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The nephelometer, the primary instrument used for accurate in-situ scattering measurements, suffers from small errors due to truncation and angular nonidealities in the light source. Because that error depends on particle size, it has been traditionally estimated from the wavelength dependence of scattering, which also depends on particle size. Absorption by particles alters the scattering wavelength dependence, so the traditional correction is in error by 1-5% for absorbing particles, particularly when that absorption is wavelength-dependent. Single-scattering albedos under 0.9 and absorption Ångstrom exponents above 2 can have errors of over 2%. This problem will occur only very near sources, including laboratory measurements of combustion aerosol. For these situations, we suggest the correction should be calculated using Mie theory and an assumed refractive index, resulting in less than a 2% error. Scattering wavelength dependence for absorbing particles should not be used as a measurement of particle size. [ABSTRACT FROM AUTHOR]

Integrating nephelometers are widely used for monitoring and research applications related to air pollution and climate. Several commercial versions of the instrument are available and are in wide use in the community. This article reports on results from a calibration and intercomparison workshop where several units of the three most widely used nephelometer models were tested with respect to their CO2 calibration accuracy and stability and nonidealities of their angular illumination function. Correction factors that result from the non-ideal illumination due to truncation of the sensing volumes in the near-forward and near-backward angular ranges and for non-Lambertian illumination from the light sources are presented, in particular for two models that have not previously been tested in this respect. The correction factors ranged from 0.95 to 1.15 depending on the model of nephelometer and aerosol size distribution. Recommendations for operational data analysis in context of these and previous performance tests are presented. [ABSTRACT FROM AUTHOR]

During the 2006 Texas Air Quality Study/Gulf of Mexico Atmospheric Composition and Climate Study (TexAQS/GoMACCS 2006) a filter-based (Particle Soot Absorption Photometer, or PSAP) and a photoacoustic-based aerosol light absorption technique were deployed and here the data are compared. The level of agreement between the two techniques with ambient aerosol depended on the abundance of organic aerosol (OA), with the ratio of OA to light absorbing carbon (LAC) mass (ROA-LAC) of particular importance. When OA mass concentration was low the agreement between the methods was within instrumental uncertainties (PSAP measuring 12% higher), however at high (ROA-LAC) (∼15-20) the difference in agreement was between 50 and 80%. This difference is similar to the bias observed in the laboratory studies of a companion paper using non-absorbing OA and LAC (Cappa et al. 2008a). It was found that most of the OA was oxidized and non-absorbing in nature. We postulate that the observed differences results from a bias in the filter-based measurements due to (a) the redistribution of liquid-like organic particulate matter (PM) around the fiber filters thereby modifying the filter surface and subsequent light scattering, and (b) the possible coating and absorption enhancement of pre-existing absorbing PM (i.e., soot) as OA deposition and redistribution occurs. We stop short or recommending a universal correction using these findings due to the magnitude of the bias showing some dependence on air mass type. Any use of this data for a correction must consider the uncertainties in measuring OA mass concentrations, LAC mass concentrations and type of OA present. The observed PSAP bias may have a significant impact on the accuracy of aerosol single scatter albedo (SSA) and LAC mass concentrations derived from filter-based aerosol absorption methods from regions impacted by large amounts of OA. Although this study was performed using the PSAP we caution users of other filter-based instrumentation to the possibility of a similar bias in those instruments. [ABSTRACT FROM AUTHOR]

A new instrument to measure the size-resolved hygroscopic growth of both sub- and super-micron atmospheric aerosol is described. It consists of two white-light optical particle counters measuring the same sample aerosol simultaneously at two different controlled relative humidities. Calibration with aerosols of different refractive index confirms the expected relative insensitivity of the instrument to index of refraction. Data obtained in the field from airborne sampling support the utility of the instrument in measuring differences in size-resolved hygroscopicity in the marine boundary layer and also in addressing the issue of kinetic limitations to aerosol condensational growth. [ABSTRACT FROM AUTHOR]

Data are reported for both airborne and wind tunnel testing of the aerosol inlet on the CIRPAS Twin Otter research aircraft. Though utilizing different instrumentation and assumptions, results from the two test venues show broad agreement. The roll off in transmission efficiency of the inlet under standard flight conditions starts at ∼⃒3.5 μ m but plateaus at 5.5 μm particle diameter at a value slightly in excess of 0.6. This value persists to the upper limit of the test range at 9 μ m and is likely due to the sub-isokinetic aspiration flow of the inlet. [ABSTRACT FROM AUTHOR]

