SCIAMACHY (Scanning Imaging Absorption spectroMeter for Atmospheric ChartographY) is the first instrument to
allow retrieval of CO by measuring absorption in the near infrared from reflected and scattered sunlight instead of from
thermal emission. Thus, in contrast to thermal-infrared satellites (MOPITT), SCIAMACHY is highly sensitive to the
lower layers of the troposphere where the sources, such as biomass burning, are located, and where the bulk of the CO is
usually found.
In many regions of the world, the burning of vegetation has a repeating seasonal pattern, but the amount of CO emitted
from biomass burning varies considerably from place to place. Here we present a study on the relationship between fire
counts and CO vertical column densities (VCD) in different regions. These results are compared with the CO VCD from
MOPITT, aerosol index, and NO2 tropospheric VCD (TVCD) from SCIAMACHY, and the coupled chemistry climate
model (CCM) ECHAM5/MESSY.
Satellite observations provide unique opportunities for the identification of trace gas sources on a global scale. We present case studies for the synergistic use of satellite observations by comparing formaldehyde (HCHO) time series with fire count measurements as well as with surface temperature to identify the tropospheric sources of HCHO. The fire counts and temperature are taken as proxy for biomass burning events and vegetation activity, respectively. Both are sources of HCHO, either direct or trough photochemical oxidation of non-methane hydrocarbons (e.g. biogenic isoprene emissions). Formaldehyde time series are derived from satellite observations made by the GOME instrument. This instrument provides almost 8 years of continuous HCHO global observations, which constitute an ideal case to calculate time series over specific regions for various trace gases.
Nine regions have been selected to investigate the influence of fire counts (biomass burning proxy) and the temperature (vegetation activity proxy) on the HCHO tropospheric columns. The chosen time series has a length of 6 years (from
July 1996 to June 2002). The results show that biogenic sources of HCHO are in many cases the strongest HCHO sources. For example over south east of the USA, the correlation with temperature was very high indicating a strong biogenic source of HCHO (through isoprene emissions). The biomass burning source typically shows more pronounced
seasonal patterns or is even of sporadic nature. Over the Amazon region, the correlation with fires is high indicating that in this area most of the HCHO is caused by biomass burning. In several other regions for both sources moderate correlation coefficients were found.
Scattered sunlight Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) has been successfully
applied to the quantitative determination of trace gas abundances in clean and polluted environments (e.g. to measure
halogen oxides in polar regions, or SO2, NO2, CH2O, glyoxal, and HONO in urban air) In particular volcanic plumes
have been analysed for SO2, BrO, and other species.
We present and discuss promising options for the quantitative analysis of all above mentioned trace gases in biomass
burning plumes, in particular in forest fire plumes. The technique allows of the total emission burden in the plume with
extremely simple and compact ground - based instruments, which also can be operated automatically. Combining the
measured column densities with wind speed data the total trace gas flux from the fire can be determined with good
accuracy. A few examples for possible applications of the technique to monitor fire properties are given.
Monocyclic aromatic hydrocarbons are regarded as one of the most important classes of hydrocarbons in the atmosphere since they are a potential risk for human health and play a key role for photochemical smog formation. Differential Optical Absorption Spectroscopy (DOAS) has been widely used for many trace gases monitoring in the atmosphere however he analysis of aromatic compounds is difficult and the spectral structures from interfering absorbers (e.g. O2) should be very careful to separate correctly. This paper describes a spectral evaluation technique of aromatic compounds from DOAS measurement and summarizes the results of field measurements of benzene toluene xylene (BTX) in Chinese cities Beijing (2000) Shanghai (2001) and Hefei (2002). High concentrations of BTX were observed in Beijing and Shanghai whereas low concentrations in Hefei. This study will provide useful information to help understanding air pollution and atmospheric chemistry processes in urban areas ofChina and thereby contributes towards an improved pollution control strategy in Chinese cities.
Polar Stratospheric Clouds (PSC) appear in the polar zones of the Earth in the winter. These clouds are known to cause enhanced chemical ozone destruction. Methods for optical remote-sensing of PSC in use or under development at the Swedish Institute of Space Physics are discussed with respect to their advantages and limitations. Especially multistatic imaging may become a valuable additional tool for PSC studies.
