Optical thickness as related to pollutant episodes and the concentration of visibility degrading pollutants

A network of six sun photometers was placed in the central and northeast United States during the months of July through October, 1931. The objective of the program was to obtain measurements of atmospheric turbidity which can be related to the concentration of visibility-degrading pollutants in the atmosphere. These measurements serve as ground truth for a program to develop remote sensing techniques for measuring the vertically integrated aerosol concentrations in pollution episodes. The sun photometers measure the direct solar radiation in four passbands: 380 nm, 500 nm, 875 nm and 940 nm. The first three passbands will be used for measuring the aerosol optical depth and the last for measuring precipitable water

Organic nitrogen in aerosols and precipitation at Barbados and Miami: Implications regarding sources, transport and deposition to the western subtropical North Atlantic

The deposition of anthropogenic nitrogen (N) species is believed to have a significant impact on the oligotrophic North Atlantic, but the magnitude of ecological effects remains uncertain because the deposition of water soluble organic N (WSON) is poorly quantified. Here we present measurements of water soluble inorganic N (WSIN) and WSON in aerosol and rain at two subtropical North Atlantic time series sites: Barbados and Miami. WSON total deposition rates ranged from 17.9 mmol m−2 yr−1 to 49.6 mmol m−2 yr−1, contributing on average only 6–14% of total N deposition, less than half the poorly constrained global average which is typically cited as 30%. On an event basis, biomass burning and dust events yielded the largest concentrations of WSON. However, biomass burning was relatively infrequent and highly variable in composition, and much of the organic N associated with dust appeared to be externally adsorbed from pollution sources. Conversely, in Miami pollution made relatively small contributions of WSON on an event basis, but impacts were relatively frequent, making pollution one of the largest sources of WSON during the year. The largest contributor to WSON was volatile basic organic N (VBON) species, which were present at concentrations 1–2 times higher than particulate WSON. Despite VBON inputs, samples associated with pollution-source trajectories yielded much more inorganic N than WSON. Consequently, we would expect that in the future as anthropogenic N emissions increase, inorganic nitrogen will remain the dominant form of N that is deposited to the western North Atlantic

Estimation of the atmospheric flux of nutrients and trace metals to the Eastern Tropical North Atlantic Ocean

Atmospheric deposition contributes potentially significant amounts of the nutrients iron, nitrogen and phosphorus (via mineral dust and anthropogenic aerosols) to the oligotrophic tropical North Atlantic Ocean. Transport pathways, deposition processes and source strengths contributing to this atmospheric flux are all highly variable in space and time. Atmospheric sampling was conducted during 28 research cruises through the Eastern Tropical North Atlantic (ETNA) over a 12 year period and a substantial dataset of measured concentrations of nutrients and trace metals in aerosol and rainfall over the region was acquired. This database was used to quantify (on a spatial- and seasonal-basis) the atmospheric input of ammonium, nitrate, soluble phosphorus and soluble and total iron, aluminium and manganese to the ETNA. The magnitude of atmospheric input varies strongly across the region, with high rainfall rates associated with the Inter-tropical Convergence Zone contributing to high wet deposition fluxes in the south, particularly for soluble species. Dry deposition fluxes of species associated with mineral dust exhibited strong seasonality, with highest fluxes associated with winter-time low-level transport of Saharan dust. Overall (wet plus dry) atmospheric inputs of soluble and total trace metals were used to estimate their soluble fractions. These also varied with season and were generally lower in the dry north than in the wet south. The ratio of ammonium plus nitrate to soluble iron in deposition to the ETNA was lower than the N:Fe requirement for algal growth in all cases, indicating the importance of the atmosphere as a source of excess iron

What controls the recent changes in African mineral dust aerosol across the Atlantic?

