Margarita Yela

Pole‐to‐pole validation of Envisat GOMOS ozone profiles using data from ground‐based and balloon sonde measurements

[1]xa0In March 2002 the European Space Agency (ESA) launched the polar-orbiting environmental satellite Envisat. One of its nine instruments is the Global Ozone Monitoring by Occultation of Stars (GOMOS) instrument, which is a medium-resolution stellar occultation spectrometer measuring vertical profiles of ozone. In the first year after launch a large group of scientists performed additional measurements and validation activities to assess the quality of Envisat observations. In this paper, we present validation results of GOMOS ozone profiles from comparisons to microwave radiometer, balloon ozonesonde, and lidar measurements worldwide. Thirty-one instruments/launch sites at twenty-five stations ranging from the Arctic to the Antarctic joined in this activity. We identified 6747 collocated observations that were performed within an 800-km radius and a maximum 20-hour time difference of a satellite observation, for the period between 1 July 2002 and 1 April 2003. The GOMOS data analyzed here have been generated with a prototype processor that corresponds to version 4.02 of the operational GOMOS processor. The GOMOS data initially contained many obviously unrealistic values, most of which were successfully removed by imposing data quality criteria. Analyzing the effect of these criteria indicated, among other things, that for some specific stars, only less than 10% of their occultations yield an acceptable profile. The total number of useful collocated observations was reduced to 2502 because of GOMOS data unavailability, the imposed data quality criteria, and lack of altitude overlap. These collocated profiles were compared, and the results were analyzed for possible dependencies on several geophysical (e.g., latitude) and GOMOS observational (e.g., star characteristics) parameters. We find that GOMOS data quality is strongly dependent on the illumination of the limb through which the star is observed. Data measured under bright limb conditions, and to a certain extent also in twilight limb, should be used with caution, as their usability is doubtful. In dark limb the GOMOS data agree very well with the correlative data, and between 14- and 64-km altitude their differences only show a small (2.5–7.5%) insignificant negative bias with a standard deviation of 11–16% (19–63 km). This conclusion was demonstrated to be independent of the star temperature and magnitude and the latitudinal region of the GOMOS observation, with the exception of a slightly larger bias in the polar regions at altitudes between 35 and 45 km.

Ground-based stratospheric NO2 monitoring at Keflavik (Iceland) during EASOE

This report presents ground based measurements of nitrogen dioxide above Keflavik, Iceland, 64[degrees]N, between December 1991, and February 1992. Using visible spectrometry, the authors observed column densities below 1 [times] 10[sup 15] molecules/cm[sup 2] inside the polar vortex.

Polar Stratospheric Cloud Observations in the 2006/07 Arctic Winter by Using an Improved Micropulse Lidar

Abstract The potential of a new improved version of micropulse lidar (MPL-4) on polar stratospheric cloud (PSC) detection is evaluated in the Arctic over Ny-Alesund (79°N, 12°E), Norway. The campaign took place from January to February 2007 in the frame of the International Polar Year (IPY) activities. Collocated Alfred Wegener Institute (AWI) Koldewey Aerosol Raman Lidar (KARL) devoted to long-term Arctic PSC monitoring is used for validation purposes. PSC detection is based on lidar retrievals of both backscattering ratio R and volume depolarization ratio δV. Two episodes were unequivocally attributed to PSCs: 21–22 January and 5–6 February 2007, showing a good correlation between MPL-4 and KARL backscattering ratio datasets (mean correlation coefficient = 0.92 ± 0.03). PSC layered structures were characterized for four observational periods coincident with KARL measurements. Also, PSC type classification was determined depending on the retrieved R and δV values as compared with those obtained by KARL l...

NO2 profiles during the CRISTA-2 experiment (August 1997) at subtropical regions

A ground-based spectrometer using the DOAS technique measuring at zenith was deployed at Izana Observatory during the 2nd CRISTA flight, in August 1997. CRISTA was flown on the ASTROPAS free-flying platform during the Space Shuttle mission STS-85, and among other species measured NO2 profiles. The twilight measurements of NO2 profiles. The twilight measurements of NO2 total column over the station obtained by the ground-based spectrometer has been used to estimate the profile of this species in the subtropics for summer and to compare with the results of CRISTA during the overpassings. The results of both data sets, and the origin of the discrepancies are discussed in the paper.

Ozone and NO2 monitoring in Southern Spain: The 1994/95 winter record low

During the winter 1994/95, three episodes occurred at El Arenosillo (37N, 6.7W) when the total ozone dropped below the 2σ level, in the months of November 94, January and February 95. These episodes are analysed in detail with the help of ozone sounding and ECMWF backtrajectories. The NO2 column measurements have been used as additional tool to get more insight of the cause of the observed low ozone. Results show that the January episode is related to the elongation and unusual position of the Arctic vortex over Europe. Temperatures in the stratosphere were at their minimum annual values and the backtrajectories present a southward strong meridional component. In addition, measurements of NO2 show values typical of winter high latitude. The episode in February, on the other hand, was of different origin. Although the total amounts were similar, the trajectory analysis shows typical air masses over the station, resulting in an absence of the secondary ozone maximum.

Ozone deficiencies measured during EASOE in Iceland: the 15.1.92 episode

UV-Vis spectrometry measurements at zenith were carried out at Keflavik (64 N, 23 W) during the EASOE Campaign in the winter of 1991/92. During the middle of January, a situation of low ozone was observed associated to a very low temperature field in the lower stratosphere. Ozonesounding on day 15 January 1992 exhibits two different patterns, up and down 70 hPa. From the potential vorticity fields and backward trajectories analysis, the different origin of the air masses at the two layers are shown. While at the lower levels the ozone deficiency is the result of low latitudes advection, at the upper levels a more structured profile of 30% less than 10 days before is found to be air from inside the vortex presumably processed by PSC. The partial chemical depletion is supported by the very low NO2 content measured at the same period at the station.