Meteor-Ablated Aluminum in the Mesosphere-Lower Thermosphere

The first global atmospheric model (WACCM-Al) of meteor-ablated aluminum was constructed from three components: The Whole Atmospheric Community Climate Model (WACCM6); a meteoric input function for Al derived by coupling an astronomical model of dust sources in the solar system with a chemical meteoric ablation model; and a comprehensive set of neutral, ion-molecule and photochemical reactions relevant to the chemistry of Al in the upper atmosphere. The reaction kinetics of two important reactions that control the rate at which Al+ ions are neutralized were first studied using a fast flow tube with pulsed laser ablation of an Al target, yielding k(AlO+ + CO) = (3.7 +/- 1.1) x 10(-10) and k(AlO+ + O) = (1.7 +/- 0.7) x 10(-10) cm(3) molecule(-1) s(-1) at 294 K. The first attempt to observe AlO by lidar was made by probing the bandhead of the B-2 sigma(+)(v ' = 0) <- X-2 sigma(+)(v '' = 0) transition at lambda(air) = 484.23 nm. An upper limit for AlO of 60 cm(-3) was determined, which is consistent with a night-time concentration of similar to 5 cm(-3) estimated from the decay of AlO following rocket-borne grenade releases. WACCM-Al predicts the following: AlO, AlOH and Al+ are the three major species above 80 km; the AlO layer at mid-latitudes peaks at 89 km with a half-width of similar to 5 km, and a peak density which increases from a night-time minimum of similar to 10 cm(-3) to a daytime maximum of similar to 60 cm(-3); and that the best opportunity for observing AlO is at high latitudes during equinoctial twilight.