Abstract
Abstract. We use the 3-D FinROSE chemistry transport model (CTM) and Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) observations to study connections between atmospheric dynamics and middle atmospheric NOx (NOx = NO + NO2) distribution. Two cases are considered in the northern polar regions: (1) descent of mesospheric NOx in February–March 2009 after a major sudden stratospheric warming (SSW) and, for comparison, (2) early 2007 when no NOx descent occurred. The model uses the European Centre for Medium-Range Weather Forecasts (ECMWF) operational data for winds and temperature, and we force NOx at the model upper altitude boundary (80 km) with ACE-FTS observations. We then compare the model results with ACE-FTS observations at lower altitudes. For the periods studied, geomagnetic indices are low, which indicates absence of local NOx production by particle precipitation. This gives us a good opportunity to study effects of atmospheric transport on polar NOx. The model results show no NOx descent in 2007, in agreement with ACE-FTS. In contrast, a large amount of NOx descends in February–March 2009 from the upper to lower mesosphere at latitudes larger than 60° N, i.e. inside the polar vortex. Both observations and model results suggest NOx increases of 150–200 ppb (i.e. by factor of 50) at 65 km due to the descent. However, the model underestimates the amount of NOx around 55 km by 40–60 ppb. According to the model results, chemical loss of NOx is insignificant during the descent period, i.e. polar NOx is mainly controlled by dynamics. The descent is terminated and the polar NOx amounts return to pre-descent levels in mid-March, when the polar vortex breaks. The break-up prevents the descending NOx from reaching the upper stratosphere, where it could participate in catalytic ozone destruction. Both ACE-FTS observations and FinROSE show a decrease of ozone of 20–30 % at 30–50 km from mid-February to mid-March. In the model, these ozone changes are not related to the descent but are due to solar activation of halogen and NOx chemistry.
Highlights
In the stratosphere odd nitrogen (NOx = NO + NO2) is produced mainly by oxidation of nitrous oxide (N2O)
We have used the FinROSE-chemistry transport model (CTM) and ACEFTS observations to study the effect of polar vortex dynamics on the distribution of NOx in the winter polar region
Two different Northern Hemisphere cases, 2007 and 2009, were considered. The latter was characterized by a strong NOx descent event in February–March, which begun after a record-breaking stratospheric warming (SSW)
Summary
Another important and highly varying source of NOx is ionizing energetic particle precipitation directly affecting a wide range of altitudes from the thermosphere down to the stratosphere (Barth, 1992; Vitt et al, 2000). Observations have shown that during winter times NOx can be effectively transported downwards inside the polar vortex (Funke et al, 2005, 2007; Hauchecorne et al, 2007; Seppalaet al., 2007; Randall et al, 2009), after NOx is produced by particle precipitation in the mesosphere-lower thermosphere (MLT). The satellite observations of NOx often cover altitudes up to middle mesosphere only, the connection to lower thermospheric NOx production has been established using VLF (Very Low-Frequency) radio propagation data in the case of the 2004 descent event (Clilverd et al, 2006). Several recent studies based on atmospheric modelling support the importance of wintertime NOx enhancements to middle
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