Aerosol Backscatter Profiles Research Articles

Aerosol hygroscopic properties as measured by lidar and comparison with in situ measurements

We demonstrate the ability of a single‐wavelength backscatter lidar to provide information on the uptake of water vapor by aerosol in a well‐mixed, cloud‐capped, boundary layer. Aerosol hydration has important consequences for the effect of aerosols on the Earth's radiation budget. A vertically pointing, airborne lidar is used to measure vertical profiles of aerosol backscatter beneath a stratocumulus cloud deck. In situ aircraft thermodynamic measurements are used to derive simultaneous profiles of relative humidity (RH) under the assumption that the boundary layer is well mixed. The change in backscatter is derived as a function of relative humidity over the range ∼85% RH to ∼98.5% RH. In situ measurements of the aerosol size distribution and composition are used to calculate the expected enhancement in backscatter due to equilibrium uptake of water vapor. Comparison between lidar backscatter enhancement as a function of RH and that derived from the in situ aerosol size distribution and composition measurements shows good agreement. Conditional sampling on strong updrafts/downdrafts indicates that aerosol backscatter tends to be higher in downdrafts than it is in updrafts for the same RH range. This is consistent with the concept of inertia of the larger hydrated particles to growth/evaporation at short timescales but may also be due to a bias in the way that lidar‐derived cloud base is interpreted in updrafts versus downdrafts. Calculations of enhancement in total scatter due to water vapor uptake with enhancement in backscatter suggest that the effects agree to within ∼20% of one another for RH < ∼95% but that they differ significantly for RH > 95%.

A new algorithm for simultaneous ozone and aerosol retrieval from tropospheric DIAL measurements

Aerosols can introduce significant errors in measurements of trace gas concentration profiles with differential absorption lidar (DIAL). Here, a solution to counteract the aerosol effect is introduced, which does not depend on additional measurements and is applicable when absorption at the off-wavelength is not negligible. It is based on a numerical solution of the coupled lidar equations and calculates the trace-gas concentration and the aerosol backscatter ratio simultaneously. Consequently, most of the errors introduced by aerosols to DIAL measurements are inherently corrected and, additionally, the aerosol backscatter profiles at the DIAL wavelengths are obtained.

A polar cloud analysis based on Micro-pulse Lidar measurements at Ny-Alesund, Svalbard and Syowa, Antarctica

In order to acquire long-term data sets of backscatter profiles of clouds and aerosols in the polar regions, we started Micro-pulse Lidar (MPL) measurements at Ny-Alesund, Svalbard in March 1998 and at Syowa Station, Antarctica in January 2001. The MPL data have been acquired and analyzed for 11 months from the Arctic measurement and for 12 months from the Antarctic measurement. Fine structures of clouds and their temporal change were observed by MPLs with a 30-m range resolution and a 1-min time average. Statistical features of the polar clouds were investigated from the Arctic and Antarctic measurements. Results from the preliminary analysis indicated some differences of seasonal cloud appearance and cloud base heights at the two polar sites. Some features obtained in this study are as follows: over Syowa Station, Antarctica, high altitude clouds tended to appear in (austral) summer, and low altitude appeared more often in (austral) spring and autumn. On the other hand, low altitude clouds in the Arctic were observed particularly in summer and early autumn. Clear sky days were not expected in this season. In both polar regions, the winter clouds were relatively thin compared to the summer clouds.

