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Summary In a sandy loam soil the stability of freshly sampled aggregates, determined by turbidimetry, showed a net decrease following the growth of maize that was partly offset when the plants dried the soil over a period of 9 days. The cause of this restabilization on drying may be the adsorption onto soil surfaces of polysaccharide materials produced in the rhizosphere.

There is much evidence that soil structure can be markedly affected by a farmer’s choice of cropping systems. For example, tillage type and intensity may substantially influence soil structure (for reviews see Cannell, 1985; Davies et al., 1982; R.S. Russell, 1977). The choice of crop may also result in significant alterations to soil structure. Many researchers have found deteriorations in soil structure under arable crops such as corn (Zea mays) (Page and Willard, 1946), potatoes (Solanum tuberosum) (Doyle and Hamlyn, 1960), wheat (Triticum aestivum) and barley (Hordeum vulgare) (Low, 1972). Improvements in soil structure have been observed under a number of ley and pasture grasses (see e.g., A.L. Clarke et al., 1967; Robinson and Jacques, 1958) and alfalfa (Medicago sativa L.) (Cooke and Williams, 1972). Figure 1 illustrates some measured changes in soil structure which can be attributed to different cropping systems. Harris et al. (1966) and E.W. Russell (1973) summarized much of the earlier research in this area.

The Angstrom wavelength exponent α, which is the slope of the logarithm of aerosol optical depth (τ a ) versus the logarithm of wavelength (λ), is commonly used to characterize the wavelength dependence of τ a and to provide some basic information on the aerosol size distribution. This parameter is frequently computed from the spectral measurements of both ground-based sunphotometers and from satellite and aircraft remote sensing retrievals. However, spectral variation of α is typically not considered in the analysis and comparison of values from different techniques. We analyze the spectral measurements of τ a from 340 to 1020 nm obtained from ground-based Aerosol Robotic Network radiometers located in various locations where either biomass burning, urban, or desert dust aerosols are prevalent. Aerosol size distribution retrievals obtained from combined solar extinction and sky radiance measurements are also utilized in the analysis. These data show that there is significant curvature in the In τ a versus In λ relationship for aerosol size distributions dominated by accumulation mode aerosols (biomass burning and urban). Mie theory calculations of α for biomass burning smoke (for a case of aged smoke at high optical depth) agree well with observations, confirming that large spectral variations in α are due to the dominance of accumulation mode aerosols. A second order polynomial fit to the In τ a versus In λ data provides excellent agreement with differences in τ a of the order of the uncertainty in the measurements (-0.01-0.02). The significant curvature in In τ a versus In λ for high optical depth accumulation mode dominated aerosols results in α values differing by a factor of 3-5 from 340 to 870 nm. We characterize the curvature in In τ a versus In λ by the second derivative α' and suggest that this parameter be utilized in conjunction with α to characterize the spectral dependence of τ a , The second derivative of In τ a versus In λ gives an indication of the relative influence of accumulation mode versus coarse mode particles on optical properties.

An ensemble-based forecast and data assimilation system has been developed for use in Navy aerosol forecasting. The system makes use of an ensemble of the Navy Aerosol Analysis Prediction System (ENAAPS) at 1 × 1°, combined with an ensemble adjustment Kalman filter from NCAR's Data Assimilation Research Testbed (DART). The base ENAAPS-DART system discussed in this work utilizes the Navy Operational Global Analysis Prediction System (NOGAPS) meteorological ensemble to drive offline NAAPS simulations coupled with the DART ensemble Kalman filter architecture to assimilate bias-corrected MODIS aerosol optical thickness (AOT) retrievals. This work outlines the optimization of the 20-member ensemble system, including consideration of meteorology and source-perturbed ensemble members as well as covariance inflation. Additional tests with 80 meteorological and source members were also performed. An important finding of this work is that an adaptive covariance inflation method, which has not been previously tested for aerosol applications, was found to perform better than a temporally and spatially constant covariance inflation. Problems were identified with the constant inflation in regions with limited observational coverage. The second major finding of this work is that combined meteorology and aerosol source ensembles are superior to either in isolation and that both are necessary to produce a robust system with sufficient spread in the ensemble members as well as realistic correlation fields for spreading observational information. The inclusion of aerosol source ensembles improves correlation fields for large aerosol source regions, such as smoke and dust in Africa, by statistically separating freshly emitted from transported aerosol species. However, the source ensembles have limited efficacy during long-range transport. Conversely, the meteorological ensemble generates sufficient spread at the synoptic scale to enable observational impact through the ensemble data assimilation. The optimized ensemble system was compared to the Navy's current operational aerosol forecasting system, which makes use of NAVDAS-AOD (NRL Atmospheric Variational Data Assimilation System for aerosol optical depth), a 2-D variational data assimilation system. Overall, the two systems had statistically insignificant differences in root-mean-squared error (RMSE), bias, and correlation relative to AERONET-observed AOT. However, the ensemble system is able to better capture sharp gradients in aerosol features compared to the 2DVar system, which has a tendency to smooth out aerosol events. Such skill is not easily observable in bulk metrics. Further, the ENAAPS-DART system will allow for new avenues of model development, such as more efficient lidar and surface station assimilation as well as adaptive source functions. At this early stage of development, the parity with the current variational system is encouraging.

