji. However, the N30 reached after 10 days substantially depends on J only if J is sufficiently larger than Jh and only if J is less than about 1 cm-3 s-1 (Arnold et al., 2006).

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