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COVID-19’s impact on society and our daily habits has been unprecedented. With a decrease in vehicular traffic and industrial production, a decrease in local emissions was expected to occur. In ord...

Decades of atmospheric research have focused on the Western North Atlantic Ocean (WNAO) region because of its unique location that offers accessibility for airborne and ship measurements, gradients in important atmospheric parameters, and a range of meteorological regimes leading to diverse conditions that are poorly understood. This work reviews these scientific investigations for the WNAO region, including the East Coast of North America and the island of Bermuda. Over 50 field campaigns and long-term monitoring programs, in addition to 715 peer-reviewed publications between 1946 and 2019 have provided a firm foundation of knowledge for these areas. Of particular importance in this region has been extensive work at the island of Bermuda that is host to important time series records of oceanic and atmospheric variables. Our review categorizes WNAO atmospheric research into eight major categories, with some studies fitting into multiple categories (relative %): Aerosols (25%), Gases (24%), Development/Validation of Techniques, Models, and Retrievals (18%), Meteorology and Transport (9%), Air-Sea Interactions (8%), Clouds/Storms (8%), Atmospheric Deposition (7%), and Aerosol-Cloud Interactions (2%). Recommendations for future research are provided in the categories highlighted above.

The Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC4RS) field mission based at Ellington Field, Texas, during August and September 2013 employed the most comprehensive airborne payload to date to investigate atmospheric composition over North America. The NASA ER-2, DC-8, and SPEC Inc. Learjet flew 57 science flights from the surface to 20 km. The ER-2 employed seven remote sensing instruments as a satellite surrogate and eight in situ instruments. The DC-8 employed 23 in situ and five remote sensing instruments for radiation, chemistry, and microphysics. The Learjet used 11 instruments to explore cloud microphysics. SEAC4RS launched numerous balloons, augmented Aerosol RObotic NETwork, and collaborated with many existing ground measurement sites. Flights investigating convection included close coordination of all three aircraft. Coordinated DC-8 and ER-2 flights investigated the optical properties of aerosols, the influence of aerosols on clouds, and the performance of new instruments for satellite measurements of clouds and aerosols. ER-2 sorties sampled stratospheric injections of water vapor and other chemicals by local and distant convection. DC-8 flights studied seasonally evolving chemistry in the Southeastern U.S., atmospheric chemistry with lower emissions of NOx and SO2 than in previous decades, isoprene chemistry under high and low NOx conditions at different locations, organic aerosols, air pollution near Houston and in petroleum fields, smoke from wildfires in western forests and from agricultural fires in the Mississippi Valley, and the ways in which the chemistry in the boundary layer and the upper troposphere were influenced by vertical transport in convective clouds.

The AErosol RObotic NETwork (AERONET) program over the past 24 years has provided highly accurate remote sensing characterization of aerosol optical and physical properties for an increasingly extensive geographic distribution that includes all continents and many island sites. The measurements and retrievals from the AERONET global network have addressed satellite and model validation needs very well, but there have been challenges in making comparisons to similar parameters from in situ surface and airborne measurements. Additionally, with improved spatial and temporal satellite remote sensing of aerosols, there is a need for higher spatial resolution ground-based remote sensing networks. An effort to address this need resulted in a number of field campaign networks called Distributed Regional Aerosol Gridded Observation Networks (DRAGONs) that were designed to provide a database for in situ and remote sensing comparison and analysis of local to meso-scale variability of aerosol properties. This paper describes the networks that that have contributed and will continue to contribute to that body of research. The research presented in this special issue illustrates the diversity of topics that has resulted from the application of data from these networks.

By modulating the Earth-atmosphere energy, hydrological and biogeochemical cycles, and affecting regional-to-global weather and climate, biomass burning is recognized as one of the major factors affecting the global carbon cycle. However, few comprehensive and wide-ranging experiments have been conducted to characterize biomass-burning pollutants in Southeast Asia (SEA) or assess their regional impact on meteorology, the hydrological cycle, the radiative budget, or climate change. Recently, BASE-ASIA (Biomass-burning Aerosols in South-East Asia: Smoke Impact Assessment) and the 7-SEAS (7-South-East Asian Studies)/Dongsha Experiment were conducted during the spring seasons of 2006 and 2010 in northern SEA, respectively, to characterize the chemical, physical, and radiative properties of biomass-burning emissions near the source regions, and assess their effects. This paper provides an overview of results from these two campaigns and related studies collected in this special issue, entitled “Observation, modeling and impact studies of biomass burning and pollution in the SE Asian Environment”. This volume includes 28 papers, which provide a synopsis of the experiments, regional weather/climate, chemical characterization of biomass-burning aerosols and related pollutants in source and sink regions, the spatial distribution of air toxics (atmospheric mercury and dioxins) in source and remote areas, a characterization of aerosol physical, optical, and radiative properties, as well as modeling and impact studies. These studies, taken together, provide the first relatively complete dataset of aerosol chemistry and physical observations conducted in the source/sink region in the northern SEA, with particular emphasis on the marine boundary layer and lower free troposphere (LFT). The data, analysis and modeling included in these papers advance our present knowledge of source characterization of biomass-burning pollutants near the source regions as well as the physical and chemical processes along transport pathways. In addition, we raise key questions to be addressed by a coming deployment during springtime 2013 in northern SEA, named 7-SEAS/BASELInE (Biomass-burning Aerosols & Stratocumulus Environment: Lifecycles and Interactions Experiment). This campaign will include a synergistic approach for further exploring many key atmospheric processes (e.g., complex aerosol–cloud interactions) and impacts of biomass burning on the surface–atmosphere energy budgets during the lifecycles of biomass-burning emissions.