Three years of routine Raman lidar measurements of tropospheric aerosols: Backscattering, extinction, and residual layer height

Abstract. We have performed a three-year series of routine lidar measurements at preselected times. The measurements were performed between 1 December 1997, and 30 November 2000, at Kühlungsborn, Germany (54°07' N, 11°46' E). Using a Rayleigh/Mie/Raman lidar system, we measured the aerosol backscatter coefficients at three wavelengths and the extinction coefficient at one wavelength. The present data analysis focuses on after-sunset Raman measurements obtained on cloud-free days. Aerosol backscatter profiles are available for altitudes above 100 m, while the majority of the extinction measurements has been restricted to heights above the residual layer. The residual layer shows an annual cycle with its maximum height in summer (2000 m) and minimum height in winter (850 m). The backscatter coefficients in the residual layer were found to be about 10 times higher than above. The mean aerosol optical depth above the residual layer and below 5 km is 0.3(±1.0) x10-2 in summer, and 1.5(±1.0) x10-2 in winter, which almost is negligible compared to values measured in during daytime in the planetary boundary layer. A cluster analysis of the backward trajectories yielded two major directions of air mass origin above the residual layer and 4 major directions inside. A marked difference between the aerosol properties dependent on the air mass origin could be found for air masses originating from the west and travelling at high wind speeds. Comparing the measured spectral dependence of the backscatter coefficients with data from the Global Aerosol Data Set, we found a general agreement, but only a few conclusions with respect to the aerosol type could be drawn due to the high variability of the measured backscatter coefficients.

Aerosol optical properties during INDOEX 1999: Means, variability, and controlling factors

As part of the Indian Ocean Experiment (INDOEX) 1999 Intensive Field Phase, measurements of aerosol properties were made on board the R/V Ronald H. Brown in the Indian Ocean north and south of the Intertropical Convergence Zone (ITCZ) in the Arabian Sea and in the Bay of Bengal. On the basis of air mass trajectories, eight air mass source regions were identified including the southern hemisphere Atlantic; southern hemisphere Indian Ocean; northern hemisphere Indian Ocean; east Indian subcontinent where trajectories came from near Calcutta, through the southern portion of India, and then to the ship; Indian subcontinent where trajectories came from across central India to the ship; Arabia; Arabia/Indian subcontinent, a mixed region where lower‐level trajectories came from Arabia and upper‐level trajectories came from India; and Arabian Sea/coastal India where trajectories came from along the coast of India to the ship. Properties of the aerosol measured in the marine boundary layer included chemical composition, number size distribution, and scattering and absorption coefficients. In addition, vertical profiles of aerosol backscatter and optical depth were measured. Presented here as a function of air mass source region are the concentrations and mass fractions of the dominant aerosol chemical components, the fraction of the extinction measured at the surface due to each component, mass extinction efficiencies of the individual components, aerosol scattering and absorption coefficients, single scattering albedo, Ångström exponent, and optical depth. All results except aerosol optical depth are reported at the measurement relative humidity of 55 ± 5%. For air masses that originated from the two southern hemisphere marine regions (southern hemisphere Atlantic and Indian Ocean), sea salt dominated the extinction by sub‐1 μm and sub‐10 μm aerosol particles. The ratios of sub‐1μm to sub‐10 μm extinction were the lowest measured of all air mass source regions (mean values of 28 and 40%) due to the dominance of the aerosol mass by supermicron sea salt. In addition, aerosol optical depths were the lowest measured averaging 0.06 ± 0.03. Non‐sea‐salt (nss) sulfate aerosol concentrations in air masses from the northern hemisphere Indian Ocean were a factor of 6 higher than those in southern hemisphere air masses, while submicron sea‐salt concentrations were comparable. Sulfate aerosol made up 40% of the sub‐1μm extinction, while sea salt dominated the sub‐10 μm extinction. Aerosol optical depths for this source region averaged 0.10 ± 0.03. A mean single scattering albedo near 0.89 and detectable black carbon (BC) concentrations (0.14 ± 0.05 μg m−3) indicated the transport of continentally derived aerosol to the ITCZ. The two regions influenced by low‐level (500 m) airflow from Arabia had higher concentrations of submicron nss sulfate, particulate organic matter (POM), and inorganic oxidized material (IOM) than were observed in the marine regions. Concentrations of supermicron IOM were comparable to supermicron sea‐salt concentrations. Nss sulfate aerosol dominated the sub‐1 μm extinction and made significant contributions to the sub‐10 μm extinction. Sea salt dominated the supermicron extinction. Mean BC contributions to submicron extinction were 8 and 12%. Single scattering albedo values averaged 0.93 ± 0.02 and 0.89 ± 0.02 for these two source regions. Aerosol optical depths averaged 0.19 ± 0.12 and 0.38 ± 0.07 with the higher value due to upper‐level (2500 m) flow from India. Regions influenced by low‐level airflow from the Indian subcontinent had the highest submicron nss sulfate, POM, BC, and IOM concentrations measured during the experiment. Supermicron sea‐salt concentrations were lower than or comparable to supermicron nitrate concentrations. Sub‐1 μm and sub‐10 μm extinction were dominated by nss sulfate aerosol although a burning component consisting of BC, KNO3, and K2SO4 made a nearly equivalent contribution. These regions had a mean single scattering albedo of 0.85 ± 0.01, the lowest measured for any region. Mean aerosol optical depths were highest (0.3 to 0.4) for regions with low‐level or upper‐level airflow from the Indian subcontinent.

