Intercontinental Dust Research Articles

Contributions of dust and biomass burning to aerosols at a Colorado mountain-top site

Abstract. Visible Multifilter Rotating Shadowband Radiometer (vis-MFRSR) data were collected at Storm Peak Laboratory (SPL), a mountain-top facility in northwest Colorado, from 1999 to 2011 and in 2013. From 2011 to 2014, in situ measurements of aerosol light scattering were also obtained. Using these data sets together, the seasonal impact of dust and biomass burning is considered for the western USA. Analysis indicates that the median contributions to spring and summer aerosol optical depth (AOD) from dust and biomass-burning aerosols across the data set are comparable. The mean AOD is slightly greater in the summer, with significantly more frequent and short-duration high AOD measurements due to biomass-burning episodes than in the spring. The Ångström exponent showed a significant increase in the summer for both the in situ and vis-MFRSR data, suggesting an increase in combustion aerosols. Spring dust events are less distinguishable in the in situ data than the column measurement, suggesting that a significant amount of dust may be found above the elevation of SPL, 3220 m a.s.l. Twenty-two known case studies of intercontinental dust, regional dust, and biomass-burning events were investigated. These events were found to follow a similar pattern, in both aerosol loading and Ångström exponent, as the seasonal mean signal in both the vis-MFRSR and ground-based nephelometer. This data set highlights the wide-scale implications of a warmer, drier climate on visibility in the western USA.

Intercontinental dispersal of bacteria and archaea by transpacific winds.

Microorganisms are abundant in the upper atmosphere, particularly downwind of arid regions, where winds can mobilize large amounts of topsoil and dust. However, the challenge of collecting samples from the upper atmosphere and reliance upon culture-based characterization methods have prevented a comprehensive understanding of globally dispersed airborne microbes. In spring 2011 at the Mt. Bachelor Observatory in North America (2.8 km above sea level), we captured enough microbial biomass in two transpacific air plumes to permit a microarray analysis using 16S rRNA genes. Thousands of distinct bacterial taxa spanning a wide range of phyla and surface environments were detected before, during, and after each Asian long-range transport event. Interestingly, the transpacific plumes delivered higher concentrations of taxa already in the background air (particularly Proteobacteria, Actinobacteria, and Firmicutes). While some bacterial families and a few marine archaea appeared for the first and only time during the plumes, the microbial community compositions were similar, despite the unique transport histories of the air masses. It seems plausible, when coupled with atmospheric modeling and chemical analysis, that microbial biogeography can be used to pinpoint the source of intercontinental dust plumes. Given the degree of richness measured in our study, the overall contribution of Asian aerosols to microbial species in North American air warrants additional investigation.

Microbial hitchhikers on intercontinental dust: high-throughput sequencing to catalogue microbes in small sand samples

Microbiological studies on the intercontinental transport of dust are confounded by the difficulty of obtaining sufficient material for analysis. Axenic samples of dust collected at high altitudes or historic specimens in museums are often so small and precious that the material can only be sacrificed when positive results are assured. With this in mind, we evaluated current methods and developed new ones in an attempt to catalogue all microbes present in small dust or sand samples. The methods used included classical microbiological approaches in which sand extracts were plated out on a variety of different media, polymerase chain reaction (PCR)-based amplification of 16S/18S rRNA sequences followed by construction of clone libraries, PCR amplification of 16S rRNA sequences followed by high-throughput sequencing (HtS) of the products and direct HtS of DNA extracted from the sand. A representative sand sample collected at Bahaï Wadi in the desert of the Republic of Chad was used. HtS with or without amplification showed the most promise and can be performed on ≤100 ng DNA. Since living microbes are often required, current best practices would involve geochemical and microscopic characterisation of the sample, followed by DNA isolation and direct HtS. Once the microbial content of the sample has been deciphered, growth conditions (including media) can be tailored to isolate the micro-organisms of interest.

Pathogens as potential hitchhikers on intercontinental dust

Desert dust seeds distant lands and waters with minerals as well as micro-organisms raising the question of whether this ancient phenomenon also spreads pathogens across the globe. Severe dust storms require strong winds blowing over land-masses that are largely devoid of vegetation, effectively limiting the scope for winds to raise pathogens into the air. Nevertheless, changing patterns of land-use, often driven by belligerency, result in refugees spreading to areas that were previously deemed barely habitable. With the help of the International Committee of the Red Cross, a number of sand/dust samples were collected from the Republic of Chad, some near refugee camps, others further removed from human influence. In parallel studies, we documented the micro-organisms present in these samples and used a number of the isolates here to test the effect of environmental constraints on their ability to survive intercontinental flight. We also added traditional pathogens to the palette of microbes and tested the effects of UV irradiation, desiccation and temperature on survival of both bacteria and fungi. A clear trend was obvious—those microbes that are coloured or able to form conidia or spores (in other words, those that are native to deserts) were well able to resist the imposed stresses. On the other hand, most pathogens were more sensitive to stresses than the environmental isolates. Toxin production in two species of Aspergillus was also investigated. Short-term desiccation (simulating environmental conditions during intercontinental travel) of sand amended with fungal spores containing sterigmatocystin leads to increased mycotoxin contents, but significant mycotoxin production was only possible under growth-permissive conditions, e.g. at higher humidity. It thus seems likely that an ever-decreasing fraction of the initial pathogen load survives as the dust recedes from its desert source and that those organisms that land on other continents are highly enriched in desert dwellers.

