Aerosol measurements over the Pacific Ocean in support of the IR aerosol backscatter program

The major efforts under NASA contract NAG8-841 included: (1) final analyses of the samples collected during the first GLOBE survey flight that occurred in November 1989 and collections and analysis of aerosol samples during the second GLOBE survey flight in May and June 1990. During the first GLOBE survey flight, daily samples were collected at four stations (Midway, Rarotonga, American Samoa, and Norfolk Island) throughout the month of November 1989. Weekly samples were collected at Shemya, Alaska, and at Karamea, New Zealand. During the second GLOBE survey flight, daily samples were collected at Midway, Oahu, American Samoa, Rarotonga, and Norfolk Island; weekly samples were collected at Shemya. These samples were all analyzed for sodium (sea-salt), chloride, nitrate, sulfate, and methanesulfonate at the University of Miami and for aluminum at the University of Rhode Island (under a subcontract). (2) Samples continued to be collected on a weekly basis at all stations during the periods between and after the survey flights. These weekly samples were also analyzed at the University of Miami for the suite of water-soluble species. (3) In August 1990, the results obtained from the above studies were submitted to the appropriate personnel at NASA Marshall Space Flight Center to become part of the GLOBE data base for comparison with data from instruments used aboard the aircraft. In addition, the data will be compared with data previously obtained at these stations as part of the Sea-Air Exchange (SEAREX) Program. This comparison will provide valuable information on the representativeness of the periods in terms of the longer term aerosol climatology over the Pacific Ocean. (4) Several publications have been written using data from this grant. The data will continue to be used in the future as part of a continuing investigation of the long-term trends and interannual variations in aerosol species concentrations over the Pacific Ocean

Long-term characterisation of the vertical structure of the Saharan Air Layer over the Canary Islands using lidar and radiosonde profiles: implications for radiative and cloud processes over the subtropical Atlantic Ocean

Every year, large-scale African dust outbreaks frequently pass over the Canary Islands (Spain). Here we describe the seasonal evolution of atmospheric aerosol extinction and meteorological vertical profiles on Tenerife over the period 2007–2018 using long-term micropulse lidar (MPL-3) and radiosonde observations. These measurements are used to categorise the different patterns of dust transport over the subtropical North Atlantic and, for the first time, to robustly describe the dust vertical distribution in the Saharan Air Layer (SAL) over this region. Three atmospheric scenarios dominate the aerosol climatology: dust-free (clean) conditions, the Saharan summer scenario (summer-SAL) and the Saharan winter scenario (winter-SAL). A relatively well-mixed marine boundary layer (MBL) was observed in the case of clean (dust-free) conditions; it was associated with rather constant lidar extinction coefficients (α) below 0.036 km−1 with minimum α (< 0.022 km−1) in the free troposphere (FT). The summer-SAL has been characterised as a dust-laden layer strongly affecting both the MBL (Δα = +48 % relative to clean conditions) and the FT. The summer-SAL appears as a well-stratified layer, relatively dry at lower levels (  % at the SAL’s base, where r is the water vapour mixing ratio) but more humid at higher levels compared with clean FT conditions (  % at 5.3 km), with a peak of α> 0.066 km−1 at ∼ 2.5 km. Desert dust is present up to ∼ 6.0 km, the SAL top based on the altitude of SAL's temperature inversion. In the winter-SAL scenario, the dust layer is confined to lower levels below 2 km altitude. This layer is characterised by a dry anomaly at lower levels (Δr∼ −38 % in comparison to the clean scenario) and a dust peak at ∼ 1.3 km height. Clean FT conditions were found above 2.3 km. Our results reveal the important role that both dust and water vapour play in the radiative balance within the summer-SAL and winter-SAL. The dominant dust-induced shortwave (SW) radiative warming in summer (heating rates up to +0.7 K d−1) is found slightly below the dust maximum. However, the dominant contribution of water vapour was observed as a net SW warming observed within the SAL (from 2.1 to 5.7 km) and as a strong cold anomaly near the SAL's top (−0.6 K d−1). The higher water vapour content found to be carried on the summer-SAL, despite being very low, represents a high relative variation in comparison to the very dry clean free troposphere in the subtropics. This relevant aspect should be properly taken into account in atmospheric modelling processes. In the case of the winter-SAL, we observed a dust-induced radiative effect dominated by SW heating (maximum heating of +0.7 K d−1 at 1.5 km, near the dust peak); both dust and atmospheric water vapour impact heating in the atmospheric column. This is the case of the SW heating within the SAL (maximum near the r peak), the dry anomaly at lower levels (Δr∼ −38 % at 1 km) and the thermal cooling (∼ 0.3 K d−1) from the temperature inversion upwards. Finally, we hypothesise that the SAL can impact heterogeneous ice nucleation processes through the frequent occurrence of mid-level clouds observed near the SAL top at relatively warm temperatures. A dust event that affected Tenerife on August 2015 is simulated using the regional DREAM model to assess the role of dust and water vapour carried within SAL in the ice nucleation processes. The modelling results reproduce the arrival of the dust plume and its extension over the island and confirm the observed relationship between the summer-SAL conditions and the formation of mid- and high-level clouds.The long-term meteorological sounding program has been supported by regular funds from AEMET. The long-term MPLNet program in Tenerife has been financed through R&D budgets from INTA and AEMET. AERONET sun photometers at Izaña have been calibrated within the AERONET Europe TNA, supported by the European Community Research Infrastructure Action under the FP7 ACTRIS (grant agreement no. 262254). This research has received support from the Institute of Physics Belgrade, through the grant by the Ministry of Education, Science, and Technological Development of the Republic of Serbia.Peer Reviewed"Article signat per 13 autors/es: África Barreto, Emilio Cuevas, Rosa D. García, Judit Carrillo, Joseph M. Prospero, Luka Ilić, Sara Basart, Alberto J. Berjón, Carlos L. Marrero, Yballa Hernández, Juan José Bustos, Slobodan Ničković, and Margarita Yela"Postprint (published version

