Global observations of HNO3 from the High Resolution Dynamics Limb Sounder (HIRDLS): First results

We present the first evaluation of the HNO3 data product (version 2.04.09) from the High Resolution Dynamics Limb Sounder (HIRDLS) on the Earth Observing System (EOS) Aura satellite. The HIRDLS instrument obtains between 5000 and 7000 HNO3 profiles per day. HIRDLS HNO3 data are generally good over the latitude range of 64°S to 80°N and pressure range 100 to 10 hPa, with some profiles, depending on latitude, having useful information between 100 to 161 hPa. The individual profile ‘‘measured’’ precision is between 10 and 15%, but can be much larger if the HNO3 abundance is low or outside the 100 hPa to 10 hPa range. Global results are compared with the HNO3 observations from version 2.2 of the EOS Aura Microwave Limb Sounder (MLS), and it is found that large-scale features are consistent between the two instruments. HIRDLS HNO3 is biased 0–20% low relative to Aura MLS in the mid-to-high latitudes and biased high in the tropical stratosphere. HIRDLS HNO3 is also compared with Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS). In these mostly highlatitude comparisons the HIRDLS HNO3 data are biased 10–30% low, depending on altitude. Finally, the HIRDLS HNO3 is compared to in situ data taken by the NOAA Chemical Ionization Mass Spectrometer (CIMS) instrument flown during the 2005 NASA Houston Aura Validation Experiment (AVE) and the ability of HIRDLS to measure HNO3 in the UTLS region is examined.