Kevin D. Perry

Long-range transport of North African dust to the eastern United States

The long-range transport of North African dust to the Middle East, Europe, South America, and the Caribbean has been well documented during the past 25 years. With the advent of routine collection and analysis of fine aerosols at national parks, monuments, and wilderness areas in the continental United States, these North African dust incursions can now be tracked, characterized, and quantified across much of the eastern half of the United States. Identification of the North African source of these dust episodes is confirmed by mass distribution measurements, a characteristic Al/Ca ratio, isentropic backward air mass trajectories, and sequential plots of the spatial distribution of the dust plumes. North African dust incursions into the continental United States persist for ∼10 days and occurred, on average, 3 times per year from 1992 to 1995. Fine soil mass usually exceeds 10 μg m−3 during these dust episodes and dominates local fine soil dust by an order of magnitude or more, even in the so-called “dust bowl” states of the central United States. Size-resolved measurements of elemental composition taken during July 1995 indicate that the mass mean diameter of the transported North African dust is <1 μm. The high mass scattering efficiency and abundant particle surface area associated with these submicron soil aerosols could have important consequences for both the radiative balance of the region and the chemistry of the local aerosols during summer when the long-range transport of North African dust to the United States is most common.

Comparison of Real-Time Instruments Used To Monitor Airborne Particulate Matter

ABSTRACT Measurements collected using five real-time continuous airborne particle monitors were compared to measurements made using reference filter-based samplers at Bakers-field, CA, between December 2, 1998, and January 31, 1999. The purpose of this analysis was to evaluate the suitability of each instrument for use in a real-time continuous monitoring network designed to measure the mass of airborne particles with an aerodynamic diam less than 2.5 μm (PM2.5) under wintertime conditions in the southern San Joaquin Valley. Measurements of airborne particulate mass made with a beta attenuation monitor (BAM), an integrating nephelometer, and a continuous aerosol mass monitor (CAMM) were found to correlate well with reference measurements made with a filter-based sampler. A Dusttrak aerosol sampler overestimated airborne particle concentrations by a factor of ~3 throughout the study. Measurements of airborne particulate matter made with a tapered element oscillating microbalance (TEOM) were found to be lower than the reference filter-based measurements by an amount approximately equal to the concentration of NH4NO3 observed to be present in the airborne particles. The performance of the Dusttrak sampler and the integrating nephelometer was affected by the size distribution of airborne particulate matter. The performance of the BAM, the integrating nephelometer, the CAMM, the Dusttrak sampler, and the TEOM was not strongly affected by temperature, relative humidity, wind speed, or wind direction within the range of conditions encountered in the current study. Based on instrument performance, the BAM, the integrating nephelometer, and the CAMM appear to be suitable candidates for deployment in a real-time continuous PM2.5 monitoring network in central California for the range of winter conditions and aerosol composition encountered during the study.

Aerosol backscatter fraction and single scattering albedo: Measured values and uncertainties at a coastal station in the Pacific Northwest

Light scattering, hemispheric backscattering, and absorption properties of submicrometer and supermicrometer aerosol particles at low relative humidity and 550 nm wavelength are investigated as a function of air mass category during a 2-month campaign at a midlatitude Pacific coastal station at Cheeka Peak, Washington. The main source of uncertainty in single scattering albedo (ω) measurements, namely, the measurement of light absorption, is addressed by the deployment of three identical absorption photometers and by relying on a recent calibration of this device using direct optical measurements. The absorption photometer measurement is corrected for response to light scattering, and measurements of sea-salt aerosol in this campaign provide a partial validation of this correction. Scattering measurements by nephelometry are also corrected for known instrumental nonidealities. Uncertainties stemming from instrumental noise, drift, calibration, and correction factors are propagated to allow comparisons among air mass categories and with other data sets and, ultimately, to constrain the values of ω and other optical properties used in climate models. Marine aerosol over the midlatitude eastern Pacific is found to be weakly absorbing for the sub-μm component and virtually nonabsorbing for the super-μm component (separated at 1 μm, low-relative humidity, aerodynamic diameter). A distinct increase in sub-μm light extinction (especially absorption) observed during 2 days of sustained marine flow appears to be Asian pollution transported across the Pacific. Low levels of gaseous NO x during this period rule out nearby combustion sources, and low levels of particulate Fe, Al, and Si rule out a significant contribution from mineral dust. Excluding this episode, both scattering and absorption properties for marine sampling conditions are similar to those observed in the clean midlatitude Southern Hemisphere (Cape Grim, Tasmania). In general, continental influence, as indicated by trends over the air mass categories, tends to raise the backscatter ratio and lower ω. Light absorption values compared to previous marine and coastal measurements confirm the range of values found by others and the highly variable nature of this quantity.

