Validation of the Aura Microwave Limb Sounder HNO 3 measurements

We assess the quality of the version 2.2 (v2.2) HNO 3 measurements from the Microwave Limb Sounder (MLS) on the Earth Observing System Aura satellite. The MLS HNO 3 product has been greatly improved over that in the previous version (v1.5), with smoother profiles, much more realistic behavior at the lowest retrieval levels, and correction of a high bias caused by an error in one of the spectroscopy files used in v1.5 processing. The v2.2 HNO 3 data are scientifically useful over the range 215 to 3.2 hPa, with single‐profile precision of ∼0.7 ppbv throughout. Vertical resolution is 3–4 km in the upper troposphere and lower stratosphere, degrading to ∼5 km in the middle and upper stratosphere. The impact of various sources of systematic uncertainty has been quantified through a comprehensive set of retrieval simulations. In aggregate, systematic uncertainties are estimated to induce in the v2.2 HNO 3 measurements biases that vary with altitude between ±0.5 and ±2 ppbv and multiplicative errors of ±5–15% throughout the stratosphere, rising to ∼±30% at 215 hPa. Consistent with this uncertainty analysis, comparisons with correlative data sets show that relative to HNO 3 measurements from ground‐based, balloon‐borne, and satellite instruments operating in both the infrared and microwave regions of the spectrum, MLS v2.2 HNO 3 mixing ratios are uniformly low by 10–30% throughout most of the stratosphere. Comparisons with in situ measurements made from the DC‐8 and WB‐57 aircraft in the upper troposphere and lowermost stratosphere indicate that the MLS HNO 3 values are low in this region as well, but are useful for scientific studies (with appropriate averaging).