Past studies have indicated that long-term averages of the aerosol number to volume ratios (defined as the number of particles larger than a certain diameter divided by the particle volume over some range less than 1μm) show little variability over the Atlantic. This work presents number to volume ratios (R) measured during the ACE-2 experiment on the land-based Sagres field site located in Southwest Portugal. The values ofRmeasured in Sagres compare reasonably well with previous measurements over the Atlantic. The main emphasis of this work is therefore to investigate more closely possible reasons for the observed stability of the number to volume ratio. Aerosol number size distributions measured in Sagres are parametrized by the sum of two log-normal distributions fitted to the accumulation and to the Aitken mode. The main factor that limits the variability ofRis that the parameters of these log-normal distributions are not always independent but show some covariance. In polluted air mass types correlations between parameters of the Aitken and accumulation mode are mostly responsible for stabilizingR. In marine air mass types the variability ofRis reduced by an inverse relationship between the accumulation-mode mean diameter and standard deviation, consistent with condensational processes and cloud processing working on the aerosol. However, despite this reduction, the variability ofRin marine air mass types is still considerable andRis linearly dependent on the number concentration of particles larger than 90 nm. This partly due to a tail of Aitken-mode particles extending to sizes larger than 90 nm. [ABSTRACT FROM AUTHOR]

Although the importance of the aerosol contribution to the global radiative budget has been recognized, the forcings of aerosols in general, and specifically the role of the organic component in these forcings, still contain large uncertainties. In an attempt to better understand the relationship between the background forcings of aerosols and their chemical speciation, marine air samples were collected off the windward coast of Oahu, Hawaii, during the Rough Evaporation Duct project (RED) using filters mounted on both the Twin Otter aircraft and the Floating Instrument Platform (FLIP) research platform. Laboratory analysis revealed a total of 17 species, including 4 carboxylic acids and 2 carbohydrates that accounted for 74% ± 20% of the mass gain observed on the shipboard filters, suggesting a possible significant unresolved organic component. The results were correlated with in situ measurements of particle light scattering (σsp) at 550 nm and with aerosol hygroscopicities. Principal component analysis revealed a small but ubiquitous pollution component affecting the σsp and aerosol hygroscopicity of the remote marine air. The Princeton Organic-Electrolyte Model (POEM) was used to predict the growth factor of the aerosols based upon the chemical composition. This output, coupled with measured aerosol size distributions, was used to attempt to reproduce the observed σsp. It was found that while the POEM model was able to reproduce the expected trends when the organic component of the aerosol was varied, due to large uncertainties especially in the aerosol sizing measurements, the σsp predicted by the POEM model was consistently higher than observed. [ABSTRACT FROM AUTHOR]

Airborne data are presented on the impact of cloud processing on the aerosol mass light-scattering efficiency. The measurements, on marine stratocumulus, suggest that cloud processing significantly enhanced the mass light-scattering efficiency in three of the five cases analysed. Enhancements were of the order of 10% for air detraining from the cloud deck relative to non-detraining air. A diagnostic modelling analysis suggested that the observed enhancements were consistent with the previously proposed explanation of in-cloud sulfate production in the particle size range for efficient light scattering. [ABSTRACT FROM AUTHOR]

An experiment was conducted to measure aerosol turbulent fluxes to a grass field. A new high-flow-rate aerosol sensor was deployed from a tower to make eddy correlation (EC) measurements of aerosol turbulent flux and deposition velocity. The EC data were screened and analysed for uncertainties associated with advection, boundary layer growth, instrument separation and counting particles. An apparent bias in the aerosol flux due to particle hygroscopic growth was evaluated from chemical and microphysical measurements and removed from the results based on derived corrections. The resulting aerosol deposition velocity for 0.52 μm diameter particles depended on atmospheric stability with values of 0.3 cm s−1 during near-neutral stability, 0.44 cm s−1 during unstable periods and 0.16 cm s−1 during stable periods with an estimated uncertainty of ±0.07 cm s−1 due to chemical composition and particle counting. [ABSTRACT FROM AUTHOR]

Ground-based aerosol optical measurements made at near-ambient relative humidity (RH) under clean marine sampling conditions are presented and compared to 1) almost identical optical measurements made at a polluted continental site and 2) optical properties calculated from measured size distributions and Mie theory. The use of Mie theory (which assumes homogeneous spheres) is justified based on the fact that the sea-salt aerosol was measured in a hydrated state. This study focuses on the extinction-to-backscatter ratio S, an optical property required to interpret remote measurements by elastic backscatter lidar. For clean marine conditions, S is found to be 25.4 ± 3.5 sr at 532 nm (central value ± 95% confidence uncertainty). Other optical properties reported include single-scattering albedo, wavelength dependence of scattering, fraction of scattering due to submicrometer particles, and hemispheric-backscatter fraction, as well as the extensive properties (e.g., scattering coefficient) upon which these intensive properties are based. In addition, correlation scale lengths are examined via the autocorrelation function. Except during deliberate drying experiments that lowered the measurement RH below 43%, S exhibited little variation with RH. A subtle but clearly detectable change in optical properties was observed at the onset of volcanically influenced sampling conditions. [ABSTRACT FROM AUTHOR]

Presents a study that proposed and tested a method to calculate cloud condensation nuclei (CCN) spectra based on aerosol number size distributions and hygroscopic growth factors. Crucial step in the calculation; Basis for the calculation of CCN spectra; Factor which enhanced the CCN concentrations.

Focuses on a study which looked at the field measurements in marine stratocumulus at a site located on the coast of Washington State. Experimental methods used in the study; Comparison of the aerosol and droplet spectra; Data and analysis used in the study; Conclusions of the study.