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

In situ measurements of the relative humidity with respect to ice ($RH_i$) and of nitric acid (HNO3) 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 ($\Delta-ice$). We propose that surface HNO3molecules prevent the ice/vapor system from reaching equilibrium by a mechanism similar to that of freezing point depression by antifreeze proteins.$\Delta-ice$represents a new link between global climate and natural and anthropogenic nitrogen oxide emissions. Including$\Delta-ice$in climate models will alter simulated cirrus properties and the distribution of upper tropospheric water vapor.