Profiling of atmospheric water vapor with MIR and LASE

Concurrent measurements of atmospheric water vapor profiles were conducted over the Atlantic Ocean on September 25, 1995 with both the Millimeter-wave Imaging Radiometer (MIR) and Lidar Atmospheric Sounding Experiment (LASE) on board the NASA ER-2 aircraft. LASE provides high precision measurements of both aerosol backscatter and water vapor profiles; aerosol backscatter has a vertical resolution of 60 m while the water vapor profiles have a resolution of 330 m in the low-to-mid troposphere and 550 m in the upper troposphere. Therefore, LASE measurements provide an excellent resource for assessing the capabilities and limitations of MIR as a water vapor profiler. Previously, the water vapor profiles retrieved from the MIR measurements have been compared with those of rawinsonde and Raman lidar observations at point locations. The frequency and extent of the comparisons made in that fashion were largely constrained by the requirement of near coincidence in time and space. The data acquired concurrently by MIR and LASE from this ER-2 aircraft flight enable the comparison of MIR-retrieved and LASE-measured moisture profiles over a long stretch of time and space. In addition, the LASE-measured profiles of aerosol backscatter provide a resource to assess the impact of clouds on the retrieval of water vapor profiles from the MIR measurements. It is shown that profiles of water vapor mixing ratio retrieved from the MIR data generally conform to those measured by the LASE; however, differences in the values of mixing ratio at individual altitude levels are quite often not small. The standard deviations of these differences are found to be about. /spl plusmn/0.98, /spl plusmn/0.84, /spl plusmn/0.95, /spl plusmn/0.42, and /spl plusmn/0.06 g/kg at altitudes of 1.25, 2.75, 4.75, 7.25, and 10.25 km. It is demonstrated that a substantial portion of these differences are due to the poor vertical resolution inherent in the profile retrieval using the MIR radiometric measurements. Additionally, MIR water vapor profiling under cloudy conditions is demonstrated, and it is shown that location and height of the low-altitude clouds estimated from the retrieval process were generally consistent with those observed by the LASE. For study cases where cirrus clouds are present, retrievals from the MIR data over-estimate the mixing ratio; this over-estimate is provoked by brightness temperature decreases that occur at 183-220 GHz within these regions. Undoubtedly, the retrieval method needs an additional procedure to account for the mm-wave scattering by cloud ice particles so that water vapor profiling can be improved within regions where cirrus clouds are present.

