Temporal coverage: 9 May 2013 - present
The Microwave Limb Sounder (MLS) Nitric Acid (HNO3) Mixing Ratio layer at 46hPa (hectopascals) indicates nitric acid levels at the vertical atmospheric pressure level of 46hPa, and is measured in parts per billion by volume (ppbv).
Nitric acid (HNO3) is a very water soluble, acidic gas. In the atmosphere it is formed by the conversion of nitrogen monoxide into nitrogen dioxide, and ultimately into nitric acid. It readily reacts with atmospheric water to produce acidic precipitation. Nitric acid also reacts with gaseous ammonia to form particulate or aerosol nitrate, which in turn is removed by wet and dry deposition of the particles. In the clean background troposphere, its removal in precipitation acts as a sink for odd hydrogen and nitrogen compounds and limits the formation of ozone.
One of the main objectives of the Aura mission in general and MLS in particular is to track the stability of stratospheric ozone. HNO3 indirectly regulates the extent, duration, and cumulative magnitude of stratospheric ozone depletion. Despite international regulations controlling the production of ozone-depleting substances, the ozone layer may not recover as expected.
The MLS Nitric Acid (HNO3) Mixing Ratio at 46hPa layer is derived from the MLS Nitric Acid (ML2HNO3_NRT) available from the Microwave Limb Sounder (MLS) instrument on the Aura satellite. The sensor resolution is 5 km, imagery resolution is 2 km, and the temporal resolution is twice daily (day and night).
References: GES DISC - ML2HNO3_NRT: MLS/Aura Near-Real-Time L2 Nitric Acid (HNO3) Mixing Ratio V004; MLS - HNO3 Product