A parameterization for the size- and composition-resolved production fluxes of nascent marine aerosol was developed from prior experimental observations and extrapolated to ambient conditions based on estimates of air entrainment by the breaking of wind-driven ocean waves. Production of particulate organic carbon (OCaer) was parameterized based on Langmuir equilibrium-type association of organic matter to bubble plumes in seawater and resulting aerosol as constrained by measurements of aerosol produced from productive and oligotrophic seawater. This novel approach is the first to parameterize size- and composition-resolved aerosol production based on explicit evaluation of wind-driven air entrainment/detrainment fluxes and chlorophyll-a as a proxy for surfactants in surface seawater. Production fluxes were simulated globally with an eight aerosol-size-bin version of the NCAR Community Atmosphere Model (CAM v3.5.07). Simulated production fluxes fell within the range of published estimates based on observationally constrained parameterizations. Because the parameterization does not consider contributions from spume drops, the simulated global mass flux (1.5 × 103 Tg y−1) is near the lower end of published estimates. The simulated production of aerosol number (1.4 × 106 m−2 s−1) and OCaer (29 Tg C y−1) fall near the upper end of published estimates and suggest that primary marine aerosols may have greater influences on the physicochemical evolution of the troposphere, radiative transfer and climate, and associated feedbacks on the surface ocean than suggested by previous model studies.

The NASA Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) mission was conducted in two 3-week deployments based in Alaska (April 2008) and western Canada (June–July 2008). Its goal was to better understand the factors driving current changes in Arctic atmospheric composition and climate, including (1) influx of mid-latitude pollution, (2) boreal forest fires, (3) aerosol radiative forcing, and (4) chemical processes. The June–July deployment was preceded by one week of flights over California (ARCTAS-CARB) focused on (1) improving state emission inventories for greenhouse gases and aerosols, (2) providing observations to test and improve models of ozone and aerosol pollution. ARCTAS involved three aircraft: a DC-8 with a detailed chemical payload, a P-3 with an extensive aerosol and radiometric payload, and a B-200 with aerosol remote sensing instrumentation. The aircraft data augmented satellite observations of Arctic atmospheric composition, in particular from the NASA A-Train. The spring phase (ARCTAS-A) revealed pervasive Asian pollution throughout the Arctic as well as significant European pollution below 2 km. Unusually large Siberian fires in April 2008 caused high concentrations of carbonaceous aerosols and also affected ozone. Satellite observations of BrO column hotspots were found not to be related to Arctic boundary layer events but instead to tropopause depressions, suggesting the presence of elevated inorganic bromine (5–10 pptv) in the lower stratosphere. Fresh fire plumes from Canada and California sampled during the summer phase (ARCTAS-B) indicated low NOx emission factors from the fires, rapid conversion of NOx to PAN, no significant secondary aerosol production, and no significant ozone enhancements except when mixed with urban pollution.

As a result of increasing attention paid to aerosols in climate studies, numerous global satellite aerosol products have been generated. Aerosol parameters and underlining physical processes are now incorporated in many general circulation models (GCMs) in order to account for their direct and indirect effects on the earth's climate, through their interactions with the energy and water cycles. There exists, however, an outstanding problem that these satellite products have substantial discrepancies, that must be lowered substantially for narrowing the range of the estimates of aerosol's climate effects. In this paper, numerous key uncertain factors in the retrieval of aerosol optical depth (AOD) are articulated for some widely used and relatively long satellite aerosol products including the AVHRR, TOMS, MODIS, MISR, and SeaWiFS. We systematically review the algorithms developed for these sensors in terms of four key elements that influence the quality of passive satellite aerosol retrieval: calibration, cloud screening, classification of aerosol types, and surface effects. To gain further insights into these uncertain factors, the NOAA AVHRR data are employed to conduct various tests, which help estimate the ranges of uncertainties incurred by each of the factors. At the end, recommendations are made to cope with these issues and to produce a consistent and unified aerosol database of high quality for both environment monitoring and climate studies.

Handheld sun photometers are typically used to make aerosol optical depth measurements while on the ground. Various investigators, in unrelated efforts, have used handheld sun photometers to make aerosol optical depth measurements from light aircraft, but the strengths and weakness of this approach have not been characterized until now. While the ease and relatively low cost of an aircraft manual sun photometer are attractive, determining if the sun photometer was correctly pointed at the sun for each measurement is the biggest challenge. This problem can be partially addressed by collecting a large number of measurements at each altitude, then manually removing the largest optical depths (misalignment always results in erroneous larger values). Examples of past aircraft manual sun photometer measurements are demonstrating that it is possible to obtain quantitative measurements if sufficient sun photometer measurements are made at each elevation. In order to improve on manual sun photometer measurements, a small webcam was attached to the side of a Microtops sun photometer, and the Microtops sun photometer was triggered by computer control. By detecting the position of the sun in the webcam image, it is possible to determine whether the sun photometer was pointed at the sun correctly when the aerosol optical depth measurement was made. Unfortunately, it was found that the Microtops sun photometer takes ∼1.1 s to scan over the five wavelength channels. This 1.1-s delay proved to be too long, preventing the proposed approach from working as the aircraft was bouncing around.

This study presents an empirical relation that links the volume extinction coefficients of water clouds, the layer integrated depolarization ratios measured by lidar, and the effective radii of water clouds derived from collocated passive sensor observations. Based on Monte Carlo simulations of CALIPSO lidar observations, this method combines the cloud effective radius reported by MODIS with the lidar depolarization ratios measured by CALIPSO to estimate both the liquid water content and the effective number concentration of water clouds. The method is applied to collocated CALIPSO and MODIS measurements obtained during July and October of 2006, and January 2007. Global statistics of the cloud liquid water content and effective number concentration are presented.

The NASA Glory mission is intended to facilitate and improve upon long-term monitoring of two key forcings influencing global climate. One of the mission's principal objectives is to determine the global distribution of detailed aerosol and cloud properties with unprecedented accuracy, thereby facilitating the quantification of the aerosol direct and indirect radiative forcings. The other is to continue the 28-yr record of satellite-based measurements of total solar irradiance from which the effect of solar variability on the Earth's climate is quantified. These objectives will be met by flying two state-of-the-art science instruments on an Earth-orbiting platform. Based on a proven technique demonstrated with an aircraft-based prototype, the Aerosol Polarimetry Sensor (APS) will collect accurate multiangle photopolarimetric measurements of the Earth along the satellite ground track within a wide spectral range extending from the visible to the shortwave infrared. The Total Irradiance Monitor (TIM) is an improved version of an instrument currently flying on the Solar Radiation and Climate Experiment (SORCE) and will provide accurate and precise measurements of spectrally integrated sunlight illuminating the Earth. Because Glory is expected to fly as part of the A-Train constellation of Earth-orbiting spacecraft, the APS data will also be used to improve retrievals of aerosol climate forcing parameters and global aerosol assessments with other A-Train instruments. In this paper, we detail the scientific rationale and objectives of the Glory mission, explain how these scientific objectives dictate the specific measurement strategy, describe how the measurement strategy will be implemented by the APS and TIM, and briefly outline the overall structure of the mission. It is expected that the Glory results will be used extensively by members of the climate, solar, atmospheric, oceanic, and environmental research communities as well as in education and outreach activities.