Numerical prediction of aerosol particle properties has become an important activity at many research and operational weather centers. This development is due to growing interest from a diverse set of stakeholders, such as air quality regulatory bodies, aviation and military authorities, solar energy plant managers, climate services providers, and health professionals. Owing to the complexity of atmospheric aerosol processes and their sensitivity to the underlying meteorological conditions, the prediction of aerosol particle concentrations and properties in the numerical weather prediction (NWP) framework faces a number of challenges. The modeling of numerous aerosol-related parameters increases computational expense. Errors in aerosol prediction concern all processes involved in the aerosol life cycle including (a) errors on the source terms (for both anthropogenic and natural emissions), (b) errors directly dependent on the meteorology (e.g., mixing, transport, scavenging by precipitation), and (c) errors related to aerosol chemistry (e.g., nucleation, gas–aerosol partitioning, chemical transformation and growth, hygroscopicity). Finally, there are fundamental uncertainties and significant processing overhead in the diverse observations used for verification and assimilation within these systems. Indeed, a significant component of aerosol forecast development consists in streamlining aerosol-related observations and reducing the most important errors through model development and data assimilation. Aerosol particle observations from satellite- and groundbased platforms have been crucial to guide model development of the recent years and have been made more readily available for model evaluation and assimilation. However, for the sustainability of the aerosol particle prediction activities around the globe, it is crucial that quality aerosol observations continue to be made available from different platforms (space, near surface, and aircraft) and freely shared. This paper reviews current requirements for aerosol observations in the context of the operational activities carried out at various global and regional centers. While some of the requirements are equally applicable to aerosol–climate, the focus here is on global operational prediction of aerosol properties such as mass concentrations and optical parameters. It is also recognized that the term “requirements” is loosely used here given the diversity in global aerosol observing systems and that utilized data are typically not from operational sources. Most operational models are based on bulk schemes that do not predict the size distribution of the aerosol particles. Others are based on a mix of “bin” and bulk schemes with limited capability of simulating the size information. However the next generation of aerosol operational models will output both mass and number density concentration to provide a more complete description of the aerosol population. A brief overview of the state of the art is provided with an introduction on the importance of aerosol prediction activities. The criteria on which the requirements for aerosol observations are based are also outlined. Assimilation and evaluation aspects are discussed from the perspective of the user requirements.

