Measurement of atmospheric formaldehyde profiles with a laser-induced fluorescence lidar
2012
Formaldehyde is a trace species that plays a key role in atmospheric chemistry. It is an important indicator of nonmethane
volatile organic compound emissions. Also, it is a key reactive intermediate formed during the photochemical
oxidation in the troposphere. Because the lifetime of formaldehyde in the atmosphere is fairly short (several hours), its
presence signals hydrocarbon emission areas. The importance of measuring formaldehyde concentrations has been
recognized by the National Academy's Decadal Survey and two of NASA's forthcoming missions the GEO-CAPE and
GACM target its measurement. There are several techniques some of which are highly sensitive (detection limit ~ 50
parts-per-trillion) for in-situ measurement of formaldehyde and many reported atmospheric measurements. However
there appear to be no reported standoff lidar techniques for range resolved measurements of atmospheric formaldehyde
profiles. In this paper, we describe a formaldehyde lidar profiler based on differential laser induced fluorescence
technique. The UV absorption band in the 352 - 357nm is well suited for laser excitation with frequency tripled
Neodymium lasers and measuring the strong fluorescence in the 390 - 500nm region. Preliminary nighttime
measurements of formaldehyde were demonstrated with a lidar using a commercial Nd:YAG laser (354.7 nm) with a
rather large linewidth (~.02 nm). The measured sensitivity was ~1 ppb at 1 km with 100 meters range resolution even
with this non-optimized system. In this paper we describe our approach for increasing the sensitivity by many orders
and for daytime operation by improving the laser parameters (power and linewidth) and optimizing the receiver.
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