The first deployment of an emission-based aerosol sodium detector (ASD), designed to chemically characterize marine aerosols on a near-real-time basis, is reported. Deployment occurred as part of the Shoreline Environment Aerosol Study (SEAS) from 16 April to 1 May 2000 at Bellows Air Force Base on the east side of Oahu, where the University of Hawaii's Department of Oceanography maintains a tower for aerosol measurements. The instrument was operated in size-unsegregated mode and measurements were made that included two extended continuous sampling periods, each of which lasted for 24 h. During this time, the ASD was compared with measurements that used aerosol volatility coupled with optical particle counting to infer sea-salt size distributions. A reasonable agreement was obtained between the instruments when sampling in clean air, suggesting that under these conditions both approaches can provide reliable sea-salt distributions. The combination of these measurements suggested that sea salt was...

Elevated pollution levels have been observed at marine boundary layer (MBL) and free troposphere (FT) sites on the island of Tenerife. These have been linked to air mass transport from continental areas, with over 40% of the measurements being directly influenced by continental sources. The influence of different meteorological patterns on this transport has been examined. Saharan dust was an important component of the FT aerosol. In the MBL, modification of aerosol light scattering and cloud nucleating properties correlated strongly with enhanced non-sea-salt (nss)SO42 levels, linked to transport from Europe. Regression analysis of cloud condensation nuclei (CCN) concentrations estimated at supersaturation levels of 0.15% and 0.75% gave CCN = 95 nssSO−24 + 139(0.15%) and CCN = 142 nssSO−24 + 256(0.75%), respectively. Regression analysis of submicron aerosol scattering at 530 nm gave a sulfate mass scattering efficiency value of 6.3 m2 g−1. Sea salt was the dominant supermicron scattering species in the MBL. A mass scattering efficiency of 2.9 m2 g−1 is estimated for Na+.

When measuring the aerosol scattering coefficient (ASC) with an integrating nephelometer, the instrumental “zero” value is established by passing filtered, particle-free air (zero air) through the instrument and setting the instrumental analog signal to zero. Ideally, the computer logged analog value of the zero air should also be zero. However, more commonly a non-zero analog value (zero offset) is obtained. This zero offset is normally regarded as a “blank” and the value is subtracted from the data values. During a one-month field experiment where we measured ASC, we consistently obtained a zero offset that ranged from 4 to 25 mV (equivalent to ASC values of 0.8 to 5×10-6 m-1). In this paper, we demonstrate that the zero offset we observed is in reality a threshold to the recorded analog data and that it should not be subtracted from the data record. Treating the zero offset as a blank could lead to an error in ASC data of as high as 21%. The zero offset observed in our experiment was not produced by the nephelometer. Rather, it was generated during the transmission of the analog signal from the instrument to an analog-to-digital converter. Thus, our results should serve as a caution for those who perform ASC or other measurements using analog data transmission.

Using an integrating nephelometer, we measured aerosol light scattering during a month-long experiment conducted in April 1994 on Barbados, West Indies (13.17°N, 59.43°W). Concurrent measurements of aerosol composition showed that the trade wind aerosol mass was alternatively dominated by sea salt and by mineral dust transported from North Africa. Dust concentrations ranged from 0.9 to 257 μg m−3, and sea salt ranged from 9.4 to 29 μg m−3. Nonsea-salt sulfate (nss SO4=) was generally a minor component with concentrations ranging from 0.5 to 4.2 μg m−3. We obtained hygroscopic growth factors (HGF) by calculating the ratio of the aerosol scattering coefficient (ASC) measured at ambient relative humidity (RH), which ranged from 67% to 83%, to the ASC measured with the sample air stream heated so as to reduce the RH to under 40%. Over the course of the experiment, HGF ranged from 1.13 to 1.69, with lower values corresponding to high dust concentration periods. By a variety of procedures, including the chemical apportioning of ASC, we derived the HGF for mineral dust; depending on the technique used, values ranged from 1.0 to 1.1. In contrast, we obtain for sea salt an HGF of 1.8±0.2, a value consistent with previously reported measurements. The low HGF values obtained for dusty periods suggest that only minor amounts of hygroscopic materials are associated with dust. Consequently, the radiative properties of North African dust in the atmosphere will be relatively insensitive to RH changes.

In order to measure condensation particles of geophysical significance and to detect locally produced airborne combustion products, we modified commercial clean room condensation particle counters and deployed these instruments on atmospheric sampling towers in remote marine regions. The modified instruments accurately counted condensation particles in continuous mode both initially and after more than a year of deployment with only minor on-site maintenance.

As the most populated region of the world, Asia is a major source of aerosols with potential large impact over vast downstream areas, Papers published in this special section describe the variety of aerosols observed in China and their effects and interactions with the regional climate as part of the East Asian Study of Tropospheric Aerosols and their Impact on Regional Climate (EAST-AIRC), The majority of the papers are based on analyses of observations made under three field projects, namely, the Atmospheric Radiation Measurements (ARM) Mobile Facility mission in China (AMF-China), the East Asian Study of Tropospheric Aerosols: An International Regional Experiment (EAST-AIRE), and the Atmospheric Aerosols of China and their Climate Effects (AACCE), The former two are U,S,-China collaborative projects, and the latter is a part of the China's National Basic Research program (or often referred to as "973 project"), Routine meteorological data of China are also employed in some studies, The wealth of general and speCIalized measurements lead to extensive and close-up investigations of the optical, physical, and chemical properties of anthropogenic, natural, and mixed aerosols; their sources, formation, and transport mechanisms; horizontal, vertical, and temporal variations; direct and indirect effects; and interactions with the East Asian monsoon system, Particular efforts are made to advance our understanding of the mixing and interaction between dust and anthropogenic pollutants during transport. Several modeling studies were carried out to simulate aerosol impact on radiation budget, temperature, precipitation, wind and atmospheric circulation, fog, etc, In addition, impacts of the Asian monsoon system on aerosol loading are also simulated.

