ABSTRACT
We report on the development of a blue light-emitting-diode-based incoherent broad-band cavity-enhanced absorption spectroscopy (IBBCEAS) instrument for the measurement of the aerosol extinction coefficient at λ=461 nm. With an effective absorption path length of 2.8 km, an optimum detection limit of 0.05 Mm-1 (5×10-10 cm-1) was achieved with an averaging time of 84 s. The baseline drift of the developed spectrometer was about ±0.3 Mm-1 over 2.5 h (1σ standard deviation). The performance of the system was evaluated with laboratory-generated monodispersed polystyrene latex (PSL) spheres. The retrieved complex refractive index of PSL agreed well with previously reported values. The relative humidity (RH) dependence of the aerosol extinction coefficient was measured using IBBCEAS. The measured extinction enhancement factor values for 200 nm dry ammonium sulphate particles at different RH were in good agreement with the modeled values. Field performance of the aerosol extinction spectrometer was demonstrated at the Hefei Radiation Observatory site.
ABSTRACT
In this paper, first time as our knowledge, we describe the development and performance evaluation of a 4.4 µm external cavity quantum cascade laser based laser heterodyne radiometer. Laser heterodyne spectroscopy is a high sensitive laser spectroscopy technique which offers the potential to develop a compact ground or satellite based radiometer for Earth observation and astronomy. An external cavity quantum cascade laser operating at 4. 4 µm, with output power up to 180 mW and narrow line width was used as a local oscillation. The external cavity quantum cascade laser offers wide spectral tuning range, it is tunable from 4.38 to 4.52 µm with model hop free and can be used for simultaneous detections of CO2, CO and N2 O. A blackbody was used as a signal radiation source. Development and fundamental theory of Laser heterodyne spectroscopy was described. The performance of the developed Laser heterodyne radiometer was evaluated by measuring of CO2 spectral at different pressures. Analyses results showed that a signal-to-noise ratio of 86 was achieved which was less than the theoretical value of 287. The spectral resolution of the developed Laser heterodyne spectroscopy is about 0.007 8 cm(-1) which could meet the requirement of high resolution spectroscopy measurement in the case of Doppler linewidth. The experiment showed that middle Infrared laser heterodyne spectroscopy system had high signal-to-noise ratio and spectral resolution, and had broad application prospect in high precision measurement of atmospheric greenhouse gas concentration and vertical profile.
ABSTRACT
Aerosol complex refractive index (ACRI) has attracted intensive attentions due to its significance in modeling aerosol radiative effects. Determinations of ACRI from surface measurements of aerosol scattering and absorption coefficients as well as number size distributions during June, 2008 based on an iterative Mie algorithm were performed. The aim of our study was to introduce an inversion approach with the merits of high time-resolutions to retrieve the optically effective ACRI, especially its imaginary part. Based on simultaneous measurements of aerosol characteristics, mean ACRI value of 1.50 ( ± 0.34)-i0.025 ( ± 0.015) at 550 nm in Hefei in summer was deducted. The lower imaginary parts with higher single scattering albedos and lower scattering Angstrom exponents were obtained for haze periods compared with nonhaze conditions with similar air-mass back-trajectories, indicating more large and scattering particles contributing to the formation of haze episodes. The derived imaginary parts of ACRI related to agricultural biomass burning were in the range from 0.013 to 0.029 at 550 nm. Significant negative correlations between retrieved imaginary parts of ACRI and measured single scattering albedos indicate that our retrieval approach is a reasonable method for determining the imaginary parts of complex refractive indices of aerosol particles.
