Multiwavelength Lidar Observations Research Articles

How much information do extinction and backscattering measurements contain about the chemical composition of atmospheric aerosol?

Abstract. We theoretically and numerically investigate the problem of assimilating multiwavelength lidar observations of extinction and backscattering coefficients of aerosols into a chemical transport model. More specifically, we consider the inverse problem of determining the chemical composition of aerosols from these observations. The main questions are how much information the observations contain to determine the particles' chemical composition, and how one can optimize a chemical data assimilation system to make maximum use of the available information. We first quantify the information content of the measurements by computing the singular values of the scaled observation operator. From the singular values we can compute the number of signal degrees of freedom, Ns, and the reduction in Shannon entropy, H. As expected, the information content as expressed by either Ns or H grows as one increases the number of observational parameters and/or wavelengths. However, the information content is strongly sensitive to the observation error. The larger the observation error variance, the lower the growth rate of Ns or H with increasing number of observations. The right singular vectors of the scaled observation operator can be employed to transform the model variables into a new basis in which the components of the state vector can be partitioned into signal-related and noise-related components. We incorporate these results in a chemical data assimilation algorithm by introducing weak constraints that restrict the assimilation algorithm to acting on the signal-related model variables only. This ensures that the information contained in the measurements is fully exploited, but not overused. Numerical tests show that the constrained data assimilation algorithm provides a solution to the inverse problem that is considerably less noisy than the corresponding unconstrained algorithm. This suggests that the restriction of the algorithm to the signal-related model variables suppresses the assimilation of noise in the observations.

A trans-Pacific Asian dust episode and its impacts to air quality in the east coast of U.S.

The transport of an intense trans-Pacific Asian dust episode to the Northeast United States (U.S.) is studied using a synergistic suite of observations and models including a ground-based lidar, AERONET-sunphotometer, satellite measurements and global aerosol transport model for New York City (40.821°N, 73.949°W). During the dust intrusion on March 17–19, 2010, the multi-wavelength lidar observations indicate dense dust plumes (∼80% of total column AOD) located between 3 and 9 km altitudes with the lower layer mixing toward the planetary boundary layer (PBL). The column AOD shows a significant increase from 0.08 to 0.38 at 532-nm while the Angstrom exponent indicates a decrease from 1.3 to 0.7. The linear particle depolarization ratio is estimated to be 0.1–0.15 and the single-scattering-albedo shows the dust-like spectral dependence with the value of 0.9–0.95 at 440-nm. The NOAA-NCEP reanalysis and HYSPLIT model indicate that this long-range transport is driven by the strong western jets and travels for 6 days to arrive the U.S. east coast versus the western and northern U.S. Both the NAAPS aerosol transport model and satellite CALIPSO observations for multiple orbits clearly illustrate the dust-dominated aerosol along the transport path. In addition, coincident increase of both particulate matter (PM) and fine soil concentrations indicate the potential impact of transported dust on the air quality that is found to be associated with a large area of sinking air along the U.S. east coast.

Indo‐Asian pollution during INDOEX: Microphysical particle properties and single‐scattering albedo inferred from multiwavelength lidar observations

We present for the first time a comprehensive data set of vertically resolved physical particle properties, and the single‐scattering albedo at 532 nm, derived from multiwavelength lidar observations of pollution plumes advected from India and Southeast Asia out over the tropical Indian Ocean during the northeast monsoon. The parameters follow from the inversion of optical data that were obtained from six‐wavelength aerosol lidar observations at Hulhule Island (4.1°N, 73.3°E), Maldives, in the framework of the Indian Ocean Experiment (INDOEX) (February/March 1999) and three follow‐up campaigns in July and October 1999 and in March 2000. Effective particle radii were 0.20 ± 0.08 μm for pollution plumes above 1‐km height, advected during the northeast monsoon. Volume concentrations ranged from 6 to 44 μm3 cm−3, and surface‐area concentrations were 95–774 μm2 cm−3. The particles showed substantial absorption when advected from northern India. The imaginary part of the wavelength‐independent complex refractive index was as large as 0.045i. The respective mean value was 0.022i ± 0.014i for observations during the northeast monsoon. The mean real part was 1.54 ± 0.11. The mean single‐scattering albedo at 532 nm was 0.90 ± 0.06. Values were as low as 0.8 during advection of air from northern India. On average, less absorbing particles were advected from southern India and Southeast Asia. The numbers indicate that the major contributor to the observed pollution from northern India is absorbing soot‐like material from, e.g., fossil fuel and firewood burning. The effective radius was well correlated with the Ångström exponents of the underlying extinction spectra. Strong correlation of the single‐scattering albedo was found not only with respect to the imaginary part of the complex refractive index but also with respect to the particle extinction‐to‐backscatter ratio. A comparison to Ångström exponents and single‐scattering albedo obtained from Sun photometer and satellite observations showed good representativity of the lidar‐derived quantities. Comparison with results from airborne in situ observations showed substantial deviations of Ångström exponents and single‐scattering albedo.