Current uncertainties in the total solar irradiance (TSI) and aerosol radiative forcings of climate are so large that they limit quantitative evaluation of climate models against global temperature change. Reducing these uncertainties is the objective of the NASA Glory mission scheduled for launch in November 2010 as part of the NASA A-Train. Glory is intended to meet the following scientific objectives: Improve the quantification of solar variability by continuing the uninterrupted 32-year satellite measurement record of TSI, facilitate the quantification of the aerosol direct and indirect forcings of climate, and provide better aerosol representations for use by other operational satellite instruments.

[1] The Tropical Composition, Cloud and Climate Coupling Experiment (TC4), was based in Costa Rica and Panama during July and August 2007. The NASA ER-2, DC-8, and WB-57F aircraft flew 26 science flights during TC4. The ER-2 employed 11 instruments as a remote sampling platform and satellite surrogate. The WB-57F used 25 instruments for in situ chemical and microphysical sampling in the tropical tropopause layer (TTL). The DC-8 used 25 instruments to sample boundary layer properties, as well as the radiation, chemistry, and microphysics of the TTL. TC4 also had numerous sonde launches, two ground-based radars, and a ground-based chemical and microphysical sampling site. The major goal of TC4 was to better understand the role that the TTL plays in the Earth's climate and atmospheric chemistry by combining in situ and remotely sensed data from the ground, balloons, and aircraft with data from NASA satellites. Significant progress was made in understanding the microphysical and radiative properties of anvils and thin cirrus. Numerous measurements were made of the humidity and chemistry of the tropical atmosphere from the boundary layer to the lower stratosphere. Insight was also gained into convective transport between the ground and the TTL, and into transport mechanisms across the TTL. New methods were refined and extended to all the NASA aircraft for real-time location relative to meteorological features. The ability to change flight patterns in response to aircraft observations relayed to the ground allowed the three aircraft to target phenomena of interest in an efficient, well-coordinated manner.

On 27 May 2010, NASA announced the first Earth Venture (EV-1) selections in response to a recommendation made by the National Research Council for low-cost investigations fostering innovation in Earth Science. The five EV-1 investigations span the Earth science focus areas of atmosphere, weather, climate, water and energy and, carbon and represent Earth science researchers from NASA as well as other government agencies, academia and industry from around the world. The five EV-1 missions are (1) Airborne Microwave Observatory of Subcanopy and Subsurface (AirMOSS); (2) Airborne Tropical TRopopause Experiment (ATTREX); (3) Carbon in Arctic Reservoirs Vulnerability Experiment (CARVE); (4) Deriving Information on Surface Conditions from Column and VERtically Resolved Observations Relevant to Air Quality (DISCOVER-AQ), and (5) Hurricane and Severe Storm Sentinel (HS3). The Earth Venture missions are managed out of the Earth System Science Pathfinder (ESSP) Program Office [3]

Current uncertainties in the total solar irradiance (TSI) and aerosol radiative forcings of climate are so large that they limit quantitative evaluation of climate models against global temperature change. Reducing these uncertainties is the objective of the NASA Glory mission scheduled for launch in November 2010 as part of the NASA A-Train. Glory is intended to meet the following scientific objectives: Improve the quantification of solar variability by continuing the uninterrupted 32-year satellite measurement record of TSI, facilitate the quantification of the aerosol direct and indirect forcings of climate, and provide better aerosol representations for use by other operational satellite instruments.

[1] Marine aerosols produced by bursting bubbles at the ocean surface are highly enriched in organic matter (OM) relative to seawater. The importance of this OM in the photochemical evolution of marine aerosols, particularly as a source of reactive oxidants, is unknown but likely significant. To investigate oxidant production, nascent aerosols were generated by bubbling zero air through flowing Sargasso seawater and photochemical production of OH radical and hydroperoxide were quantified in aqueous aerosol extracts exposed to solar radiation. Extrapolation to ambient conditions indicates that OM photolysis was the primary in situ source for both OH (1.1 × 10−8 M s−1) and hydroperoxides (1.7 × 10−8 M s−1) in nascent aerosols; NO3− photolysis was the primary source in aged, acidified aerosols (1.4 × 10−7 and 4.1 × 10−8 M s−1, respectively). In situ OH photoproduction was comparable to gas-phase uptake whereas H2O2 photoproduction was slower. Results provide important constraints for poorly quantified oxidant sources in marine aerosols.

In January 2007, the National Research Council (NRC) released the first decadal survey addressing Earth science entitled "Earth Science and Applications from Space: National Imperatives for the Next Decade and Beyond". The study, initiated in 2004, conducted a decadal survey to generate consensus recommendations from the Earth and environmental science and applications communities regarding a systems approach to the space-based and ancillary observations encompassing the research programs of NASA, the related operational programs of the National Oceanic and Atmospheric Administration (NOAA), and associated programs, such as Landsat, a joint initiative of the U.S. Geological Survey (USGS) and NASA. Among its many recommendations, were that NOAA and NASA should undertake a set of 17 missions, phased over the next decade in three year groupings. Of these 17 missions, 15 were designated to NASA. The four NASA Phase 1 missions are currently in Pre-Phase A study at different levels of development with SMAP, a soil moisture monitoring mission, targeting a launch date of 2013 and ICESat-II, intended to continue the record initiated by ICESat-I to monitor ice sheet height changes for climate change diagnosis, targeting a launch date of 2015. The CLARREO solar and earth radiation monitoring mission and the DESDynI Earth surface and ice deformation monitoring mission are preparing to enter Pre-Phase A in 2009. The five NASA Phase 2 missions are: SWOT, a wide swath altimeter mission measuring ocean, lake, and river water levels; HyspIRI, a hyperspectral mission for measuring land surface composition for agriculture and mineral characterization and vegetation types for ecosystem health; ASCENDS, a day/night, all-latitude, all-season CO2 column measuring mission; ACE, an aerosol and cloud profiling mission for climate and water cycle research with an ocean color measuring capability for open ocean biogeochemistry; and GEO-CAPE, a geostationary mission for measuring atmospheric gas columns for air quality forecasts and ocean color for coastal ecosystem health and climate emissions. Risk reduction and concept maturity studies are underway for these Phase 2 missions with the goal to improve their understanding and scope to enable the Earth Science Division (ESD) to make programmatic decisions for FY10 and beyond. An overview of their science requirements, system concept, technology readiness, and study status will be provided.

