Validation of the Aura Microwave Limb Sounder HNO3 Measurements

We assess the quality of the version 2.2 (v2.2) HNO3 measurements from the
Microwave Limb Sounder (MLS) on the Earth Observing System Aura satellite. The MLS
HNO3 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 HNO3 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 HNO3 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 HNO3 measurements from
ground-based, balloon-borne, and satellite instruments operating in both the infrared
and microwave regions of the spectrum, MLS v2.2 HNO3 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 HNO3 values are low in this region as well, but are
useful for scientific studies (with appropriate averaging).