The online-coupled Weather Research and Forecasting model with Chemistry (WRF-Chem) is used to simulate the direct and semi-direct radiative impacts of smoke particles over the Southeast Asian Maritime Continent (MC, 10° S–10° N, 90–150° E) during October 2006 when a significant El Niño event caused the highest biomass burning activity since 1997. With the use of an OC (organic carbon) / BC (black carbon) ratio of 10 in the smoke emission inventory, the baseline simulation shows that the clouds can reverse the negative smoke forcing in cloud-free conditions to a positive value. The net absorption of the atmosphere is largely enhanced when smoke resides above a cloud. This led to a warming effect at the top of the atmosphere (TOA) with a domain and monthly average forcing value of ~ 20 W m−2 over the islands of Borneo and Sumatra. Smoke-induced monthly average daytime heating (0.3 K) is largely confined above the low-level clouds, and results in a local convergence over the smoke source region. This heating-induced convergence transports more smoke particles above the planetary boundary layer height (PBLH), hence rendering a positive effect. This positive effect contrasts with a decrease in the cloud fraction resulting from the combined effects of smoke heating within the cloud layer and the more stable boundary layer; the latter can be considered as a negative effect in which a decrease of the cloud fraction weakens the heating by smoke particles above the clouds. During the nighttime, the elevated smoke layer lying above the clouds in the daytime is decoupled from the boundary layer, and the enhanced downdraft and shallower boundary layer lead to the accumulation of smoke particles near the surface. Because of monthly smoke radiative extinction, the amount of solar input at the surface is reduced by as much as 60 W m−2, which leads to a decrease in sensible heat, latent heat, 2 m air temperature, and PBLH by a maximum of 20 W m−2, 20 W m−2, 1 K, and 120 m, respectively. During daytime, the cloud changes over continents mostly occur over the islands of Sumatra and Borneo where the low-level cloud fraction decreases more than 10%. However, the change of local wind, including sea breeze, induced by the smoke direct radiative effect leads to more convergence over the Karimata Strait and the south coastal area of Kalimantan during both daytime and nighttime; consequently, the cloud fraction there is increased up to 20%. The sensitivities with different OC / BC ratios show the importance of the smoke single-scattering albedo for the smoke semi-direct effects. Lastly, a conceptual model is used to summarize the responses of clouds, smoke, temperature, and water vapor fields to the coupling of smoke direct effect below and above clouds over the Southeast Asian Maritime Continent.

Over the past several years, there has been a rapid development in the number and quality of global aerosol models intended for operational forecasting use. Indeed, most centers with global numerical weather prediction (NWP) capabilities have some program for aerosol prediction. These aerosol models typically have differences in their underlying meteorology as well as aerosol sources, sinks, microphysics and transformations. However, like similar diversity in aerosol climate models, the aerosol forecast models have fairly similar overall bulk error statistics for aerosol optical thickness (AOT)-one of the few aerosol metrics that is globally available. Experience in climate and weather prediction has shown that in situations such as this where there are several independent models, a multi-model ensemble or consensus will be top performing in many key error metrics. Further, multi-model ensembles provide a highly valuable tool for forecasters attempting to predict severe aerosol events. Here we present the first steps in developing a global multi-model aerosol forecasting ensemble intended for eventual operational and basic research use. Drawing from members of the International Cooperative for Aerosol Prediction (ICAP) latest generation of quasi-operational aerosol models, five day AOT forecasts are analyzed for December 2011 through November 2012 from four institutions: ECMWF, JMA, NASA GSFC, and NRL/FNMOC. For dust, we also include the NOAA NGAC product in our analysis. The Barcelona Supercomputing Centre (NMMC) and UK Met office dust product have also recent become available with ICAP, but have insufficient data to be included in this analysis period. A simple consensus ensemble of member and mean AOT fields for modal species (e.g., fine and coarse mode, and a separate dust ensemble) is used to create the ICAP Multi-Model Ensemble (ICAP-MME). The ICAP-MME is run daily at 0Z for 6 hourly forecasts out to 120 h. Basing metrics on comparisons to 21 regionally representative Aerosol Robotic Network (AERONET) sites, all models generally captured the basic aerosol features of the globe. However, there is an overall AOT low bias among models, particularly for high AOT events. Biomass burning regions have the most diversity in seasonal average AOT. The southern oceans, though low in AOT, nevertheless also have high diversity. In regard to root mean square error, as expected the ICAP-MME placed first over all models worldwide, and was typically first or second in ranking against all models at individual sites. These results are encouraging; as more global operational aerosol models come on line, we expect their inclusion in a robust operational multi-model ensemble will provide valuable aerosol forecasting guidance.