daily mean surface solar radiation by � 30–40 W m � 2 , but barely changed solar reflection at the top of the atmosphere. Aerosol loading, particle size and composition vary considerably with location and season. The MODIS AOD data from Collection 5 (C5) agree much better with ground data than earlier releases, but considerable discrepancies still exist because of treatments of aerosol SSA and surface albedo. Four methods are proposed/adopted to derive the SSA by means of remote sensing and in situ observation

inorganic aerosol constituents were similar to those in ambient air. Ca 2+ was significantly enriched relative to seawater (median factor = 1.2). If in the form of CaCO3, these enrichments would have important implications for pH-dependent processes. Other inorganic constituents were present at ratios indistinguishable from those in seawater. Soluble organic carbon (OC) was highly enriched in all size fractions (median factor for all samples = 387). Number size distributions exhibited two lognormal modes. The number production flux of each mode was linearly correlated with bubble rate. At 80% RH, the larger mode exhibited a volume centroid of 5-mm diameter and included 95% of the inorganic sea-salt mass; water comprised 79% to 90% of volume. At 80% RH, the smaller mode exhibited a number centroid of 0.13-mm diameter; water comprised 87% to 90% of volume. The median mass ratio of organic matter to sea salt in the smallest size fraction (geometric mean diameter = 0.13 mm) was 4:1. These results support the hypothesis that bursting bubbles are an important global source of CN and CCN with climatic implications. Primary marine aerosols also influence radiative transfer via multiphase processing of sulfur and other climate-relevant species.

The largest uncertainty in the radiative forcing of climate change over the industrial era is that due to aerosols, a substantial fraction of which is the uncertainty associated with scattering and absorption of shortwave (solar) radiation by anthropogenic aerosols in cloud-free conditions (IPCC, 2001). Quantifying and reducing the uncertainty in aerosol influences on climate is critical to understanding climate change over the industrial period and to improving predictions of future climate change for assumed emission scenarios. Measurements of aerosol properties during major field campaigns in several regions of the globe during the past decade are contributing to an enhanced understanding of atmospheric aerosols and their effects on light scattering and climate. The present study, which focuses on three regions downwind of major urban/population centers (North Indian Ocean (NIO) during INDOEX, the Northwest Pacific Ocean (NWP) during ACE-Asia, and the Northwest Atlantic Ocean (NWA) during ICARTT), incorporates understanding gained from field observations of aerosol distributions and properties into calculations of perturbations in radiative fluxes due to these aerosols. This study evaluates the current state of observations and of two chemical transport models (STEM and MOZART). Measurements of burdens, extinction optical depth (AOD), and direct radiative effect of aerosols (DRE – change in radiative flux due to total aerosols) are used as measurement-model check points to assess uncertainties. In-situ measured and remotely sensed aerosol properties for each region (mixing state, mass scattering efficiency, single scattering albedo, and angular scattering properties and their dependences on relative humidity) are used as input parameters to two radiative transfer models (GFDL and University of Michigan) to constrain estimates of aerosol radiative effects, with uncertainties in each step propagated through the analysis. Constraining the radiative transfer calculations by observational inputs increases the clear-sky, 24-h averaged AOD (34±8%), top of atmosphere (TOA) DRE (32±12%), and TOA direct climate forcing of aerosols (DCF – change in radiative flux due to anthropogenic aerosols) (37±7%) relative to values obtained with 'a priori' parameterizations of aerosol loadings and properties (GFDL RTM). The resulting constrained clear-sky TOA DCF is −3.3±0.47, −14±2.6, −6.4±2.1 Wm−2 for the NIO, NWP, and NWA, respectively. With the use of constrained quantities (extensive and intensive parameters) the calculated uncertainty in DCF was 25% less than the 'structural uncertainties' used in the IPCC-2001 global estimates of direct aerosol climate forcing. Such comparisons with observations and resultant reductions in uncertainties are essential for improving and developing confidence in climate model calculations incorporating aerosol forcing.

[1] Using data collected during the Puerto Rico Dust Experiment (PRIDE), we examine the effect of non-spherical phase functions on dust aerosol retrievals from geostationary satellites. We utilize a statistical model based on Scanning Electron Micrograph (SEM) analysis of individual dust particles. Using T-matrix calculations and assuming that dust particles are randomly-orientated oblate spheroids we compute the dust phase function and scattering properties. Although the non-spherical function developed in this study compares well with the synthetic non-spherical phase functions [Liu et al., 2003], new retrievals using the non-spherical phase function only show slight improvement at scattering angles (Θ) from 110° ∼ 150°, and become worse for Θ between 150° ∼ 170°. However the retrievals are greatly improved at all angles when retrieval algorithms account for the combined effect of both spherical and non-spherical phase functions.

[1] Substantial spatial and temporal variabilities in chemical and physical properties of aerosols complicate attempts to model associated influences on global climate. Although wet deposition is the major mechanism by which most aerosols are removed from the atmosphere, direct effects of precipitation on radiative properties of the atmosphere are not well understood. To address this issue, attenuation coefficients for total insoluble constituents (ACt) and for nonvolatile (at 500°C) insoluble constituents (ACnv; primarily crustal dust) of precipitation at Bermuda and Barbados were measured at six wavelengths between 414 and 859 nm. Coefficients for volatile (at 500°C) insoluble constituents (ACv; primarily carbonaceous species) were calculated by difference. Between April and September, ACt at Bermuda was dominated by mineral constituents transported from North Africa, whereas carbonaceous species from North America were relatively low and exhibited no systematic seasonal variability. ACt and ACnv at Barbados were dominated by mineral dust, especially between April and September. Relative to ACv, ACnv decreased more rapidly with increasing wavelength. The wavelength dependencies of ACs for volatile and nonvolatile constituents at Bermuda were statistically indistinguishable from those at Barbados. The optical and chemical characteristics of precipitation were compared with scattering and absorption by ambient aerosols in associated air parcels to evaluate the influence of scavenging on the radiative properties of air. Although discernible relationships were evident, the small number of cases limited their applicability as reliable empirical predictors. However, these data do provide useful constraints for validating models of aerosol scavenging and wet removal on the optical properties of the troposphere.

