Observations Pertaining to the Impact of Nitric Acid on Cloud Droplet Nucleation

Very soluble trace gases, such as HNO3, have been suggested to play a role in the control of the nucleation of cloud droplets (e.g. Kulmula et al., 1993). If the absorption of the soluble gas can make a significant contribution to the total amount of soluble material in the droplet before the solution particle activates, then it will affect the rate of water uptake by the particles and potentially the number of particles activated. Measurements were conducted below and in towering cumulus from the Canadian Convair 580 during the 2004 ICARTT summer intensive, based out of Cleveland. The measured quantities include cloud droplet number concentration, vertical gust velocity and HNO3 below cloud. The amount of HNO3 taken up by the cloud droplets is determined from the difference between the nitrate measured in dried residuals of the cloud droplets and the nitrate measured in bulk samples of the cloudwater. Observations from six flights interpreted with the aid of an adiabatic aerosol-cloud parcel model, which includes the effect of nitric acid absorption on the growth rate of the cloud droplets, are used to look for evidence of an effect of the HNO3 on the cloud droplet number concentrations.