A novel long-path differential optical absorption spectroscopy (DOAS) apparatus for measuring tropospheric trace gases and the first results from its use are presented: We call it the multibeam instrument. It is the first active DOAS device that emits several light beams simultaneously through only one telescope and with only one lamp as a light source, allowing simultaneous measurement along multiple light paths. In contrast to conventional DOAS instruments, several small mirrors are positioned near the lamp, creating multiple virtual light sources that emit one light beam each in one specific direction. The possibility of error due to scattering between the light beams is negligible. The trace-gas detection limits of NO2, SO2, O3, and H2CO are similar to those of the traditional long-path DOAS instrument.
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Note: Important contributions to the residual structures owing to the different instrumental components of the ensemble (the multibeam system, spectrometer, and detector) and the 2σ detection limit resulting from structures for measurements in the wavelength region between 295 and 365 nm and a total light path of 5 km. For the calculations, spectra containing 100 coadded scans and seven coadded lines on the CCD chip, saturated by 70%, were used.
Parts in 109 by volume.
White noise used to calculate the detection limit.
Depending on the intensity of the considered light beam compared with the other light beams (see also Subsection 3.B.4).
White noise, smoothed to resemble the residuum structure, is used to calculate the detection limit; see also Subsection 3.B.3.
Structures from the lamp, the grating of the spectrometer, and the detector sensitivity.
Table 2
Specifications and References for the Trace-Gas Absorption Cross Sections Used in This Study
Fourier-transform spectroscopy.
Measured with a grating spectrometer.
Table 3
Overview of Fit Parameters Calculated by a Linear Fit between the Trace-Gas Slant Column Densities of One Light Beam and the Average Values Derived with Three Light Beams along the Same Light Path during the Intercalibration Measurement
Species
Axis Intercept/Offset (ppbv)
Slope
Residual 1σ (10−3)
Detection Limit (ppbv)
Beam 1
NO2
1.54
0.97
0.55
0.86
SO2
−0.04
0.96
0.55
0.28
O3
−2.16
0.97
0.55
9.48
H2CO
−0.006
0.97
0.55
1.27
Beam 2
NO2
−0.28
1.01
0.75
1.18
SO2
0.02
0.99
0.75
0.38
O3
0.53
1.02
0.75
12.95
H2CO
−0.002
0.98
0.75
1.73
Beam 3
NO2
−1.29
1.03
0.56
0.88
SO2
0.01
1.05
0.56
0.28
O3
1.59
1.01
0.56
9.65
H2CO
0.007
1.04
0.56
1.29
Tables (3)
Table 1
Important Contributions to the Residual Structure and Their Resulting Detection Limits
Note: Important contributions to the residual structures owing to the different instrumental components of the ensemble (the multibeam system, spectrometer, and detector) and the 2σ detection limit resulting from structures for measurements in the wavelength region between 295 and 365 nm and a total light path of 5 km. For the calculations, spectra containing 100 coadded scans and seven coadded lines on the CCD chip, saturated by 70%, were used.
Parts in 109 by volume.
White noise used to calculate the detection limit.
Depending on the intensity of the considered light beam compared with the other light beams (see also Subsection 3.B.4).
White noise, smoothed to resemble the residuum structure, is used to calculate the detection limit; see also Subsection 3.B.3.
Structures from the lamp, the grating of the spectrometer, and the detector sensitivity.
Table 2
Specifications and References for the Trace-Gas Absorption Cross Sections Used in This Study
Fourier-transform spectroscopy.
Measured with a grating spectrometer.
Table 3
Overview of Fit Parameters Calculated by a Linear Fit between the Trace-Gas Slant Column Densities of One Light Beam and the Average Values Derived with Three Light Beams along the Same Light Path during the Intercalibration Measurement