Abstract
Abstract. Over 20 years of stratospheric NO2 vertical column density (VCD) data from ground-based zenith DOAS spectrometers were used for trend analysis, specifically, via multiple linear regression. Spectrometers from the Network for the Detection of Atmospheric Composition Change (NDACC) cover the subtropical latitudes in the Northern Hemisphere (Izaña, 28° N), the southern Subantarctic (Ushuaia, 55° S) and Antarctica (Marambio, 64° S, and Belgrano, 78° S). The results show that for the period 1993–2014, a mean positive decadal trend of +8.7 % was found in the subtropical Northern Hemisphere stations, and negative decadal trends of −8.7 and −13.8 % were found in the Southern Hemisphere at Ushuaia and Marambio, respectively; all trends are statistically significant at 95 %. Belgrano only shows a significant decadal trend of −11.3 % in the summer/autumn period. Most of the trends result from variations after 2005. The trend in the diurnal build-up per hour (DBU) was used to estimate the change in the rate of N2O5 conversion to NO2 during the day. With minor differences, the results reproduce those obtained for NO2. The trends computed for individual months show large month-to-month variability. At Izaña, the maximum occurs in December (+13.1 %), dropping abruptly to lower values in the first part of the year. In the Southern Hemisphere, the polar vortex dominates the monthly distributions of the trends. At Marambio, the maximum occurs in mid-winter (−21 %), whereas at the same time, the Ushuaia trend is close to its annual minimum (−7 %). The large difference in the trends at these two relatively close stations suggests a vortex shift towards the Atlantic/South American area over the past few years. Finally, the hemispheric asymmetry obtained in this work is discussed in the framework of the results obtained by previous works that considered tracer analysis and Brewer–Dobson circulation. The results obtained here provide evidence that the NO2 produced by N2O decomposition is not the only cause of the observed trend in the stratosphere and support recent publications pointing to a dynamical redistribution starting in the past decade.
Highlights
Stratospheric nitrogen dioxide (NO2) has been a subject of interest since the late sixties, when the use of supersonic aircraft flying at stratospheric levels was being considered
The residuals and the trend were plotted together to search for common structures that could represent anomalies not captured by the model (Fig. 2)
These authors suggested a meridional displacement of the subtropical barriers as the cause of the hemispheric asymmetry, since they could mimic the observed trends by shifting the subtropical mixing barriers to the south by 5◦ at altitudes below 30 km, no explanation was provided for the displacement beyond what is possible due to low-frequency natural variations
Summary
Stratospheric nitrogen dioxide (NO2) has been a subject of interest since the late sixties, when the use of supersonic aircraft flying at stratospheric levels was being considered. Studies on the impact of nitrogen oxides on the ozone in the stratosphere were initiated (Johnston, 1971). The interest in this atmospheric layer was enhanced in the mid-seventies, powered by the discovery of the ozone depletion potential of halogenated compounds (Stolarski and Cicerone, 1974). Nitrogen oxides interact with ozone both directly and indirectly. Nitric oxide (NO) reacts with ozone, forming NO2 and O2. NO is recovered by NO2 reaction with atomic oxy-
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