Below-cloud scavenging of tropospheric ammonia

Below-cloud scavenging of tropospheric ammonia gas by raindrops was investigated by observing the change in the NH₃(g) concentration measured before and during long precipitation events. The removal was parameterized as a first-order process; the scavenging coefficient, defined here as the first-order rate coefficient, was computed by two different methods:

(1) the average “measured” scavenging coefficient was calculated by taking into account the average “pre-event” and “event” connertions and the duration of the event,

(2) the average “theoretical” scavening coefficient was calculated by considering the diffusion of the gas to the raindrop, the intensity of precipitation during each hour of the event, and the raindrop size distribution.

The scavenging coefficient was computed for fourteen precipitation events. The median “measured” scavenging coefficient was 1.3 × 10^-4 s^-1, which is equivalent to an e-folding time of 2.1 hours. Note, however, that these values spanned two orders of magnitude. A comparison of the “measured” to “theoretical” scavening coefficients showed that these two quantities were within an order of magnitude of each other for most cases. The precipitation-scavenged ammonia was calculated as the difference between the gas concentrations at the beginning and end of the event. For a majority of the events, the scavenged ammonia contributed less than 10% of the total ammonium found in precipitation.