Aerosol climatology for the planetary boundary layer derived from regular lidar measurements

Regular aerosol extinction and backscatter measurements using a UV Raman Lidar have been performed for almost 3 years in Hamburg in the frame of the German Lidar Network. A set of 92 aerosol extinction and 164 aerosol backscatter profiles has been used for statistical investigations. Mean values and variances of the aerosol extinction and backscatter in the boundary layer have been calculated. Large fluctuations during the whole year have been found. The measured aerosol extinction over Hamburg shows a seasonal cycle with highest values in early fall and a second less prominent peak in spring. An analysis of the data using back trajectories showed a dependence of the aerosol extinction on the origin of the air mass. The residence time of the air mass over industrialized areas was found to be an important parameter for the measured aerosol extinction at Hamburg. However, only a small part of the total variability could be explained by the air mass origin. For 75 cases of aerosol extinction measurements under cloud-free conditions, the aerosol backscatter profile and therefore, the lidar ratio as a function of altitude could be determined. Winter measurements of the lidar ratio are often close to model results for maritime aerosol, the summer measurements are close to the model results for urban or continental aerosols. The high quality of the data has been proven by intercomparisons with other lidar systems and with star photometer measurements of the aerosol optical depth during the Lindenberg Aerosol Characterization Experiment (LACE'98) field campaign.

Development of a tunable IR lidar system

A differential absorption lidar system (DIAL) based on a continuously tunable optical parametric amplifier (OPA) pumped by a Nd : YAG laser (200 mJ at λ=355 nm) operating at a maximum pulse repetition rate of 100 Hz has been developed. The system provides continuously tunable coherent radiation in the Visible–near IR range (0.4–2.5 μm), allowing to perform DIAL measurements in a spectral region where most of atmospheric constituents and pollutants display absorption lines. The spectral width of the OPA system is line-narrowed by using a master oscillator dye laser as seeder, achieving a linewidth of 0.04 cm −1 (FWHM), a spectral purity larger than 99% and a frequency stability better than 1 pm h −1, with an output energy in the IR of 1–10 mJ. The OPA system was used to perform DIAL measurements in the lower troposphere. Preliminary results in terms of water vapor content and aerosol backscattering profiles are presented and discussed.

Variability in the stratospheric background aerosol over Mauna Loa Observatory

The stratospheric aerosol layer above Mauna Loa Observatory (MLO), Hawaii, has been at low background levels for the past 5 years. This is the first time that an extended non‐volcanic background aerosol period has been observed since modern measurements began in the early 1960s. Lidar backscatter at 532 nm shows a distinct maximum in winter and minimum in summer. The five annual cycles have included three easterly phases and two westerly phases of the quasibiennial oscillation (QBO). Differences in aerosol backscatter versus altitude profiles are seen for different QBO phases. There is also a switching of about 25% in the magnitude of the aerosol backscatter on a weekly time scale with varying particle size derived from multiwavelength data. Assumption of a tropical particle source at background suggests that the differing particle regimes are tropical and midlatitude.

Springtime enhancement of upper tropospheric aerosol at 45°S

Monthly sonde data for Lauder in Central Otago, New Zealand show profiles of aerosol backscatter from the surface to over 30 km altitude. The tropospheric data vary by season, with greater aerosol backscatter throughout the free troposphere in springtime. Aerosol mixing ratios in layers in the upper troposphere at these times are often much higher than anywhere else above the boundary layer, suggesting that they arise from horizontal transport. Ozone measurements from the sonde show correspondence in vertical structure to the backscatter data and also seasonal enhancement. The latter correlates with aerosol, but competing causes of ozone enhancement make the correspondence indistinct. High concentrations of carbon monoxide are observed by Fourier transform spectroscopy in spring, and altitude profiles derived from line shape suggest that the peak in CO occurs in the same altitude range as the aerosol enhancement.

