Showing papers by "Folkard Wittrock published in 2004"

MAX-DOAS measurements of atmospheric trace gases in Ny- ˚ Alesund - Radiative transfer studies and their application

Abstract: . A new approach to derive tropospheric concentrations of some atmospheric trace gases from ground-based UV/vis measurements is described. The instrument, referred to as the MAX-DOAS, is based on the well-known UV/vis instruments, which use the sunlight scattered in the zenith sky as the light source and the method of Differential Optical Absorption Spectroscopy (DOAS) to derive column amounts of absorbers like ozone and nitrogen dioxide. Substantial enhancements have been applied to this standard setup to use different lines of sight near to the horizon as additional light sources (MAX - multi axis). Results from measurements at Ny-Alesund (79° N, 12° E) are presented and interpreted with the full-spherical radiative transfer model SCIATRAN. In particular, measurements of the oxygen dimer O4 which has a known column and vertical distribution in the atmosphere are used to evaluate the sensitivity of the retrieval to parameters such as multiple scattering, solar azimuth, surface albedo and refraction in the atmosphere and also to validate the radiative transfer model. As a first application, measurements of NO2 emissions from a ship lying in Ny-Alesund harbour are presented. The results of this study demonstrate the feasibility of long term UV/vis multi axis measurement that can be used to derive not only column amounts of different trace gases but also some information on the vertical location of these absorbers.

MAX-DOAS measurements of formaldehyde in the Po-Valley

Abstract: . During the first measurement campaign of the EU FORMAT project in summer 2002 near Milan, northern Italy, ground-based scattered light differential optical absorption spectroscopy (DOAS) measurements were performed using a new multi-axis instrument. From the data set of this four week measurement period, the detailed analysis results of three days, 12–14 August, are presented exemplary. Slant column densities for formaldehyde (HCHO) and the oxygen dimer (O4) have been retrieved, employing fitting windows from 335 nm to 357 nm and 350 nm to 360 nm respectively. In order to convert slant into vertical columns radiative transfer calculations were perfomred using aerosol parameters derived from the actual O4 measurements. By analysing the measurements from different viewing directions (zenith, 4x off-axis) vertical profile information, and in particular mixing ratios for the boundary layer have been derived for the first time for HCHO with a multi-axis DOAS (MAX-DOAS) instrument. HCHO vertical columns are in the range of 5 to 20·1015 molec/cm2 with an relative error of about 15%. This corresponds to HCHO mixing ratios in the boundary layer of 0.7 ppb to 4.2 ppb, which is in excellent agreement with simultaneous measurements from both a Hantzsch in-situ and a long-path DOAS instrument operated at the same place.

BrO emission from volcanoes: A survey using GOME and SCIAMACHY measurements

Abstract: [1] Volcanic eruptions are known to be a major source of SO2 and some reactive halogen species notably HCl and HF. Recent studies have however observed the presence of large amounts of BrO with tight correlation to SO2 in a volcanic plume by ground-based spectroscopic measurements. In this work, upper limits of BrO columns have been estimated for a number of volcanic eruptions observed in measurements made by two satellite instruments - the Global Ozone Monitoring Experiment (GOME), and the new SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY). The results obtained for the eruptions covered by satellite overpasses over the Etna, Nyamuragira, Popocatepetl, and Reventador volcanoes show no correlation between enhanced volcanic SO2 during large eruptions and the corresponding BrO columns. Evidence for BrO enhancement was also not found in the vicinity of the Soufriere Hills volcanoes. The possible reasons for the differences between ground-based and satellite observations are considered.

