Dual-channel two-tone frequency modulation tunable diode laser spectrometer for ground-based and airborne trace gas measurements
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We describe a new, dual-channel Tunable Diode Laser (TDL) field spectrometer for the rapid, simultaneous measurement of two species. The instrument uses two-tone frequency modulation (TTFM) with signal detection at approximately equals 12 MHz. Multiplexing is achieved using a dichroic optical element and a mechanical chopper which blocks each beam alternately. A control program running on a dedicated digital signal processor (DSP) allows the registration of the full absorption line shape each millisecond and simultaneous zero overhead on-line data reduction using a multiple linear regression algorithm. Gas exchange through the compact multireflection cell (2.71 volume, total path 53 m.) takes place in approximately equals 200 ms and thus determines the instrument response time. Applications of the spectrometer in airborne and ground based field measurement campaigns are described.