Zenith-sky Spectrometers Research Articles

Variability and trends in stratospheric NO<sub>2</sub> in Antarctic summer, and implications for stratospheric NO<sub>y</sub>

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.

A new software suite for NO 2 vertical profile retrieval from ground-based zenith-sky spectrometers

Here we present an operational method to improve accuracy and information content of ground-based measurements of stratospheric NO 2. The motive is to improve the investigation of trends in NO 2, and is important because the current trend in NO 2 appears to contradict the trend in its source, suggesting that the stratospheric circulation has changed. To do so, a new software package for retrieving NO 2 vertical profiles from slant columns measured by zenith-sky spectrometers has been created. It uses a Rodgers optimal linear inverse method coupled with a radiative transfer model for calculations of transfer functions between profiles and columns, and a chemical box model for taking into account the NO 2 variations during twilight and during the day. Each model has parameters that vary according to season and location. Forerunners of each model have been previously validated. The scheme maps random errors in the measurements and systematic errors in the models and their parameters on to the retrieved profiles. Initialisation for models is derived from well-established climatologies. The software has been tested by comparing retrieved profiles to simultaneous balloon-borne profiles at mid-latitudes in spring.

Observations of NO3 concentration profiles in the troposphere

A zenith sky spectrometer has been deployed at a number of remote locations in the Northern and Southern Hemispheres. The technique of differential optical absorption spectroscopy was used to determine the column density of the nitrate radical (NO3) as a function of solar zenith angle (SZA) during sunrise. The tropospheric column densities show a large degree of day‐to‐day variability and indicate that a significant fraction of tropospheric NO3 exists in the free troposphere (FT). The change in the NO3 column density with SZA was then used to retrieve the vertical concentration profile of NO3, by applying an optimal estimation method [see Coe et al., 2002]. This analysis indicates that there is often substantially more NO3 in the upper boundary layer and lower FT than at the surface. Back‐trajectory calculations and a simple box model are used to account for the observations.

Ozone measurements by zenith-sky spectrometers: An evaluation of errors in air-mass factors calculated by radiative transfer models

Calculations of air-mass factors (AMFs) for ground-based zenith-sky UV-visible spectrometers are presented and discussed. Causes and size of errors in AMFs of ozone in the visible are evaluated. Errors can be caused by approximations in the calculation (intensity-weight approximation, ignoring the finite field of view of the instrument); by approximation in the scheme of the calculation (single scattering, ignoring refraction); or by variable geophysical parameters (vertical profile of constituents). These relative errors in AMF cause identical relative errors in vertical columns of ozone deduced from measurements by zenith-sky spectrometers. The mean of the relative errors of ozone AMFs due to using one set of AMFs for all seasons and locations is ±2.4% when averaged over the commonly used range of solar zenith angles.

Ozone and NO2 air‐mass factors for zenith‐sky spectrometers: Intercomparison of calculations with different radiative transfer models

Calculations of air‐mass factors (AMFs) for ground‐based zenith‐sky UV‐visible spectrometers are now well developed in laboratories where stratospheric constituents are measured with this technique. An intercomparison between results from the different radiative transfer models used to calculate AMFs at twilight is presented here. The comparison was made for ozone AMFs at 510 nm and for NO2 AMFs at 440 nm. Vertical profiles were specified. Results are presented firstly for calculations in a pure Rayleigh atmosphere, then including background aerosols. Relative differences between calculated AMFs from different models cause relative errors in vertical columns of ozone and NO2 measured by zenith‐sky spectrometers. For commonly used averages over solar zenith angles, these relative errors are ±2.3% in the vertical column of ozone and ±1.1% in the vertical column of NO2. Refinements to the calculations, suggested by the intercomparison, should reduce these errors to ±1.0% for ozone and ±0.5% for NO2.