Dust from Africa strongly perturbs the radiative balance over the Atlantic, with emissions that are highly variable from year to year. We show that the aerosol optical depth (AOD) of dust over the mid-Atlantic observed by the AVHRR satellite has decreased by approximately 10% per decade from 1982 to 2008. This downward trend persists through both winter and summer close to source and is also observed in dust surface concentration measurements downwind in Barbados during summer. The GEOS-Chem model, driven with MERRA re-analysis meteorology and using a new dust source activation scheme, reproduces the observed trend and is used to quantify the factors contributing to this trend and the observed variability from 1982 to 2008. We find that changes in dustiness over the east mid-Atlantic are almost entirely mediated by a reduction in surface winds over dust source regions in Africa and are not directly linked with changes in land use or vegetation cover. The global mean all-sky direct radiative effect (DRE) of African dust is −0.18 Wm−2 at top of atmosphere, accounting for 46% of the global dust total, with a regional DRE of −7.4 ± 1.5 Wm−2 at the surface of the mid-Atlantic, varying by over 6.0 Wm−2 from year to year, with a trend of +1.3 Wm−2 per decade. These large interannual changes and the downward trend highlight the importance of climate feedbacks on natural aerosol abundance. Our analysis of the CMIP5 models suggests that the decreases in the indirect anthropogenic aerosol forcing over the North Atlantic in recent decades may be responsible for the observed climate response in African dust, indicating a potential amplification of anthropogenic aerosol radiative impacts in the Atlantic via natural mineral dust aerosol.Massachusetts Institute of Technology (Charles E. Reed Faculty Initiative Fund)National Science Foundation (U.S.) (AGS-1238109)National Science Foundation (U.S.) (AGS- 0962256)National Science Foundation (U.S.) (NASA NNX12AP45G

Comparisons of trace constituents from ground stations and the DC-8 aircraft during PEM-West B

Chemical data from ground stations in Asia and the North Pacific are compared with data from the DC-8 aircraft collected during the Pacific Exploratory Measurements in the Western Pacific Ocean (PEM-West B) mission. Ground station sampling took place on Hong Kong, Taiwan, Okinawa, and Cheju; and at three Pacific islands, Shemya, Midway, and Oahu. Aircraft samples were collected during 19 flights, most over the western North Pacific. Aluminum was used as an indicator of mineral aerosol, and even though the aircraft did sample Asian dust, strong dust storms were not encountered. The frequency distribution for non-sea-salt sulfate (nss SO4=) in the aircraft samples was bimodal: the higher concentration mode (∼1 μg m−3) evidently originated from pollution or, less likely, from volcanic sources, while the lower mode, with a peak at 0.040 μg m−3, probably was a product of biogenic emissions. In addition, the concentrations of aerosol sulfate varied strongly in the vertical: arithmetic mean SO4=concentrations above 5000 m ( = 0.21±0.69 μg m−3) were substantially lower than those below ( = 1.07±0.87 μg m−3), suggesting the predominance of the surface sources. Several samples collected in the stratosphere exhibited elevated SO4=, however, probably as a result of emissions from Mount Pinatubo. During some boundary layer legs on the DC-8, the concentrations of CO and O3 were comparable to those of clean marine air, but during other legs, several chemically distinct air masses were sampled, including polluted air in which O3was photochemically produced. In general, the continental outflow sampled from the aircraft was substantially diluted with respect to what was observed at the ground stations. Higher concentrations of aerosol species, O3, and CO at the Hong Kong ground station relative to the aircraft suggest that much of the continental outflow from southeastern Asia occurs in the lower troposphere, and extensive long-range transport out of this part of Asia is not expected. In comparison, materials emitted farther to the north apparently are more susceptible to long-range transport

Aerosol and pollutant transport and their impact on radiative forcing over the tropical Indian ocean during the January - February 1996 pre-INDOEX cruise