[1] An intensive field campaign of the Atlanta Supersite Experiment was carried out at a ground-based measurement site on Jefferson Street in midtown Atlanta, Georgia, from 3 August to 1 September 1999. This paper examines the semicontinuous particulate organic and elemental measurements that were made as a part of the experiment. Measurements were made using a Rutgers University/Oregon Graduate Institute in situ thermal-optical carbon analyzer, Rupprecht and Patashnick 5400 ambient carbon particulate monitor, Radiance Research particle soot absorption photometer, Aerosol Dynamics flash vaporization carbon analyzer, and Magee Scientific AE-16 Aethalometer. The intersampler precision with which semicontinuous particulate total carbon (TC), organic carbon (OC), and elemental carbon (EC) were measured is 7, 13, and 26%, expressed as pooled coefficients of variation of 2, 3, and 4 instruments, respectively. Correlations between pairs of OC measurements are moderate (R 2 = 54-73%), and correlations between pairs of EC measurements are high (R 2 = 74-97%). Differences in reported OC concentrations are small compared to differences in EC concentrations; intersampler EC concentration differences result from differences (1) in the operational definitions of OC and EC, (2) in the calibration of optical instruments, and (3) because EC values are closer to limits of detection. This agreement between semicontinuous samplers is quite good, especially in light of previous particulate carbon comparisons. Reasons for measurement differences and benefits of automated time-resolved measurements are discussed.

[1] Airborne mineral dust can impact visibility, climate, biogeochemical processes, and possibly human health. The magnitude of the impact of dust depends on particle size. We measured the size distributions of airborne mineral dust over the Canary Islands during July 1995 and Puerto Rico during July 2000. Dust size distributions do not appear lognormal. Stokes gravitational settling overestimates losses of large dust particles during atmospheric transport from North Africa over the tropical North Atlantic and Caribbean. Normalized mineral dust size distributions of particles smaller than 7.3 μm over the Canary Islands and Puerto Rico were indistinguishable, indicating these particles were not preferentially removed during atmospheric transport. However, mineral dust aerosols larger than 7.3 μm were preferentially removed during atmospheric transport. Larger particles were more efficiently removed. A simple empirical model setting the vertical velocity of dust particles equal to the Stokes gravitational settling velocity minus an upward velocity of ∼0.33 cm s−1 accurately predicts changes in dust size distribution during atmospheric transport. Thus it appears some atmospheric process(es) partially counteracts gravitational settling.

[1] A multitude of sensitivity studies in the literature point to the importance of proper chemical and morphological characterization of particles when the radiative impacts of airborne dusts are modeled. However, the community data set is based on heterogeneous measurement methods relying on varying aerodynamic, chemical, morphological, and optical means. During the Puerto Rico Dust Experiment, size distributions of dust particles from Africa were measured using a variety of aerodynamic, optical, and geometric means. Consistent with the literature, comparisons of these size distributions showed quite dissimilar results. “Measured” volume median diameters varied from 2.5 to 9 μm for various geometric, aerodynamic, optical, and optical inversion methods. Aerodynamic systems showed mixed performance. Column integrated size distributions inverted from AERONET Sun/sky radiance data produced somewhat reasonable results in the coarse mode when given proper constraints and taken in the proper context. The largest systematic errors were found in optical particle counters due to insensitivities to particle size in the 4–10 μm region with further complications due to dust particle morphology and index of refraction issues. As these methods can produce quite dissimilar size distributions, considerable errors in calculated radiative properties can occur if incorrectly modeled into dust parameters. None of the methods compared in this study can adequately reproduce the measured mass extinction or mass scattering efficiency of the dust using spherical geometry methods. Given all of the uncertainties in the sizing methods, we promote the use of fundamental and quantifiable descriptors of particles such as mass as a function of aerodynamic diameter.

[1] We simulate Saharan dust transport during the Puerto Rico Dust Experiment (June–July 2000) with a three-dimensional aerosol transport model driven by assimilated meteorology. The model does a reasonable job of locating the dust plume as it emerges from Africa but transports it somewhat farther south in the western North Atlantic Ocean than is seen in satellite imagery. The model is able to simulate low-level, uniformly mixed, and elevated vertical dust layer profiles over Puerto Rico similar to observations made in PRIDE. We determine that the variability in the dust vertical profile across the North Atlantic Ocean is most strongly associated with descent of the dust by sedimentation and downward vertical winds during transit rather than low-level transport directly from source regions. Wet removal plays a key role in modulating this process. Assuming our dust is 3.5% iron by mass, we estimate July 2000 iron deposition into the North Atlantic Ocean to be between 0.71 and 0.88 Tg, which is consistent with estimates derived from observed surface dust mass concentrations. We estimate that if annual dust deposition remains constant at five times our July 2000 estimates, there is an accumulation of 1 m of sediment from Saharan dust over the Florida peninsula every one million years.

0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0330Atmospheric Composition and Structure: Geochemical cycles; 0365 Atmospheric Composition andStructure: Troposphere—composition and chemistry; 1640 Global Change: Remote sensing; 3399Meteorology and Atmospheric Dynamics: General or miscellaneous; K

[1] The Puerto Rico Dust Experiment (PRIDE) took place in Roosevelt Roads, Puerto Rico from 26 June to 24 July 2000 to study the radiative and physical properties of African dust aerosol transported into the region. PRIDE had the unique distinction of being the first major field experiment to allow direct comparison of aerosol retrievals from the Moderate Imaging Spectroradiometer (MODIS) with Sun photometer and in situ aerosol measurements. Over the ocean the MODIS algorithm retrieves aerosol optical depth (AOD) as well as information about the aerosols' size distribution. During PRIDE, AODs derived by MODIS in the red wavelengths (0.66 μm) compare closely with Sun photometers. However, MODIS-derived AODs are too large in the blue and green wavelengths (0.47 and 0.55 μm) and too small in the near infrared (0.87 μm). This error in AOD spectral dependence results in retrieved particle size distributions that are small compared to in situ measurements and smaller still when compared to Sun photometer sky radiance inversions. The differences in size distributions may be, in part, associated with MODIS' simplification of dust as spherical particles. Analysis of this PRIDE data set is a first step toward derivation of realistic models for future MODIS retrievals.

