Variability and trends in stratospheric NO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; in Antarctic summer, and implications for stratospheric NO&amp;lt;sub&amp;gt;y&amp;lt;/sub&amp;gt;

P. A. Cook,H. K. Roscoe

doi:10.5194/acp-9-3601-2009

Abstract

Abstract. NO2 measurements during 1990–2007, obtained from a zenith-sky spectrometer in the Antarctic, are analysed to determine the long-term changes in NO2. An atmospheric photochemical box model and a radiative transfer model are used to improve the accuracy of determination of the vertical columns from the slant column measurements, and to deduce the amount of NOy from NO2. We find that the NO2 and NOy columns in midsummer have large inter-annual variability superimposed on a broad maximum in 2000, with little or no overall trend over the full time period. These changes are robust to a variety of alternative settings when determining vertical columns from slant columns or determining NOy from NO2. They may signify similar changes in speed of the Brewer-Dobson circulation but with opposite sign, i.e. a broad minimum around 2000. Multiple regressions show significant correlation with solar and quasi-biennial-oscillation indices, and weak correlation with El Nino, but no significant overall trend, corresponding to an increase in Brewer-Dobson circulation of 1.4±3.5%/decade. There remains an unexplained cycle of amplitude and period at least 15% and 17 years, with minimum speed in about 2000.

Highlights

The circulation whereby air enters the stratosphere in the tropics, continues toward the winter pole, and returns to the troposphere via tropopause folds at mid-latitudes, is known as the Brewer-Dobson circulation after its discoverers (Brewer, 1949; Dobson, 1956)
Because there is no single path of scattered light, the effective slant path length through the atmosphere must be calculated via radiative transfer code. The ratio of this slant path length to the vertical is known as the Air Mass Factor (AMF), equal to about 2 at Solar Zenith Angle (SZA) 60◦ and about 18 at 90◦, but AMF depends on the wavelength, and on the vertical profiles of air density and NO2
On a Langley plot the slant measurements of NO2 are plotted against the associated AMFs to obtain a straight line, the gradient of which gives the vertical column and the intercept gives the negative of the actual amount of NO2 in the reference spectrum plus the artefact

Summary

Introduction

The circulation whereby air enters the stratosphere in the tropics, continues toward the winter pole, and returns to the troposphere via tropopause folds at mid-latitudes, is known as the Brewer-Dobson circulation after its discoverers (Brewer, 1949; Dobson, 1956). It is known to be driven by the breaking of planetary-scale Rossby waves and of gravity waves, against the mean zonal winds, in the stratosphere and mesosphere Changes in the speed of the Brewer-Dobson circulation will affect the stratospheric concentrations of longer-lived trace gases whose sources are in the troposphere (e.g. CFCs, CH4, N2O), as well as of their eventual stratospheric products (e.g. reactive chlorine gases, H2O, reactive nitrogen gases). Changes in its speed will affect the tropospheric concentrations of trace gases with large sources in the stratosphere (e.g. ozone). Because these gases are important to atmospheric climate and chemistry, it is important to know what changes have occurred to the Brewer-Dobson circulation in the past and will occur in future. Artefact, is larger if there are fewer pixels within the slit width of the spectrometer

Apparatus and spectral analysis

Langley plots and their chemical modification

The chemical modification scheme and daily analysis

Interpretation and conclusion