Evidence That Nitric Acid Increases Relative Humidity in Low-Temperature Cirrus Clouds

Abstract

In situ measurements of the relative humidity with respect to ice (RH _i ) and of nitric acid (HNO ₃ ) were made in both natural and contrail cirrus clouds in the upper troposphere. At temperatures lower than 202 kelvin, RH _i values show a sharp increase to average values of over 130% in both cloud types. These enhanced RH _i values are attributed to the presence of a new class of HNO ₃ -containing ice particles (Δ-ice). We propose that surface HNO ₃ molecules prevent the ice/vapor system from reaching equilibrium by a mechanism similar to that of freezing point depression by antifreeze proteins. Δ-ice represents a new link between global climate and natural and anthropogenic nitrogen oxide emissions. Including Δ-ice in climate models will alter simulated cirrus properties and the distribution of upper tropospheric water vapor.