As part of the multinational Lake Owens Dust Experiment (LODE), we have studied the generation of dust storms on the south sand sheet of Owens (dry) Lake, California, an anthropogenically desiccated playa reported to be the single greatest source of particulate matter in North America. During March 1993, we performed an intensive field study including eight significant dust storms, building on our prior work (1978–1984) and preliminary studies (1991–1992). We studied sources and magnitude of coarse saltating particles, the meteorological conditions that allow them to become mobile across the flat playa of Owens (dry) Lake, and how the motion of saltating particles across different types of playa surfaces results in the generation of PM10 dusts (aerosol particles smaller than 10 μm aerodynamic diameter). Saltating grains of lacustrine sand and broken crust abrade and disaggregate the playa surface into fine aerosols, and the resulting PM10 concentrations recorded during major dust storms are among the highest ever recorded in North America. On 23 March 1993, we measured a 2 h concentration on the playa of 40 620 μg m−3, as far as we can determine the highest ambient PM10 value ever recorded in the U.S.A. Abrasion of salt-silt-clay crusts by saltation is shown to be responsible for all but a small part of one dust storm. The quantity ‘sand run’, saltating particle transport multiplied by wind run, is shown to be very closely correlated with dust aerosol concentration. Finally, we have established that on-lake bed studies are essential for quantitative prediction of dust events on the Owens (dry) Lake bed, despite the difficult conditions encountered.

The physical and optical properties of biomass burning aerosols in a savanna region in south central Africa (Zambia) were analyzed from measurements made during the Zambian International Biomass Burning Emissions Experiment (ZIBBEE) during August-September 1997. Due to the large spatial extent of African savannas and the high frequency of occurrence of burning in the annual dry seasons, characterization of the optical properties of the resultant biomass burning aerosols is important for the study of atmospheric radiative processes and for remote sensing of both surface and atmospheric properties in these regions. Aerosol Robotic Network Sun-sky radiometer spectral measurements of direct Sun observations and directional sky radiances were utilized to infer spectral aerosol optical depths (τa), aerosol size distributions, and single-scattering albedos. During the primary ZIBBEE study period, which coincided with the peak period of biomass burning in the region, there was a high correlation between the measured τa and the total column water vapor or precipitable water vapor (PWV), suggesting transport of smoke aerosol from regions with higher PWV. Size distribution retrievals of the biomass burning smoke show that the accumulation mode dominated and a comparison with smoke from Amazonia (Bolivia) shows a shift toward smaller particles for African savanna smoke. This may be the result of differences in mode of combustion (flaming versus smoldering), fuel type and moisture content, and the aging processes of the aerosol. The single-scattering albedo (ω0) of the aerosols were retrieved using several approaches, yielding average values of ω0 at ∼550 nm during ZIBBEE varying from ∼0.82 to ∼0.85, thus showing good agreement within the retrieval uncertainty of ∼0.03 of these methods. In general, ω0 was relatively constant as a function of aerosol loading, with very little change occurring for τa at 440 nm ranging from 0.7 to 1.7. African savanna smoke exhibits significantly higher absorption than smoke from Amazonian forested regions and also a greater rate of decrease of ω0 with increasing wavelength. Variations in the spectral change of the Angstrom wavelength exponent were also investigated with respect to the degree of aerosol absorption and changes in the accumulation mode size distributions.

In the vicinity of the North Carolina Outer Banks we observed both steady onshore flow conditions and a continental air mass transition into a marine boundary layer. Using the CIRPAS Twin Otter aircraft, we measured changes in the column burden of sea salt as the air mass was advected out to sea. We also measured the flux of whitecap-generated sea-salt particles in neutrally stable atmosphere at wind speeds of 4, 8, and 12 m s -1 . Production of salt particles as small as 0.27 μm in diameter was observed. Furthermore, we measured salt particle size distributions at various wind speeds during along shore wind and near steady state conditions. Using these measurements as a frame of reference, we discuss the very large differences in the reported size and flux of sea salt presented in the literature. The disagreement in reported salt fluxes is larger for smaller-sized particles (almost an order of magnitude) and is most likely due to assumptions made when the fluxes were computed, especially the particle dry deposition velocity and air mass history. However, for giant salt particles with short atmospheric lifetimes (> ∼10 μm in diameter), there is general agreement between fluxes and size distributions measured in this study and previous ones. Reported salt particle size distributions in the literature also vary considerably under similar steady wind and stability conditions. From these and our results it is clear that no more than half of the variance in salt particle concentration can be explained by wind speed alone, suggesting that the idea of steady state in the marine boundary layer rarely exists at midlatitudes.