Comparison of aerosol optical depth inferred from surface measurements with that determined by Sun photometry for cloud‐free conditions at a continental U.S. site

Evaluation of the forcing of climate by aerosol scattering of shortwave radiation in cloud‐free conditions (direct aerosol forcing) requires knowledge of aerosol optical properties on relevant spatial and temporal scales. It is convenient to measure these properties at the surface. However, before these measurements can be used to quantitatively estimate direct climate forcing, it is necessary to determine the extent to which these properties are representative of the entire atmospheric column. In this paper we compare aerosol optical depth (AOD) determined by Sun photometry at the Southern Great Plains (SGP) Atmospheric Radiation Measurement (ARM) site in north central Oklahoma for several cloud‐free days with estimates of AOD based on two methods. First, the aerosol extinction measured at the surface (taken as the sum of the aerosol scattering and absorption coefficients at instrumental relative humidity of∼20%) is multiplied by the mixing height determined from temperature profiles from radiosonde measurements. Even under conditions of vigorous midday mixing this approach underestimates AOD by as much as 70% using dry aerosol measurements and by roughly 40% when hygroscopic growth of aerosol under ambient relative humidity is taken into account. This discrepancy is attributed primarily to underestimation of aerosol column extinction, as confirmed by examination of normalized aerosol backscatter profiles obtained from micropulse lidar (MPL), which show substantial contributions of aerosol loading above the atmospheric boundary layer. The second approach uses MPL profiles of normalized aerosol backscatter to estimate the vertical profile of aerosol extinction using surface values. The resulting AOD's are on average 30% less than measured values. This discrepancy is attributed to hygroscopic growth of aerosols in the atmospheric column. The results show that at the SGP site even under conditions of vigorous mixing in the atmospheric boundary layer the aerosol optical depth cannot be estimated with surface measurements of aerosol extinction unless information on the vertical profile of aerosol extinction is taken into account.

ACE-2 multiple angle micro-pulse lidar observations from Las Galletas, Tenerife, Canary Islands

Multiple-angle micro-pulse lidar (MPL) observations were made at Las Galletas on Tenerife, Canary Islands during the Aerosol Characterization Experiment-2 (ACE-2) conducted June−July, 1997. A principal objective of the MPL observations was to characterize the temporal/spatial distributions of aerosols in the region, particularly to identify and profile elevated Saharan dust layers which occur intermittently during the June−July time period. Vertical and slant angle measurements taken 16 and 17 July characterize such an occurrence, providing aerosol backscatter, extinction, and optical depth profiles of the dust layer between 1 and 5 km above mean sea level (MSL). Additionally, horizontal measurements taken in Las Galletas throughout the 6-week period provide a time profile of the varying aerosol extinction at the surface. This profile exhibits the alternating periods of clean maritime air and pollution outbreaks that typified the region. Horizontal measurements also provide some evidence suggesting the possible influx of Saharan dust from the free troposphere to the surface. This paper presents estimates of aerosol optical properties retrieved from the multi-angle MPL measurements in addition to an outline of the methodologies employed to obtain these results.

Ground-based lidar measurements of aerosols during ACE-2: instrument description, results, and comparisons with other ground-based and airborne measurements

A micro-pulse lidar system (MPL) was used to measure the vertical and horizontal distribution of aerosols during the Aerosol Characterization Experiment 2 (ACE-2) in June and July of 1997. The MPL measurements were made at the Izaña observatory (IZO), a weather station located on a mountain ridge (28°18’ N, 16°30’ W, 2367 m asl) near the center of the island of Tenerife, Canary Islands. The MPL was used to acquire aerosol backscatter, extinction, and optical depth profiles for normal background periods and periods influenced by Saharan dust from North Africa. System tests and calibration procedures are discussed, and an analysis of aerosol optical profiles acquired during ACE-2 is presented. MPL data taken during normal IZO conditions (no dust) showed that upslope aerosols appeared during the day and dissipated at night and that the layers were mostly confined to altitudes a few hundred meters above IZO. MPL data taken during a Saharan dust episode on 17 July showed that peak aerosol extinction values were an order of magnitude greater than molecular scattering over IZO, and that the dust layers extended to 5 km asl. The value of the dust backscatter−extinction ratio was determined to be 0.027±0.007 sr−1. Comparisons of the MPL data with data from other co-located instruments showed good agreement during the dust episode.