[1] Using 30 days of half-hourly, high temporal resolution GOES 8 imager data and radiative transfer calculations, dust aerosol optical thickness (AOT) was retrieved over the Atlantic Ocean (14� N � 26� N, 73� W–63� W) during the Puerto Rico Dust Experiment (PRIDE). Dust aerosol size distributions and complex index of refraction inferred from ground-based measurements (1.53–0.0015i at 0.55 mm), which were used in Mie calculations and a plane-parallel discrete ordinate radiative transfer model (DISORT) to compute look up tables for AOT retrievals. Using a combination of spectral, spatial, and temporal tests, a dust detection algorithm was developed from the GOES 8 imager data. The degradation of the signal response relative to the prelaunched calibration of the GOES 8 visible channel was 39% in July 2000 and the GOES 8 AOT detection limit was estimated to be 0.04 in AOT (0.67 mm). The satellite-retrieved AOT were then compared with AOT values derived from ground-based Sun photometer (SP) sites. The comparison showed that GOES 8 retrieved AOT are in good agreement with the SP derived values, with linear correlation coefficient of 0.91 and 0.80 for the two sites. The GOES 8 monthly mean 0.67 mm AOT (0.19 ± 0.13, 0.22 ± 0.12) over the two SP sites matched the monthly mean SPAOT values (0.23 ± 0.13, 0.22 ± 0.10). The linear correlation between the GOES 8 retrieved AOT and the aircraft derived values from particle probe data and airborne Sun photometer AATS-6 measurements were 0.88 and 0.83, respectively. Besides the uncertainties from the nonspherical effect of dust aerosols, sensitivity studies showed that the uncertainties (t) of the GOES 8 retrieved AOT values were mainly from the uncertainties due to the imaginary part of refractive index (t = ±0.05) and surface reflectance [t = ±(0.02 � 0.04)]. This paper demonstrates the application of geostationary satellites to detect and retrieve dust AOT even at low to moderate AOTs. The GOES 8 imager with high temporal resolutions also captures aerosol diurnal variation in this study that can further reduce the uncertainties in the current aerosol forcing estimations caused by the high temporal variations of AOT, thereby playing a complementary role with global AOT retrievals from polar orbiting satellites. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 3359 Meteorology and Atmospheric Dynamics: Radiative processes; 3360 Meteorology and Atmospheric Dynamics: Remote sensing; KEYWORDS: dust aerosol, diurnal optical depth, GOES 8, radiative forcing

For 26 days in mid-June and July 2000, a research group comprised of U.S. Navy, NASA, and university scientists conducted the Puerto Rico Dust Experiment (PRIDE). In this paper we give a brief overview of mean meteorological conditions during the study. We focus on findings on African dust transported into the Caribbean utilizing Navajo aircraft and AERONET Sun photometer data. During the study midvisible aerosol optical thickness (AOT) in Puerto Rico averaged 0.25, with a maximum less than 0.5 and with clean marine periods of _0.08. Dust AOTs near the coast of Africa (Cape Verde Islands and Dakar) averaged _0.4, 30% less than previous years. By analyzing dust vertical profiles in addition to supplemental meteorology and MPLNET lidar data we found that dust transport cannot be easily categorized into any particular conceptual model. Toward the end of the study period, the vertical distribution of dust was similar to the commonly assumed Saharan Air Layer (SAL) transport. During the early periods of the study, dust had the highest concentrations in the marine and convective boundary layers with only a, weak dust layer in the SAL being present, a state usually associated with wintertime transport patterns. We corroborate the findings of Maring et al. that in most cases, there was an unexpected lack of vertical stratification of dust particle size. We systematically analyze processes which may impact dust vertical distribution and determine and speculate that dust vertical distribution predominately influenced by flow patterns over Africa and differential advection couple with mixing by easterly waves and regional subsidence.

calculated with a Mie code yielding Eap = 9.5 ± 1.5 m 2 g 1 , while EC mass summed from the impactor stages in comparison to measured sap gives Eap = 9.3 ± 3.2 m 2 g 1 .M ie light-scattering calculations using inputs of measured mass and EC size distributions give geometric mean light scattering and absorption Dp = 0.54 and 0.13 mm, respectively, and show the dominance of the submicrometer diameter particles to light extinction in the urban environment. Based on the measured aerosol optical depth in Atlanta, da (500 nm) = 0.44 ± 0.22, and other radiative measurements, a best estimate of the average direct aerosol radiative forcing at the top of the atmosphere (a measure of the climate significance) is F= 11 ± 6 W m 2 in Atlanta. This value is an order of magnitude greater than global mean estimates for aerosols underscoring the influence of aerosol particles on radiative transfer in the urban environment. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0394 Atmospheric Composition and

[1] As part of the Puerto Rico Dust Experiment (PRIDE), airborne and surface dust particle samples from Africa were collected and subjected to bulk elemental and single-particle analysis. Airborne samples were collected on polycarbonate filters at various altitudes and underwent single-particle scanning electron microscopy with energy dispersive analysis with X-rays (EDAX) to derive elemental ratios of key soil elements. Particle chemistry was related to size and morphological characteristics. At the principle surface site, particles were collected on a Davis Rotating Drum (DRUM) cascade impactor strips in eight stages from 0.1 to 12 μm at 4 hour time resolution. These samples were subjected to X-ray florescence (XRF) to determine bulk elemental composition from Al through Zn. The elemental data showed good correlation between the DRUM and the aircraft samples. Cluster analysis of single-particle data resulted in 63 statistically significant clusters. Several clusters can be easily related to their parent mineralogical species. However, as dust particles are to a large extent aggregates, most clusters are based on a continuum of varied mineralogical species and cannot be easily categorized. With 60,500 total particles counted from the airborne filters, a statistically significant number of large particles could be analyzed. Estimated mean surface area modal diameter is ∼5 μm, with an average aspect ratio of 1.9. An apparent change in source region is seen in the morphological data and non alumino-silicate minerals but is not seen in the aluminum to silicon ratio. We suspect homogenization during long-range transport.