An elevated large-scale dust veil from the Taklimakan Desert: Intercontinental transport and three-dimensional structure as captured by CALIPSO and regional and global models

Abstract. An intense dust storm occurred during 19–20 May 2007 over the Taklimakan Desert in northwestern China. Over the following days, the space-borne lidar CALIOP tracked an optically thin, highly elevated, horizontally extensive dust veil that was transported intercontinentally over eastern Asia, the Pacific Ocean, North America, and the Atlantic Ocean. A global aerosol transport model (SPRINTARS) simulated the dust veil quite well and provided a three-dimensional view of the intercontinental dust transport. The SPRINTARS simulation revealed that the dust veil traveled at 4–10 km altitudes with a thickness of 1–4 km along the isentropic surface between 310 and 340 K. The transport speed was about 1500 km/day. The estimated dust amount exported to the Pacific was 30.8 Gg, of which 65% was deposited in the Pacific and 18% was transported to the North Atlantic. These results imply that dust veils can fertilize open oceans, add to background dust, and affect the radiative budget at high altitudes through scattering and absorption. The injection mechanism that lifts dust particles into the free atmosphere is important for understanding the formation of the dust veil and subsequent long-range transport. We used a regional dust transport model (RC4) to analyze the dust emission and injection over the source region. The RC4 analysis revealed that strong northeasterly surface winds associated with low pressures invaded the Taklimakan Desert through the eastern corridor. These winds then formed strong upslope wind along the high, steep mountainsides of the Tibetan Plateau and blew large amounts of dust into the air. The updraft lifted the dust particles farther into the upper troposphere (about 9 km above mean sea level, MSL), where westerlies are generally present. The unusual terrain surrounding the Taklimakan Desert played a key role in the injection of dust to the upper troposphere to form the dust veil.

Source identification for fine aerosols in Mammoth Cave National Park

In this study, positive matrix factorization (PMF) was applied to the chemical composition data of the ambient PM 2.5 collected at the Mammoth Cave National Park, an IMPROVE site in Kentucky. Eight individual carbon fractions, four organic carbons (OCs), pyrolyzed organic carbon (OP) and three elemental carbons (ECs), were provided to the analysis. Nine sources including the well-distinguished gasoline emission and diesel emission were identified. Also, the back trajectories indicated the crustal factor in this study were likely caused by Saharan dust storms in the summer. The apportionment of nine sources was: gasoline emission (6.7%), diesel emission (3.1%), summer secondary sulfate (49.0%), winter secondary sulfate (0.6%), OP-rich secondary sulfate (16.2%), secondary nitrate (2.8%), Intercontinental dust plus soil (4.9%), wood smoke (13.6%), and aged sea salt (3.2%). The results of this study will help regularize the pollution control strategies in rural areas of Kentucky and upper mid-western US while demonstrating the feasibility of applying carbon fractions to the source apportionment of rural upper-Midwestern areas.

Receptor Modeling of Toronto PM2.5 Characterized by Aerosol Laser Ablation Mass Spectrometry

Urban Toronto fine particulate matter (PM2.5) was physically and chemically characterized by online aerosol laser ablation mass spectrometry (LAMS) between January 2002 and February 2003. The mass spectra from the analysis of individual aerosol particles were classified according to chemical composition by a neural network approach called adaptive resonance theory (ART-2a). Temporal trends of the hourly analysis rate of over 120 different particles types were constructed and subjected to positive matrix factorization (PMF). This receptor modeling technique enabled the identification of nine distinct emission sources responsible for these particle types: biogenic, mixed crustal, organic nitrate, construction dust, Toronto soil/road salt, secondary salt, wood burning, intercontinental dust, and an unknown source of aluminum fluoride dust. Episodic events occurred with the wood burning, intercontinental dust, and unknown dust sources. This is the first paper reporting the application of PMF to single-particle spectral data.

Long-range sources of Toronto particulate matter (PM2.5) identified by Aerosol Laser Ablation Mass Spectrometry (LAMS)

Abstract An aerosol laser ablation mass spectrometer (LAMS) was operated continuously in downtown Toronto for a period of 11 months. The mass spectra of individual particles were classified according to their chemical composition using neural net-based adaptive resonance theory (ART-2a). The types of particles thereby identified were used as the basis for classifying particles from a 10-day sampling period within the 11-month sampling study. The particles within a class were further segregated by size, and the number of particles analyzed per hour was calculated for each of the resulting composition and sized based classes. ART-2a was used to combine these classes into groups based on similarity in their temporal trends. Two unusual sources were thereby identified: forest fire particles and intercontinental dust. The forest fire particles primarily exhibited potassium and potassium sulphate, while the intercontinental dust exhibited crustal metals, like titanium, aluminum, and iron. Back trajectory analysis of the air masses that transported these particles to Toronto revealed their origins as Northern Canada and Africa. From the chemical composition and size of the particles, a forest fire episode in Western Canada was identified during the 11-month sampling study on 1 June 2002. Other episodes of particles originating from Africa were not observed. These findings demonstrated the use of temporal variations in single particle chemistry to identify sources.

Case study of a Chinese dust plume reaching the French Alps

By combining reconstruction of airmass back‐trajectories from dust deposition sites in Europe and measurements of the (Nd) isotopic composition of deposited dust particles, potential sources of different Saharan dust events can be identified. The study of “red dust” events collected in France allowed us to identify distinct North African source areas (e.g. Lybia vs. Mauritania). Surprisingly, the airmass trajectory of one dust event (March 6, 1990) was distinct from the others, and revealed a Chinese origin. The Nd isotopic composition of this dust was consistent with the range of isotopic compositions of Chinese loess. Moreover, an atmospheric global model simulation reveals that a dust plume left China before February 25, 1990, flew over North America around the February/March transition and reached the French Alps by March 6, 1990, revealing that intercontinental dust and pollutant transport may occur across the Pacific Ocean and the North Atlantic at the Westerlies latitudes.