Wind-Flow Patterns in the Grand Canyon as Revealed by Doppler Lidar

Many interesting flow patterns were found in the Grand Canyon by a scanning Doppler lidar deployed to the south rim during the 1990 Wintertime Visibility Study. Three are analyzed in this study: 1) flow reversal in the canyon, where the flow in the canyon was in the opposite direction from the flow above the canyon rim; 2) under strong, gusty flow from the southwest, the flow inside and above the canyon was from a similar direction and coupled; and 3) under light large-scale ambient flow, the lidar found evidence of local, thermally forced up- and down-canyon winds in the bottom of the canyon. On the days with flow reversal in the canyon, the strongest in-canyon flow response was found for days with northwesterly flow and a strong inversion at the canyon rim. The aerosol backscatter profiles were well mixed within the canyon but poorly mixed across the rim because of the inversion. The gusty southwest flow days showed strong evidence of vertical mixing across the rim both in the momentum and in the aerosol backscatter profiles, as one would expect in turbulent flow. The days with light ambient flow showed poor vertical mixing even inside the canyon, where the jet of down-canyon flow in the bottom of the canyon at night was often either cleaner or dirtier than the air in the upper portions of the canyon. In a case study presented, the light ambient flow regime ended with an intrusion of polluted, gusty, southwesterly flow. The polluted, high-backscatter air took several hours to mix into the upper parts of the canyon. An example is also given of high-backscatter air in the upper portions of the canyon being mixed rapidly down into a jet of cleaner air in the bottom of the canyon in just a few minutes.

Extracting vertical winds from simulated clouds with ground-based coherent Doppler lidar

The performance of mean velocity estimators is determined by computer simulations for solid-state coherent Doppler lidar measurements of wind fields at a cloud interface with deterministic profiles of velocity and aerosol backscatter. Performance of the velocity estimates is characterized by the standard deviation about the estimated mean and the bias referenced to the input velocity. A new class of estimators are required for cloud conditions, as traditional techniques result in biased estimates. We consider data with high signal energy that produces negligible random outliers.

Dissipation and mixing of a small‐scale stratospheric intrusion in the upper troposphere

Ground‐based lidar measurements of ozone mixing ratio and aerosol backscatter profiles above Fritz Peak Observatory near Boulder, Colorado (39.9° N, 105.3°W), are used in conjunction with nearby radiosonde profiles and GOES‐9 satellite water vapor imagery to describe the mixing of a dry, ozone‐rich streamer of stratospheric air into the upper troposphere over the western United States on June 30 and July 1, 1997. The satellite images show that the streamer extended over more than 2000 km from the eastern Pacific Ocean to the central United States beneath the subtropical jet with a width of ∼200 km over Colorado. The streamer persisted with little change for more than 6 hours of lidar observations and then abruptly disappeared to be replaced by a region of unusually low ozone. The disappearance of the streamer is attributed to localized convective activity induced by the incursion of a moist, ozone‐poor subtropical air mass between the streamer and the convective boundary layer. This activity disrupted the streamer and, together with shear‐induced mixing beneath the descending subtropical jet, merged ∼6.5 × 1031 molecules of ozone into the free troposphere over Colorado between 8 and 10 km. These results illustrate the effectiveness of small‐scale processes for the irreversible transfer of stratospheric air into the upper troposphere.

Raman lidar measurements of aerosol extinction and backscattering: 2. Derivation of aerosol real refractive index, single‐scattering albedo, and humidification factor using Raman lidar and aircraft size distribution measurements