Long‐range transport of anthropogenic aerosols to the National Oceanic and Atmospheric Administration baseline station at Mauna Loa Observatory, Hawaii

Size-segregated measurements of aerosol mass and composition are used to determine the composition and seasonal variations of natural and anthropogenic aerosols at Mauna Loa Observatory (MLO) from 1993 through 1996. Although the springtime transport of Asian dust to MLO is a well-documented phenomenon, this study shows that fine anthropogenic aerosols, including sulfur, black carbon, and enriched trace metals such as As, Cu, Pb, and Zn, are also routinely transported to MLO each spring. It is estimated that at least one third of the sulfate measured at MLO during the spring is anthropogenic. In addition, indirect measurements indicate that the organic aerosol concentrations are often comparable to the sulfate concentrations. This study also combines size- and time-resolved aerosol composition measurements with isentropic, backward air-mass trajectories and gas measurements of 222Rn, CH4, CO, and CO2 to identify some potential source regions of the anthropogenic aerosols. Three types of long-range transport episodes are identified: (1) anthropogenic aerosols mixed with Asian dust, (2) Asian pollution with relatively small amounts of soil dust, and (3) biomass burning emissions from North America. This study shows that anthropogenic aerosols and gases can be efficiently transported to MLO from both Asia and North America during the spring.

Further evidence for particle nucleation in clear air adjacent to marine cumulus clouds

Observational evidence is presented for the nucleation of condensation nuclei (CN) in the clear air adjacent to an isolated, marine, cumulus cloud. Two separate regions of particle nucleation are identified: one located above the cloud top, and the second located downwind of the cloud near the level of the anvil outflow. The regions of high CN concentrations were located in extremely clean marine air, with unactivated aerosol surface area (excluding the nucleation mode) <2 μm2 cm−3, air temperature −31°C, and higher relative humidities than the undisturbed environment. Vertical profile measurements downwind of the cloud showed that CN concentrations at the level of the anvil outflow (4.9 km) were 8 times greater than at any other level between the surface and 5.3 km. A conceptual model is formulated in which aerosol particles, sulfur dioxide (SO2), sulfuric acid vapor (H2SO4), dimethyl sulfide (DMS), and ozone (O3) from the boundary layer are entrained into the cumulus cloud. Total aerosol number concentrations and unactivated aerosol surface area decrease with height in the cloud due to Brownian diffusion and diffusiophoresis of cloud interstitial aerosol to hydrometeors, coalescence scavenging by cloud droplets, collisional scavenging by ice particles, and subsequent removal by precipitation. The air that is detrained from the cloud raises the relative humidity and vents the clean air, SO2, H2SO4, DMS, and O3 to the near-cloud environment. Hydroxyl radicals then oxidize the SO2 and DMS to H2SO4. Under the conditions of high relative humidity, low total aerosol surface area, low temperatures, and high SO2 concentrations near cloud top, significant concentrations of new particles can be produced by homogeneous-bimolecular nucleation of sulfuric acid solution droplets from H2SO4 and H2O vapor molecules. The concentration of CN as a function of time is calculated for the case described in this paper using a bimodal integral nucleation model. The model results show that significant numbers of CN could have been produced within a few hours by the homogeneous-bimolecular nucleation of sulfuric acid solution droplets under the observed conditions provided the concentration of SO2 near cloud top was enhanced by vertical transport.

Intercontinental Dispersal of Bacteria and Archaea by Transpacific Winds

ABSTRACT 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.

Modeling the atmospheric distribution of mineral aerosol: Comparison with ground measurements and satellite observations for yearly and synoptic timescales over the North Atlantic

We present here a 3-year simulation (1990 to 1992) of the atmospheric cycle of Saharan dust over the Atlantic with an off-line three-dimensional transport model. The results of the simulation have been compared with selected relevant measurements. Careful attention has been paid to the spatial and temporal consistency between the observations and the model results. Satellite observations of optical thickness and the model show a closely similar latitudinal shift and change of the aerosol plume extent from month to month over 3 years. This is explained by the dominant role of the large-scale transport, well described by the European Centre for Medium-Range Weather Forecasts winds, a sufficiently consistent description of aerosol physics along with a detailed prognostic source function. A feature not captured perfectly by the model is the winter maximum in observed optical depth, which is south of the satellite observation window. This underestimate in the very southern tropical region in winter suggests that additional aerosol sources become important, such as Sahelian dust and carbonaceous aerosols from biomass burning, not included in our simulation. However, spring and autumn simulated optical thickness is 50% less than that observed, while it is only 30% less in summer and winter. This is found for both the subtropical and the tropical Atlantic Ocean, which points to a general underestimate by the model, not just because of aerosol sources missing in the Sahel region. Another seasonal feature is discussed for Sal Island where measurements suggest that low-level dust transport in winter is replaced by a pronounced high-level Saharan dust layer in summer. The model reproduces this pattern except that there is also significant low level transport in summer, associated mainly with peculiar simulated dust transport events from the western Sahara. On a synoptic scale the frequency of dust outbreaks over the North Atlantic and of major dust deposition events in Spain and a dust vertical profile measured by a lidar over the Azores region are reproduced by the model.