Temporal and spatial variability of Icelandic dust emissions and atmospheric transport

Icelandic dust sources are known to be highly active, yet there exist few model simulations of Icelandic dust that could be used to assess its impacts on the environment. We here present estimates of dust emission and transport in Iceland over 27 years (1990–2016) based on FLEXDUST and FLEXPART simulations and meteorological re-analysis data. Simulations for the year 2012 based on high-resolution operational meteorological analyses are used for model evaluation based on PM2. 5 and PM10 observations in Iceland. For stations in Reykjavik, we find that the spring period is well predicted by the model, while dust events in late fall and early winter are overpredicted. Six years of dust concentrations observed at Stórhöfði (Heimaey) show that the model predicts concentrations of the same order of magnitude as observations and timing of modelled and observed dust peaks agrees well. Average annual dust emission is 4.3 ± 0.8 Tg during the 27 years of simulation. Fifty percent of all dust from Iceland is on average emitted in just 25 days of the year, demonstrating the importance of a few strong events for annual total dust emissions. Annual dust emission as well as transport patterns correlate only weakly to the North Atlantic Oscillation. Deposition amounts in remote regions (Svalbard and Greenland) vary from year to year. Only limited dust amounts reach the upper Greenland Ice Sheet, but considerable dust amounts are deposited on Icelandic glaciers and can impact melt rates there. Approximately 34 % of the annual dust emission is deposited in Iceland itself. Most dust (58 %), however, is deposited in the ocean and may strongly influence marine ecosystems.We acknowledge funding provided by the Swiss National Science Foundation (grant 155294) and travel grants provided by the Nordic Centre of Excellence eSTICC (Nordforsk 57001). OA and PDW were supported by Icelandic Research Fund (Rannis) grant no. 152248-051 and PDW by The Recruitment Fund of the University of Iceland. The station at Stórhöfði was initially established with support from the US National Atmospheric and Oceanic Administration to JMP and later sampling and analysis with support various grants from the US National Science Foundation (AGS-0962256).Peer Reviewe

Predicting the mineral composition of dust aerosols: Insights from elemental composition measured at the Izaña Observatory

Regional variations of dust mineral composition are fundamental to climate impacts but generally neglected in climate models. A challenge for models is that atlases of soil composition are derived from measurements following wet sieving, which destroys the aggregates potentially emitted from the soil. Aggregates are crucial to simulating the observed size distribution of emitted soil particles. We use an extension of brittle fragmentation theory in a global dust model to account for these aggregates. Our method reproduces the size-resolved dust concentration along with the approximately size-invariant fractional abundance of elements like Fe and Al in the decade-long aerosol record from the Izaña Observatory, off the coast of West Africa. By distinguishing between Fe in structural and free forms, we can attribute improved model behavior to aggregation of Fe and Al-rich clay particles. We also demonstrate the importance of size-resolved measurements along with elemental composition analysis to constrain models.This research was supported by the Department of Energy (DE-SC0006713), the NASA Modeling, Analysis and Prediction Program, and the Aerosol Global Atmospheric Watch program of Izaña Observatory, which has been funded by AEMET and several research projects of the Ministry of Economy and Competitiveness of Spain and the European Regional Development Fund (ERDF) including POLLINDUST (CGL2011-26259) and AEROATLAN (CGL2015-66229-P)

Geochemical and mineralogical evidence for Sahara and Sahel dust additions to Quaternary soils on Lanzarote, eastern Canary Islands, Spain

Africa is the most important source of dust in the world today, and dust storms are frequent on the nearby Canary Islands. Previous workers have inferred that the Sahara is the most important source of dust to Canary Islands soils, with little contribution from the Sahel region. Soils overlying a late Quaternary basalt flow on Lanzarote, Canary Islands, contain, in addition to volcanic minerals, quartz and mica, exotic to the island!s bedrock. Kaolinite in the soils also likely has an exotic origin. Trace-element geochemistry shows that the soils are derived from varying proportions of locally derived basalt and African dust. Major-element geochemistry, clay mineralogy and interpretation of satellite imagery suggest that dust additions to the Canary Islands come not only from the Sahara Desert, but also from the Sahel region

Radiative properties of aerosols in Saharan dust outbreaks using ground-based and satellite data: applications to radiative forcing