Using high resolution spectroscopy of the atmospheric oxygen A-band rotational absorption lines (760-780 run}, the probability distribution of optical pathlengths were derived for clear and cloudy skies. A large enhancement of the mean optical pathlength and a systematic change in the form of the probability distribution is found for the cloudy sky. Our observations are compatible with Gamma type distributions. The measurements are compared with a radiative transfer model in order to validate the treatment of the multiple Mie-scattering in clouds. Our measurements show a significant discrepancy compared to a model (DISORT) assuming plane parallel homogenous cloudiness. Larger cloudy sky optical paths may contribute to explain the intensely debated anomalous cloud absorption. These measurements can serve as direct input for atmospheric heating calculations. Keywords: multiple Mie scattering, heterogeneous clouds, oxygen A-band, Gamma distribution
The `multiplex advantage' of diode array detectors over optomechanical spectral scanning devices promises to improve DOAS instruments by reducing measurement time about two orders of magnitude. Alternatively the signal to noise ratio can be improved by adding several scans. Unfortunately, the use of diode arrays gives rise to new problems. Two thin layers on the semiconductor (the protective SiO2 layer plus deposits of vapor) produce spectral interference structures and a thermal recombination current in the diode junction is superimposed to the light signal. Interferences in the protective layer of the diode array impose spectral structures, which are subjected to slow changes due to deposition of vapor upon the diode array and rapid changes caused by varying illumination of the array due to air turbulence in the light path. Detailed model calculations reproduce the etalon structure measured in the laboratory by assuming the existence of a layer of SiO2 (index of refraction n approximately equals 1.6) and a second layer with n approximately equals 1.3 (probably ice). A model considering the geometry of the spectrograph detector system is presented, which describes the influence of changing illumination of the diodes (i.e., caused by atmospheric turbulence) on the etalon structure. The dark current can, in principle, be reduced by cooling the diode array. However, low temperatures increase the complexity of the detector and enhance deposition of vapor, aggravating the etalon problem. The dark current, depending on the charge of the diode, is a complex function of light intensity and exposure time. Thus, subtracting the signal of the darkened diode array to remove the dark current signal leads to apparent nonlinearities. A model for the behavior of the dark current is presented, which allows the use of diode arrays without extensive cooling.
Multireflection cells based on the development by White (1942, 1976) are becoming increasingly popular for studying the chemical composition of the ambient atmosphere by long path UV-visible absorption spectroscopy (DOAS). This is partly due to the special property of the design to eliminate negative effects of vibration and atmospheric fluctuations on the optical quality of the cell. The main parameters limiting the number of reflections (and thus achievable optical length) as well as the vibration tolerance are the size of the main mirror and the number of loci that can be fitted on its surface. Also limiting can be the total reflectivity of the mirrors. Here we present a design variant simplifying the optical alignment of the cell by replacing two pairs of about 90-deg mirrors by quartz prisms and adding a third prism to further increase the stability of the system by a factor of two. An application of a practical design based on those principles to the simultaneous measurement of NO2, NO3, and oxygen dimers (O4) is presented.
DOAS measurements of NO3, NO2, and O3 were performed on the French Atlantic coast in June 1989. The use of two light paths of 6 km length covering the altitude intervals of 5 m - 10 m and 5 m - 40 m allowed the observation of vertical NO3 concentration gradients. Since the NO3 precursors (NO2 and O3) did not show significant differences on the two light paths altitude dependent sinks must be considered. Simultaneously measured DMS concentrations at 1 m and 40 m altitude showed a vertical gradient with higher values at ground level during all nights. For 3 (of 10) nights measured DMS concentrations and gradients could explain the observed NO3 behavior.
The resonant and non-resonant photoacoustic (PA) detection of atmospheric trace gases is discussed with special respect to field measurements. Continuous in-situ methane measurements in a cow shed by a non-resonant PA detector applying an HeNe laser are reported, which allow the model based determination of the methane production rate of cows.
The episodes when the boundary-layer ozone concentrations in the Arctic drop from the normal 30-40 ppb to less than 3 ppb levels have been found to be associated with high concentrations of 'filterable bromine', and were suggested (Barrie et al., 1988; Bottenheim et al., 1990) to be caused by BrO radicals present in the filterable bromine. This paper describes direct measurements of BrO made in April 1992 during the Polar Sunrise Experiment in the Canadien Arctic, using long-path differential optical absorption spectroscopy (LP-DOAS). A new design of the LP-DOAS spectrometer was employed, based on the principle of Platt and Perner (1983), which uses a holographic flat field grating, a slotted disk as scanning device, and a photomultiplier as a detector. A comparison of ozone concentrations measured with the LP-DOAS with in-situ measurements made by a short-path UV-absorption instrument showed agreement within a few ppb. Measurements for April 1992 indicated BrO concentrations between about 3 ppb to 17 ppb. However, no anticorrelation was found between concentrations of ozone and BrO.
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