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FIGURE 7.13 Calculated lifetime of NO, as a function of relative humidity at several locations in California. The points marked with an upward arrow indicate that these represent lower limits to the lifetimes (adapted from Piatt et at., 1984).

fornia desert (Piatt et al., 1984). The lifetimes were calculated based on the following reaction scheme:

N03 Loss of N03.

Under conditions where the N03 concentration remained constant for a period of time, a steady state was assumed, i.e., d[N03]/dt = 0 = &8[N02][03] - *1oss[N03], from which the effective first-order rate constant for loss of N03, k|oss, could be calculated. The lifetime of N03 is then given by rN(>i = 1 /&k)SS = [N03]/&8[N02][03]. Since kH is known (DeMore et al., 1997; Atkinson et al., 1997a, 1997b), simultaneous measurements of N02, 03, and N03 allow one to calculate tno:,

It is striking that the lifetimes fall to very small values as the relative humidity (RH) approaches 50%. At larger values of RH, not only is the gas-phase concentration of water larger, but condensation to form an aqueous liquid film on surfaces becomes more important. One cannot distinguish from data whether N03 is itself being taken up into a liquid film, whether N205 is being removed, or whether both processes are occurring.

Evidence for the uptake of N03 by aqueous solutions has been sought in both laboratory and field studies. A lower limit for the mass accommodation coefficient for N03 on liquid water of > 2.5 X 10"3 was reported by Thomas et al. (1989) and Mihelcic et al. (1993). Li et al. (1993) followed the formation of particulate nitrate in a rural area and, by comparing their measurements to model predictions, suggested that the mass accommodation coefficient for N03 on aqueous (NH4)2S04-NH4HS04-H2S04 aerosols is approximately unity, i.e.; N03 is taken up into the particle on every collision.

However, Rudich et al. (1996a, 1996b), Imamura et al. (1997), and Thomas et al. (1998) have measured the uptake of N03 on pure water as well as on aqueous solutions containing CI", Br", I", N02, HSO-^, S03 , HCOO", CH3COO", and OH". The uptake coefficient for pure water at 273 K was reported to be 2.0 X 10"4 (Rudich et al., 1996a). Larger values were obtained with the salt solutions due to reactions of N03 in solution; for example, a lower limit of 2 X 10"3 was measured for the uptake on O.f M NaCl at 293 K (Thomas et al., 1998), in good agreement with the value of 2.7 X 10"3 at 273 K reported by Rudich et al. (1996a).

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