Column closure studies are a tool to assess whether in situ and remote measurements of aerosol optical properties on a given aircraft are mutually consistent. In this paper we describe aerosol and water vapor column closure studies on the basis of instrumentation flown aboard the NCAR C‐130 aircraft in the ACE‐Asia field experiment in March–May 2001. For in situ observations, aerosol particles were sampled through a newly designed low‐turbulence inlet (LTI). In 28 profiles extending to altitudes of up to 8 km, the in situ observations of scattering and absorption were compared to measurements with the six‐channel NASA Ames Airborne Tracking Sun photometer (AATS‐6). The comparison of Sun photometer and in situ‐derived layer aerosol optical depth (AOD) at 550 nm showed agreement (closure) within the measurement uncertainties in 25 out of 28 case studies. The average difference in layer AOD derived from the two methods was 0.03, corresponding to an average difference of 11.5%. The uncertainties in AATS‐6‐derived layer AOD ranged between 5 and 59% (with a mean of 22%), and for the first time included an estimate for the uncertainty in layer AOD caused by possible horizontal variability in AOD encountered in the vertical profile. The average uncertainty in AATS‐6‐derived layer AOD due to possible horizontal variability alone was 19%. The uncertainties in in situ‐derived layer AOD were between 10 and 55% (with a mean of 19%). Stratification of the extinction closure data by ambient relative humidity (RH) revealed that in situ‐derived aerosol extinction at low ambient relative humidity (<20% RH) tended to be slightly less than Sun photometer‐derived aerosol extinction, while in situ‐derived aerosol extinction at higher relative humidity was slightly greater than the Sun photometer‐derived values. Stratification of the extinction closure data by the fine mode fraction of scattering indicated a modest enhancement of coarse mode extinction in the combined LTI/plumbing system. Analogous closure studies for layer water vapor and water vapor density showed that AATS‐6 measured these quantities with very high accuracy, with correlation coefficients of 0.989 and 0.955 (rms differences of 10% and 33%), respectively.

[1] We present the results of aerosol forecast during the ACE-Asia field experiment in spring 2001, using the Georgia Tech/Goddard Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) model and the meteorological forecast fields from the Goddard Earth Observing System Data Assimilation System (GEOS DAS). The model provides direct information on aerosol optical thickness and concentrations for effective flight planning, while feedbacks from measurements constantly evaluate the model for successful model improvements. We verify the model forecast skill by comparing modelpredicted aerosol quantities and meteorological variables with those measured by the C-130 aircraft. The GEOS DAS meteorological forecast system shows excellent skills in predicting winds, relative humidity, and temperature, with skill scores usually in the range of 0.7-0.99. The model is also skillful in forecasting pollution aerosols, with most scores above 0.5. The model correctly predicted the dust outbreak events and their transPacific transport, but it constantly missed the high dust concentrations observed in the boundary layer. We attribute this ''missing'' dust source to desertification regions in the Inner Mongolia Province in China, which have developed in recent years but were not included in the model during forecasting. After incorporating the desertification sources, the model is able to reproduce the observed boundary layer high dust concentrations over the Yellow Sea. We demonstrate that our global model can not only account for the largescale intercontinental transport but also produce the small-scale spatial and temporal variations that are adequate for aircraft measurements planning.

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Abstract. The extinction-to-backscatter ratio S is a crucial parameter for quantitative interpretation of lidar data, yet empirical knowledge of $ for tropospheric aerosols is extremely limited. Here we review that knowledge and extend it using a recently developed in situ technique that employs a 180 ø backscatter nephelometer. This technique allows robust quantification of measurement uncertainties and permits correlations with other aerosol and meteorological properties to be explored. During 4 weeks of nearly continuous measurements in central Illinois, $ was found to vary over a wide range, confirming previous indications that geographical location by itself is not necessarily a good predictor. The data suggest a modest dependence of S on relative humidity, but this explains only a small portion of the variation. Most variation was associated with changes between two dominant ak mass types: rapid transport from the northwest and regional stagnation. The latter category displayed much higher aerosol concentrations and a systematically higher and more tightly constrained range of S. Averages and standard deviations were 64 __ 4 sr for the stagnant category and 40 q-9 sr for the rapid transport category. Considering the 95% confidence precision uncertainty of the measurements, the difference between these averages is at least 13 sr and could be as large as 35 sr. The wavelength dependence of light scattering, as measured by a conventional nephelometer, is shown to have some discriminatory power with respect to $.