Influences of Isolated Cumulus Clouds on the Humidity of Their Surroundings

Abstract Measurements are described of the distributions of humidity in the clear air surrounding small to medium sized, isolated cumulus clouds. Wind shear is the most important factor in determining the radial distribution of enhanced humidity around cumulus clouds. Significant humidity enhancements occur in the clear air on the downshear and cross-shear sides of cumulus clouds; comparatively little humidity enhancement occurs on the upshear side. The horizontal extents of the humidity perturbations in the clear air increase with cloud age. Stable atmospheric layers can have a significant effect on the vertical distribution of the humidity field; enhancements in humidity are generally confined below any stable layer. Four detailed case studies are used to evaluate the performance of the cumulus detrainment criteria suggested by Bretherton and Smolarkiewicz and Taylor and Baker. Although these criteria were formulated for clouds in an environment with no vertical wind shear, they predict the altitudes of...

The Use of STIM and PESA to Measure Profiles of Aerosol Mass and Hydrogen Content, Respectively, across Mylar Rotating Drums Impactor Samples

A method has been developed for measuring profiles of aerosol mass on thin (480 w g/cm 2 ) Apiezon-L coated Mylar films employed in rotating drum aerosol impactor samplers using the ion beam analysis technique scanning transmission ion microscopy (STIM). The greased Mylar films are excellent impaction substrates and possess excellent uniformity in projected density, making them an ideal substrate for STIM analysis. The uniformity in projected density of a film enables STIM with a 3 MeV proton beam to produce profiles of aerosol mass with an accuracy of better than 90% and a mass sensitivity approaching 10 w g/cm 2 . Further, we have extended proton elastic scattering analysis (PESA) to the same films, achieving measurement of an organic surrogate. Although the films contain ∼ 20 w g/cm 2 hydrogen, the spatial uniformity in film hydrogen content enables PESA with a 3 MeV proton beam to produce profiles of hydrogen arising solely from the aerosols with an accuracy to within - 1 w g/cm 2 and a mass sensitivity of ∼ 1 w g/cm 2 . These measurements when combined with synchrotron-x-ray fluorescence (S-XRF) measurements on the same film allow mass closure, sum of species versus measured mass, a key quality assurance protocol, to be approached. All 3 techniques were applied to very fine and ultra-fine particles collected in Fresno, CA, November, 2000 by slotted DRUM samplers. Temporal resolution in the resulting profiles was h 6 h. The dramatic changes in composition versus size and time, and new types of elemental correlations unseen in PM 2.5 filters, will be major assets in correlating aerosols and health impacts, visibility degradation, and the effects of aerosols on climate.

Effects of Outdoor Pyrotechnic Displays on the Regional Air Quality of Western Washington State

Data from a PM25 (i.e., Dp < 2.5 mm) particulate matter monitoring network was used to quantify the effects of outdoor pyrotechnic displays on the regional air quality of western Washington State. Linear regression and principal component analysis demonstrated that the fine par-ticulate matter generated by these displays was primarily composed of Sr, K, V, Ti, Ba, Cu, Pb, Mg, Al, S, Mn, Zn,and soot. The maximum 24-hour averaged PM2.5 mass concentration apportioned to the pyrotechnic displays by absolute principal component scores regression analysis was 18.5 mg/m3. The majority of this mass (54%) was composed of K and S, which originated from the combustion of black powder. The distribution of smoke emissions from the displays closely resembled the population distribution of western Washington. The PM2.5 aerosol monitoring network tracked the pyrotechnic smoke plume for a period of two days as it was advected by low-level winds. The geometric mass mean diameter of the K particles was ~0.7 mm after transport of ~100 km. In the absence of rain, which is the primary sink for particles of this size, the particulate matter generated by the pyrotechnic displays could have an atmospheric residence time of more than one week.