We report on measurements of atmospheric transmission (ATT) and aerosol optical depth (AODT) made at three wavelengths (368, 500, and 778 nm) with a spectroradiometer placed on Tenerife (28.5°N, 16.3°W), Canary Islands. Using the National Oceanic and Atmospheric Administration (NOAA) advanced very high resolution radiometer (AVHRR) channel 1, we also measured the aerosol optical depth (AODS) and albedo over a region of the North Atlantic Ocean extending from 15°–35°N to 12°–25°W. We observe large changes in ATT and AODT when dust outbreaks pass over this region. Using all these data, we derive the asymmetry factor (g), the single-scattering albedo (ω), and the local mean AODT and we compute the direct radiative forcing ΔF attributable to mineral dust. The local radiative forcing obtained is over the ocean ΔF = −9.7 W/m2 and for the land ΔF = −4.5 W/m2 with an error of ±25%. Extending these results to global-scale averages, we obtain values of ΔF of −1.22 W/m2 over the ocean and −0.57 W/m2 over land. The forcings attributable to dust are comparable in magnitude to those reported in the literature for anthropogenic sulphate and for biomass burning aerosols.We wish to thank the Gobierno Autónomo de Canarias for its financial support by contract 4/95, the Comisi6n Interministerial de Ciencia y Tecnologia (CICYT) by contract CLI97- 0453, and the University of La Laguna by contract 1802260003. A portion of this work was carried out as part of the Atmosphere/Ocean Chemistry Experiment (AEROCE) and supported by the National Science Foundation grants ATM-9414808, ATM-9414812, and ATM- 9414846

African biomass burning is a substantial source of phosphorus deposition to the Amazon, Tropical Atlantic Ocean, and Southern Ocean

The deposition of phosphorus (P) from African dust is believed to play an important role in bolstering primary productivity in the Amazon Basin and Tropical Atlantic Ocean (TAO), leading to sequestration of carbon dioxide. However, there are few measurements of African dust in South America that can robustly test this hypothesis and even fewer measurements of soluble P, which is readily available for stimulating primary production in the ocean. To test this hypothesis, we measured total and soluble P in long-range transported aerosols collected in Cayenne, French Guiana, a TAO coastal site located at the northeastern edge of the Amazon. Our measurements confirm that in boreal spring when African dust transport is greatest, dust supplies the majority of P, of which 5% is soluble. In boreal fall, when dust transport is at an annual minimum, we measured unexpectedly high concentrations of soluble P, which we show is associated with the transport of biomass burning (BB) from southern Africa. Integrating our results into a chemical transport model, we show that African BB supplies up to half of the P deposited annually to the Amazon from transported African aerosol. This observational study links P-rich BB aerosols from Africa to enhanced P deposition in the Amazon. Contrary to current thought, we also show that African BB is a more important source of soluble P than dust to the TAO and oceans in the Southern Hemisphere and may be more important for marine productivity, particularly in boreal summer and fall

Temporal variability of summer-time ozone and aerosols in the free troposphere over the eastern North Atlantic

In the free troposphere over Tenerife in the summer, O3 concentrations are anti-correlated with major pollutant aerosols (nss-SO = 4 and NO−3) and with 210Pb, a tracer for boundary layer sources. In contrast, O3 is highly correlated with 7Be, a product of cosmic ray interactions in the upper troposphere and stratosphere. This suggests that natural O3 sources (i.e. the stratosphere) might be playing an important role. Nonetheless our results do not preclude the possibility that substantial amounts of pollution-related O3 could be transported in the free troposphere. However, to be consistent with our results, the transport mechanisms would have to incorporate efficient processes for the removal of pollutant aerosol species and 210Pb

Retrieving the global distribution of the threshold of wind erosion from satellite data and implementing it into the Geophysical Fluid Dynamics Laboratory land–atmosphere model (GFDL AM4.0/LM4.0)

Dust emission is initiated when surface wind velocities exceed the threshold of wind erosion. Many dust models used constant threshold values globally. Here we use satellite products to characterize the frequency of dust events and land surface properties. By matching this frequency derived from Moderate Resolution Imaging Spectroradiometer (MODIS) Deep Blue aerosol products with surface winds, we are able to retrieve a climatological monthly global distribution of the wind erosion threshold (Vthreshold) over dry and sparsely vegetated surfaces. This monthly two-dimensional threshold velocity is then implemented into the Geophysical Fluid Dynamics Laboratory coupled land–atmosphere model (AM4.0/LM4.0). It is found that the climatology of dust optical depth (DOD) and total aerosol optical depth, surface PM10 dust concentrations, and the seasonal cycle of DOD are better captured over the “dust belt” (i.e., northern Africa and the Middle East) by simulations with the new wind erosion threshold than those using the default globally constant threshold. The most significant improvement is the frequency distribution of dust events, which is generally ignored in model evaluation. By using monthly rather than annual mean Vthreshold, all comparisons with observations are further improved. The monthly global threshold of wind erosion can be retrieved under different spatial resolutions to match the resolution of dust models and thus can help improve the simulations of dust climatology and seasonal cycles as well as dust forecasting