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[1] Aircraft in situ and Raman lidar profiles of aerosol light extinction ( ep ) and 180Њ backscattering (␤ p ) are compared for 6 days during the Indian Ocean Experiment (INDOEX). The measurements of ep and ␤ p were made from the National Center for Atmospheric Research C-130 aircraft using two integrating nephelometers to measure light scattering and one Radiance Research Particle Soot Absorption Photometer to measure light absorption. Particulate 180Њ backscattering was measured in situ using a new instrument, the 180Њ backscatter nephelometer. The Institute for Tropospheric Research Raman lidar was located on the island of Hulule (4.18ЊN, 73.53ЊE), and all of the in situ profiles presented are from descents into the Hulule airport. Aerosol optical depth was also measured from Hulule using a Sun photometer, and these data are included in the intercomparison. On average, the lidar-derived values of ep and ␤ p are ϳ30% larger than the in situ-derived values to a 95% confidence interval. Possible reasons for the overall discrepancy are (1) a low bias in the in situ measurements because of losses in the C-130 Community Aerosol Inlet; (2) underestimation of the humidification effect on light extinction in the in situ measurements; (3) overestimation of ep and ␤ p in the lidar because of subvisible cloud contamination; (4) errors in data processing that could be biasing either measurement, though the lidar retrievals are especially sensitive to this type of error. Temporal and spatial variability also appear to be the source of at least some of the discrepancy in two of the six cases, none of which are well collocated.

The snow surfaces of the high plateaus of the East Antarctic and Greenland ice sheets are used to determine multi-year drift in the sensitivity of the visible channel of the Advanced Very High Resolution Radiometer (AVHRR) on the polar-orbiting satellites NOAA-9, 10, and 11. Bidirectional re ectance distribution functions are empirically derived for the months of October -February (Antarctica) and April-August (Greenland) using a simpli ed atmospheric model. The bidirectional re ectance of the snow surface should not change from year to year for near-nadir satellite views. Therefore, drift in the derived bidirectional re ectance distribution function is interpreted as drift in channel sensitivity.Several factors make the snow surface of an ice sheet suitable as a calibration target for visible and near-UV channels. (1) In this spectral region, snow has a very high albedo (>97% ) that is invariant with grain size and incidence angle.(2) On the high plateaus the temperatures are always far below freezing so the surface consists of cold ne-grained snow, and there is negligible contamination.(3) The ice sheet surfaces are uniform and at across large areas. (4) Ozone is the only signi cant variable absorber in this spectral region, and its absorption can be accounted for if the ozone amount is known. (5) Cloud detection and removal is not necessary, because the thin clouds over the high ice sheets apparently do not alter the near-nadir re ectance, as they do over dark surfaces.Our analysis indicates that the visible channel on NOAA-9 degraded linearly over the 3.5-year lifetime of the instrument by 5.3±0.1% per year. NOAA-10 showed non-linear behaviour that could be tted with a fourth-order polynomial. Data from NOAA-11 prior to the eruption of Mt Pinatubo showed a linear increase in sensitivity of 2.3±0.2% per year. The derived drifts are not sensitive to the choice of spatial and temporal averaging scales, the choice of months and gridboxes, and whether or not a cloud-rejection scheme is used. *née Doherty.

[1] The Taiwan air quality model incorporating a dust module was applied to calculate masses of supermicron (diameter greater than 1 mm) and submicron particles (diameter less than 1 mm) and their dust fractions during the ACE-Asia airborne experiments over the northwestern Pacific. The results showed that the calculated vertical profiles of supermicron particle concentrations matched reasonably well with the observations obtained from 19 research aircraft missions. During dust storm events and at dust concentrated altitudes, the calculated dust fractions in the supermicron particles were usually greater than 90%, and the dust was concentrated in the lower troposphere mainly below 6 km. Without dust storm, dust was still the major component of the supermicron particles above boundary layer. In contrast to supermicron particles, the model results showed that the major component of the submicron particles observed during aircraft experiments was mostly from pollution. The calculated vertical profiles of submicron particle concentrations were sensitive to the emission inventory of air pollutants over east Asia. The correlation between observed anthropogenic volatile organic compound and submicron particles were used to identify the pollution fraction in the submicron particles, and the results were consistent with the model calculations of dust fraction. The model results showed that the dust fractions in the submicron particles were usually less than 28% in the boundary layer. During dust storm events the dust fractions were usually greater than 40% but can be as low as 24% when significant amount of pollutants were present.