European pollution outbreaks during ACE 2: Microphysical particle properties and single‐scattering albedo inferred from multiwavelength lidar observations

We present vertically resolved physical particle properties and the single‐scattering albedo at 532 nm of pollution plumes advected from the European continent out over the Atlantic Ocean. The parameters follow from the inversion of optical data, which were obtained from six‐wavelength aerosol lidar observations near Sagres (37°N, 9°W), Portugal, in the framework of Aerosol Characterization Experiment 2 (ACE 2) (June–July 1997). Particle effective radii were 0.15 ± 0.06 μm, volume concentrations ranged from 6 to 27 μm3cm−3, and surface area concentrations were 80 to 1200 μm2cm−3. The particles in general showed negligible absorption, with the mean imaginary part of the wavelength‐independent complex refractive index being at 0.009i± 0.010i. The mean real part was 1.56 ± 0.07. A mean value of 0.95 ± 0.06 was obtained for the single‐scattering albedo at 532 nm. The numbers indicate that the major contributor to the observed pollution was nonabsorbing ammonium‐sulfate‐like material. A very small fraction was contributed by absorbing sootlike material. Correlation analysis showed that effective radius was well correlated with the Ångström exponents of the underlying extinction spectra. Furthermore, correlations of the single‐scattering albedo with the particle extinction‐to‐backscatter ratio and with the imaginary part of the complex refractive index were found. This result shows that the first two parameters contain information about the chemical state of the observed particles.

Constraint Inversion Algorithm of lidar equation for deriving aerosol optical property

A key question of the backward integration algorithm to lidar equation is how to determine the far-end boundary value. This paper develops a Constraint Inversion Algorithm (CIA) for deriving the value and then the aerosol extinction profile from lidar signals, which uses the ground-level horizontal lidar signals as the constraint information. The smaller the wavelength is, the more sensitive to the variation of aerosol extinction to backscatter ratio solved by CIA. According to the property an algorithm is further proposed to simultaneously retrieve the aerosol extinction profile, the size distribution and the imaginary part of its reflective index from the multi-wavelength lidar observations. CIA is tested in the inversion simulations with satisfactory result.

Multiwavelength lidar observations of the decay phase of the stratospheric aerosol layer produced by the eruption of Mount Pinatubo in June 1991

A small three-wavelength (355-, 532-, and 1064-nm) lidar system at NASA Langley Research Center in Hampton, Virginia, has been used since 1992 to make measurements on stratospheric aerosols. The data have been processed to study the decay rate of the stratospheric aerosol layer formed after the eruption of Mount Pinatubo in 1991 and its modulation, the aerosol effective radius, and the column mass loading. The stratospheric aerosol decay curves show annual and biennial cycles as well as short-term changes. At 532 nm, the decay time constant was 302 days for the period from February 1992 to August 1994 and had increased to 645 days for the period from September 1994 to December 1997. By 1996 the integrated stratospheric aerosol backscatter had fallen to levels (7.7 x 10(-5) sr(-1) at 532 nm) close to those seen in 1979 and 1989-1991. This decreasing trend was still continuing in 1997, showing no evidence for any anthropogenic contribution to the stratospheric aerosol.