Nitrates blaze a trail

US researchers find that HNO₃ may change climate models.

On a clear day, the condensation trails that aircrafts leave in their wake fuel many a holiday dream. To an international team of climate researchers, however, they may well bring to mind its discovery of a new class of ice particles which contain HNO₃ and which could dramatically alter climate models.

Otherwise known as contrails, the condensation trails can mix with natural cirrus clouds which interact with visible and infrared radiation and are a principal component of the water cycle and climate system. Contrails have a far higher concentration of ice particles than natural cirrus clouds.

The researchers from the US, Germany and Switzerland measured the relative humidity (RH) of a variety of contrails and cloud samples to investigate the possibility that chemical impurities might enhance RH. They identified HNO ₃, which is produced indirectly from lightning and burning fossil fuels, at concentrations ranging from 100 to over 1000 parts per trillion. Most of the NO and NO ₂ produced in the burning processes in the lower troposphere is pumped to the upper troposphere through convection to form HNO₃.

The researchers suggest that by adsorbing to ice particles, HNO₃ molecules increase RH by up to 100 per cent and estimate that increasing the RH by 35 per cent has the same effect on water vapour as increasing cloud temperatures by 2K.

They suggest that HNO₃ combines with surface water molecules to form nitric acid trihydrate, which interferes with the uptake of molecules on the ice surface, preventing the ice-vapour system from reaching equilibrium.

According to Ru-shan Gao, from the NOAA Aeronomy laboratory in Boulder, Colorado, who led the team, the work ’points out a possible physical effect that could affect climate models significantly’.

Emma Davies