[1] We developed the Light Aircraft Aerosol Package (LAAP) to make measurements of aerosol properties up to ∼4000 m asl. The LAAP reliably measured, with empirical and theoretical corrections, aerosol microphysical and chemical properties. During the Puerto Rico Dust Experiment (PRIDE), June–July 2000, we measured aerosol properties over the tropical North Atlantic. On the basis of aerosol filter samples and measured aerosol size distributions, we (1) modeled sea-salt aerosol concentration with altitude, characterizing the rapid decrease in sea-salt concentration with altitude; (2) found no evidence of sea salt above the marine boundary layer (MBL); and (3) observed mineral dust in the Saharan Air Layer (SAL) to exceed 170 μg m−3. Substantial variability in the vertical distribution and small differences in the size distribution of dust between the MBL and SAL suggested dust properties were more dependent on the separate transport histories than the interaction between the MBL and SAL near Puerto Rico. Relatively constant dust size distributions with altitude suggested either (1) no relofting of dust aerosols once they get into the “deposition layer” or (2) dry deposition removes dust particles independent of particle size. Dust size distributions measured at low (∼25%) and high (∼93%) relative humidity were indistinguishable. A simple model calculation suggests dust particles over the tropical North Atlantic during summer increase their geometric diameter

[1] The first of the U.S. Environmental Protection Agency's (EPA) Particulate Matter (PM) Supersites projects was established in Atlanta, GA, during the summer of 1999 in conjunction with the Southern Oxidants Study. The short-term primary focus was a one month intensive field campaign to evaluate advanced PM measurement methods for measuring PM mass and the chemical and physical properties of PM. Long-term objectives are being met through coordination and cooperation with existing programs in Atlanta and the southeastern United States. Three categories of PM instruments were deployed during August 1999: time-integrated or discrete filter-based methods like those used in EPA's PM2.5 Chemical Speciation Network; continuous or semicontinuous species specific methods, most of which are still in development; and single particle mass spectrometers, the most advanced methods looking at the chemical composition of single particles. The focus of this paper is on comparison of the discrete filter-based methods. Samples were collected by 12 discrete filter-based samplers on an every other day basis during the study period at the Jefferson Street Southeastern Aerosol Research and Characterization (SEARCH) study site. Samples were analyzed for PM2.5 mass, sulfate, nitrate, ammonium, organic carbon, elemental carbon, and trace elements, the latter by XRF. Samplers used a variety of filters; denuder-filter combinations in the case of nitrate and organic carbon, particle size fractionating devices, and flow rates. Ambient concentrations for most species were sufficiently above detection limits for testing comparability among samplers, with nitrate being the most notable exception for the major components having an average reported value of 0.5 μg/m 3 . Several trace species, e.g., As and Pb, also were often below limits of detection of the analysis method. Results indicate that real differences exist among the samplers tested for most species, with sulfate and ammonium being the exceptions, under the conditions tested. Differences are due to sampler design, and in the case of elemental carbon, also due to the use of different chemical analysis methods. Comparability among most of the samplers for a given species was: mass (±20%); sulfate (±10%); nitrate (±30-35%); ammonium (±10-15%); organic carbon either with or without denuders (±20%) or including samplers both with and without denuders (±35-45%); elemental carbon (±20 to ±200%, the latter if different analysis methods are used); and minor and trace elements (±20-30%). A net organic carbon-sampling artifact on quartz-fiber filters was estimated from the comparison of denuded versus undenuded samples and is in the range of 1-4 μg/m 3 .

[1] As part of Puerto Rico Dust Experiment (PRIDE), a Piper Navajo research aircraft, equipped with particle probes and an airborne Sun photometer, was deployed to Puerto Rico in July 2000. During the study, mid-visible optical depths in Puerto Rico due to dust reached 0.5. In the middle of the summer transport season, the vertical distributions of dust were similar to that commonly assumed in the region with dust concentrated in the Saharan Air Layer (SAL) aloft. However, during the first half of the study period, dust had the highest concentrations in the marine and convective boundary layers, with lower dust concentrations above the trade inversion despite the presence of a strong SAL. Supporting meteorology suggests that the state of the monsoon on the coast of Africa influences the nature of the vertical distribution of dust in the Caribbean.

The Indian Ocean Experiment (INDOEX) measurements on land, sea, and in the air were designed to provide complementary assessment of chemical, physical, and optical properties of the haze aerosol over the Indian Ocean. Differences in platform requirements and objectives resulted in diverse techniques, measurements, and analyses being employed. In order to best interpret the properties of the INDOEX aerosol, comparisons of data by platform, air mass origin, and light scattering intensity were undertaken. These revealed significant variability in platform averages of aerosol extensive properties (e.g., mass, light scattering, and absorption) but less variability in intensive properties (e.g., mass scattering efficiency, single scattering albedo, backscatter fraction, and Ångström exponent) and the ratios of constituents. In general, ratios of chemical species were found to show greater variability than properties of the size distributions or aerosol optical properties. Even so, at higher haze concentrations with higher scattering values, various determinations of the mass scattering efficiency (MSE) at 33% relative humidity converged on values of about 3.8 ± 0.3 m^2 g^(−1), providing a firm constraint upon the description and modeling of haze optical properties. MSE values trended lower with more dilute haze but became more variable in clean air or regions of low concentrations. This cross-platform comparison resolved a number of measurement differences but also revealed that regional characterization from different platforms results in differences linked to variability in time and space. This emphasizes the need to combine such efforts with coordinated satellite and modeling studies able to characterize large-scale regional structure and variability. These comparisons also indicate that “closure” between chemical, microphysical, and optical properties across platforms to better than about 20% will require significant improvements in techniques, calibration procedures, and comparison efforts.

ANTARCTICA is believed to be a relatively pristine continent, mainly because of its remote location and the atmospheric circulation patterns that limit the transport of industrial aerosols into the Antarctic polar cell1–4. This perception is apparently supported by the extremely low concentrations of lead in Antarctic surface waters―an observation that has been interpreted as showing insig-nificant contamination by anthropogenic lead5. The isotopic com-position of lead in other natural waters has been used as a tracer of the sources of lead, and in particular to identify anthropogenic inputs6,7. Here we apply this approach to Antarctic surface waters, and show that despite the low concentrations of lead in these waters (which are confirmed by our measurements), their isotopic compo-sition reveals a significant contribution of lead from industrial sources. The extremely low concentrations of lead in these waters appear to be due to biological scavenging of the lead during periods of intense primary production.