Aerosol backscattering and extinction profiles measured by the NASA Goddard Space Flight Center Scanning Raman Lidar (SRL) during the remote cloud sensing (RCS) intensive operations period (IOP) at the Department of Energy Atmospheric Radiation Measurement (ARM) southern Great Plains (SGP) site during two nights in April 1994 are discussed. These profiles are shown to be consistent with the simultaneous aerosol size distribution measurements made by a PCASP (Passive Cavity Aerosol Spectrometer Probe) optical particle counter flown on the University of North Dakota Citation aircraft. We describe a technique which uses both lidar and PCASP measurements to derive the dependence of particle size on relative humidity, the aerosol real refractive index n, and estimate the effective single‐scattering albedo ω0. Values of n ranged between 1.4–1.5 (dry) and 1.37–1.47 (wet); ω0 varied between 0.7 and 1.0. The single‐scattering albedo derived from this technique is sensitive to the manner in which absorbing particles are represented in the aerosol mixture; representing the absorbing particles as an internal mixture rather than the external mixture assumed here results in generally higher values of ω0. The lidar measurements indicate that the change in particle size with relative humidity as measured by the PCASP can be represented in the form discussed by Hanel [1976] with the exponent γ = 0.3 ± 0.05. The variations in aerosol optical and physical characteristics captured in the lidar and aircraft size distribution measurements are discussed in the context of the meteorological conditions observed during the experiment.

Raman lidar measurements of aerosol extinction and backscattering: 1. Methods and comparisons

This paper examines the aerosol backscattering and extinction profiles measured at night by the NASA Goddard Space Flight Center Scanning Raman Lidar (SRL) during the remote cloud sensing (RCS) intensive operations period (IOP) at the Department of Energy Atmospheric Radiation Measurement (ARM) southern Great Plains (SGP) site in April 1994. These lidar data are used to derive aerosol profiles for altitudes between 0.015 and 5 km. Since this lidar detects Raman scattering from nitrogen and oxygen molecules as well as the elastic scattering from molecules and aerosols, it measures both aerosol backscattering and extinction simultaneously. The aerosol extinction/backscattering ratio varied between approximately 30 sr and 75 sr at 351 nm. Aerosol optical thicknesses derived by integrating the lidar profiles of aerosol extinction measured at night between 0.1 and 5 km are found to be about 10–40% lower than those measured by a Sun photometer during the day. This difference is attributed to the contribution by stratospheric aerosols not included in the lidar estimates as well as to diurnal differences in aerosol properties and concentrations. Aerosol profiles close to the surface were acquired by pointing the lidar nearly horizontally. Measurements of aerosol scattering from a tower‐mounted nephelometer are found to be 40% lower than lidar measurements of aerosol extinction over a wide range of relative humidities even after accounting for the difference in wavelengths. The reasons for this difference are not clear but may be due to the inability of the nephelometer to accurately measure scattering by large particles.

HNO3 and PSC Measurements from the Transall: Sequestering of HNO3 in the Winter of 1994/95

In the winter of 1994/95 the TRANSALL research aircraft performed several flights in the region of the Arctic vortex during the period of low stratospheric temperatures. The results of simultaneous measurements of HNO3 column amounts by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) and of aerosol backscatter profiles by the Ozone Lidar EXperiment (OLEX) are presented for two typical flight scenarios across the polar vortex boundary on December 17, 1994 and January 11/12, 1995. On December 17 and January 12, the column amounts of gaseous HNO3 decreased significantly in regions with low stratospheric temperatures. This decrease was correlated with the extent of the polar stratospheric clouds. Depolarisation measurements showed that type Ib PSCs were observed primarily, but equilibrium calculations for H2SO4/HNO3/H2O aerosols seem to underestimate the observed HNO3 sequestering.

Analysis techniques for the recovery of winds and backscatter coefficients from a multiple-channel incoherent Doppler lidar

The University of Michigan has developed an incoherent-detection Doppler lidar system that continuously measures vertical profiles of horizontal winds and aerosol backscatter. An overview of the instrument is given, followed by a description of improvements that have been made to control the system stability. Most notably, an active feedback system has been implemented to improve the laser frequency stability. A detailed forward model of the instrument is developed that includes many subtle effects, such as detector nonlinearity. A nonlinear least-squares inversion method is then described that permits the recovery of Doppler shift and aerosol backscatter without requiring assumptions about the molecular component of the signal. Examples of wind and aerosol backscatter profiles are shown to illustrate the capabilities of the fitting method.