Measurements of aerosol bulk composition, optical depth, size distribution and the incoming solar radiation flux were carried out over the coastal waters of India, the Arabian sea and the tropical Indian Ocean during a cruise conducted in January 1996. Aerosol concentrations were relatively high throughout much of the cruise, even when the ship was at considerable distances from land. In this paper, we link the observed spatial variations and meridional gradients in the measurements to monsoonal and inter-hemispheric transport across the ITCZ using a high resolution global reanalysis that highlights the winter monsoon. We show that the northeast monsoonal low level flow can transport sulfates, mineral dust and other aerosols from the Indian sub-continent to the ITCZ within 6–7 days. These transports result in an increase the aerosol optical depth (AOD) at the equator by as much as 0.2 and a decrease in the solar radiative forcing at the sea surface by about 10–20 Wm-2. The high concentrations of continental aerosols are a result of three factors: strong (about 6–10 m/s) near-surface northerly flow; a shallow boundary layer of about 400 to 800 m thick, which traps the pollutants; subsidence, associated with the northeast monsoon, which suppresses rainfall over most of the Arabian sea and thus minimizes the wet removal process. In addition dust can be transported in the middle troposphere from the Arabian desert to the cruise region 4000 km away with a transit time of 2–3 days. There is strong evidence of interhemispheric transports effected by eddies that wrap around the ITCZ. These eddies bring clean southern hemisphere air to about 10°N in the Indian Ocean and carry polluted continental air into the southern-hemisphere. In this manner, substantial amounts of aerosols and other pollutants can be routinely transported to the southern-hemisphere Indian Ocean during the northeast monsoon. Thus, in order to understand the connection between continental emissions and impacts over the Indian Ocean, it is necessary to focus on the rôle of the northeast monsoon in the large-scale atmospheric circulation over this region

Twitter journal clubs and continuing professional development: An analysis of a #MedRadJClub tweet chat

Introduction Online Twitter journal clubs are a recent and popular innovation with the potential to increase research awareness and inform practice. The medical radiation sciences' MedRadJournalClub (MJRC) is a Twitter-based event that attracts a global group of participants at the monthly chats. An analysis of a recent MedRadJournalClub discussion evaluated the perceived benefits and limitations of medical radiation practitioners participating in an online journal club. Methods The February 2017 chat used for analysis was based on the Journal of Medical Imaging and Radiation Sciences article by Currie et al. “Twitter Journal Club in Medical Radiation Science” that examines the educational theory behind learning and evidencing professional development through MRJC and social media. The data consisted of chat tweets which were collated using the Twitter advanced search function using the #medradjclub. An initial reviewed was performed to exclude irrelevant content. A second review was then undertaken to categorize the main theme of the tweet. The data were then subjected to thematic analysis which yielded seven different categories. Results The main benefits included global access due to the online nature of MRJC that has facilitated networking and collaboration. Open access to recently published research was another key benefit. The character limitation of a tweet was the most common constraint, and the dynamic nature of the twitter conversation requires multi-tasking that may be difficult. Conclusion Our analysis indicated that participants use MedRadJournalClub as a source of continuing professional development with some evidence that this is directly informing clinical and educational practice

Links between topography, wind, deflation, lakes and dust: The case of the Bodélé Depression, Chad

The Bodélé Depression, Chad is the planet's largest single source of dust. Deflation from the Bodélé could be seen as a simple coincidence of two key prerequisites: strong surface winds and a large source of suitable sediment. But here we hypothesise that long term links between topography, winds, deflation and dust ensure the maintenance of the dust source such that these two apparently coincidental key ingredients are connected by land-atmosphere processes with topography acting as the overall controlling agent. We use a variety of observational and numerical techniques, including a regional climate model, to show that: 1) contemporary deflation from the Bodélé is delineated by topography and a surface wind stress maximum; 2) the Tibesti and Ennedi mountains play a key role in the generation of the erosive winds in the form of the Bodélé Low Level Jet (LLJ); 3) enhanced deflation from a stronger Bodélé LLJ during drier phases, for example, the Last Glacial Maximum, was probably sufficient to create the shallow lake in which diatoms lived during wetter phases, such as the Holocene pluvial. Winds may therefore have helped to create the depression in which erodible diatom material accumulated. Instead of a simple coincidence of nature, dust from the world's largest source may result from the operation of long term processes on paleo timescales which have led to ideal conditions for dust generation in the world's largest dust source. Similar processes plausibly operate in other dust hotspots in topographic depressions