Circumpolar transport and air-surface exchange of atmospheric mercury at Ny-Ålesund (79° N), Svalbard, spring 2002

Abstract: Mercury in different environmental compartments has been measured at Ny- ˚ Alesund (78 54 0 N, 11 53 0 E) dur- ing an intensive campaign, 17 April to 14 May 2002. Time- resolved speciated determination of mercury in the atmo- sphere and snow was conducted at the Norwegian research station at the Zeppelin mountain, 474 m above the sea level, and at the Italian research facility Dirigibile Italia, 12 m above the sea level. Total Gaseous Mercury (TGM) was present in the range <0.1 to 2.2 ng m 3 during the cam- paign. Three mercury depletion events, identified as periods with decreased TGM concentrations, were observed. At the lower altitude, TGM concentrations following such events were found to exhibit both higher magnitude and larger vari- ability in comparison to results from the Zeppelin station. Oxidised mercury species in air and fall-out with snow as well as mercury attached to particles were also measured and their concentrations were found to be anti-correlated with TGM in air. concentrations of total Hg in snow (Hg-tot) showed a large ( 15◊) increase in response to Gaseous El- emental Mercury Depletion Events (GEMDEs, range 1.5- 76.5 ng L 1 ). Solid evidence for photo-stimulated emissions of Hg 0 (g) from the snow pack in conjunction to depletion events were obtained from gradient measurements as well as from flux chamber measurements. Steep diurnal concen- tration variations of Hg 0 (aq) in surface seawater were also found to concur with changing solar radiation. The con- centration of Hg 0 (aq) in seawater was found to be in the range 12.2-70.4 pg L 1 , which corresponds to supersatura- tion. Hence, the seawater surface constituted a source emit- ting elemental mercury. The concentrations of RGM (reac- tive gaseous mercury), Hg-p (particulate mercury), and BrO column densities (detected by DOAS) were very low except for a few individual samples during the major Hg 0 depletion event. BrO vertical column densities obtained by the remote satellite ESR-2 and trajectory analysis indicate that the air masses exhibiting low Hg 0 concentrations originated from areas with high BrO densities.

Global observations of formaldehyde

Abstract: Formaldehyde (HCHO) indicates and supports photochemical activity in the atmosphere. Large amounts are expected to found in industrial areas and during biomass burning. HCHO is a major intermediate in the degradation of methane (and many other hydrocarbons). In the absence of heterogenous losses, essentially every methane molecule is converted to HCHO. Therefore it is found throughout the troposphere. It is destroyed via photolysis and reaction with OH. In continental boundary layers, non-methane hydrocarbons (NMHCs) emitted by biogenic and anthropogenic sources dominate over methane also as a source of HCHO.

First validation of sciamachy bro columns

Abstract: Retrievals of BrO columns using version 5.01 of the SCIAMACHY NRT processor as well as independent non-operational algorithms are compared with GOME and with ground-based measurements from two Arctic stations. NRT BrO columns are found to agree with GOME and non-operational data sets to within 20% for moderate and large slant columns, while for slant columns smaller than 1.5x10 14 molec/cm 2 , the NRT product reports larger values by 20 to 100%. First comparisons with ground-based measurements in Harestua (60°N) and Ny-Alesund (79°), tentatively evaluated at the time of the SCIAMACHY overpasses, provide encouraging results although more work is still needed for a detailed quantitative validation of BrO.

A scientific no2 product from sciamachy: first results and validation

Abstract: A scientific NO2 retrieval developed at the University of Bremen was applied to all available SCIAMACHY nadir spectra from August 2002 to May 2004. The NO2 columns show the expected seasonal, latitudinal and regional variations and a good internal consistency. The precision of the individual measurements was assessed by analyzing the scatter of the results within certain areas and an excellent standard deviation of below 3x10 14 molec cm -2 was found. Comparison of the SCIAMACHY columns with measurements from the German DOAS and FTS validation network shows very good agreement with the exception of high latitudes in summer, where a systematic under- estimation is apparent and polluted sites, where the different sensitivity of the measurement systems to tropospheric absorptions plays a role.

Use of Satellite Data to Constrain Ozone Budgets in Global Tropospheric Chemistry Models

Abstract: Satellite measurements of tropospheric composition are becoming of increasing importance. Important precursors of ozone such as carbon monoxide and NO2 can now be measured. Careful consideration of the methods used to compare satellite data to models has been required because of such issues as the method to calculate the tropospheric column of species which have substantial columns in the troposphere, averaging kernels and the overpass time of the satellite for those species which have a strong diurnal variation.