259 research outputs found
Influences on the fraction of hydrophobic and hydrophilic black carbon in the atmosphere
Black carbon (BC) is a short term climate forcer that directly warms the atmosphere, slows convection, and hinders quantification of the effect of greenhouse gases on climate change. The atmospheric lifetime of BC particles with respect to nucleation scavenging in clouds is controlled by their ability to serve as cloud condensation nuclei (CCN). To serve as CCN under typical conditions, hydrophobic BC particles must acquire hygroscopic coatings. However, the quantitative relationship between coatings and hygroscopic properties for ambient BC particles is not known nor is the time scale for hydrophobic-to-hydrophilic conversion. Here we introduce a method for measuring the hygroscopicity of externally and internally mixed BC particles by coupling a single particle soot photometer with a humidified tandem differential mobility analyzer. We test this technique using uncoated and coated laboratory generated model BC compounds and apply it to characterize the hygroscopicity distribution of ambient BC particles. From these data we derive that the observed number fraction of BC that is CCN active at 0.2% supersaturation is generally low in an urban area near sources and that it varies with the trajectory of the airmass. We anticipate that our method can be combined with measures of air parcel physical and photochemical age to provide the first quantitative estimates for characterizing hydrophobic-to-hydrophilic conversion rates in the atmosphere.Peer reviewe
The interaction of frictional slip and adhesion for a stiff sphere on a compliant substrate
How friction affects adhesion is addressed. The problem is considered in the
context of a very stiff sphere adhering to a compliant, isotropic, linear
elastic substrate, and experiencing adhesion and frictional slip relative to
each other. The adhesion is considered to be driven by very large attractive
tractions between the sphere and the substrate that can act only at very small
distances between them. As a consequence, the adhesion behavior can be
represented by the Johnson-Kendall-Roberts model, and this is assumed to
prevail also when frictional slip is occurring. Frictional slip is considered
to be resisted by a uniform, constant shear traction at the slipping interface,
a model that is considered to be valid for small asperities and for compliant
elastomers in contact with stiff material. A model for the interaction of
friction and adhesion, known to agree with some experimental data, is utilized.
This model is due to Johnson, and its adhesion-friction interaction is assumed
to stem, upon shrinkage of the contact area, from a postulated reversible
energy release associated with frictional slip. This behavior is considered to
arise from surface microstructures generated or eliminated by frictional slip,
where these microstructures store some elastic strain energy in a reversible
manner. The associated reversible energy release rate is derived from the
energy exchanges that occur in the system. The Johnson model, and an asymptotic
analysis of it for small amounts of frictional slip, is shown to be consistent
with the reversible energy release rate that we identify.Comment: 11 page
Airborne observations of aerosol microphysical properties and particle ageing processes in the troposphere above Europe
In-situ measurements of aerosol microphysical properties were performed in May 2008 during the EUCAARI-LONGREX campaign. Two aircraft, the FAAM BAe-146 and DLR Falcon 20, operated from Oberpfaffenhofen, Germany. A comprehensive data set was obtained comprising the wider region of Europe north of the Alps throughout the whole tropospheric column. Prevailing stable synoptic conditions enabled measurements of accumulating emissions inside the continental boundary layer reaching a maximum total number concentration of 19 000 particles cm<sup>−3</sup> stp. Ultra-fine particles as indicators for nucleation events were observed within the boundary layer during high pressure conditions and after updraft of emissions induced by frontal passages above 8 km altitude in the upper free troposphere. Aerosol ageing processes during air mass transport are analysed using trajectory analysis. The ratio of particles containing a non-volatile core (250 °C) to the total aerosol number concentration was observed to increase within the first 12 to 48 h from the particle source from 50 to 85% due to coagulation. Aged aerosol also features an increased fraction of accumulation mode particles of approximately 40% of the total number concentration. The presented analysis provides an extensive data set of tropospheric aerosol microphysical properties on a continental scale which can be used for atmospheric aerosol models and comparisons of satellite retrievals
Evolution of trace gases and particles emitted by a chaparral fire in California
Biomass burning (BB) is a major global source of trace gases and particles. Accurately representing the production and evolution of these emissions is an important goal for atmospheric chemical transport models. We measured a suite of gases and aerosols emitted from an 81 hectare prescribed fire in chaparral fuels on the central coast of California, US on 17 November 2009. We also measured physical and chemical changes that occurred in the isolated downwind plume in the first ~4 h after emission. The measurements were carried out onboard a Twin Otter aircraft outfitted with an airborne Fourier transform infrared spectrometer (AFTIR), aerosol mass spectrometer (AMS), single particle soot photometer (SP2), nephelometer, LiCor CO_2 analyzer, a chemiluminescence ozone instrument, and a wing-mounted meteorological probe. Our measurements included: CO_2; CO; NO_x; NH_3; non-methane organic compounds; organic aerosol (OA); inorganic aerosol (nitrate, ammonium, sulfate, and chloride); aerosol light scattering; refractory black carbon (rBC); and ambient temperature, relative humidity, barometric pressure, and three-dimensional wind velocity. The molar ratio of excess O_3 to excess CO in the plume (ΔO_3/ΔCO) increased from −5.13 (±1.13) × 10^(−3) to 10.2 (±2.16) × 10^(−2) in ~4.5 h following smoke emission. Excess acetic and formic acid (normalized to excess CO) increased by factors of 1.73 ± 0.43 and 7.34 ± 3.03 (respectively) over the same time since emission. Based on the rapid decay of C_2H_4 we infer an in-plume average OH concentration of 5.27 (±0.97) × 10^6 molec cm^(−3), consistent with previous studies showing elevated OH concentrations in biomass burning plumes. Ammonium, nitrate, and sulfate all increased over the course of 4 h. The observed ammonium increase was a factor of 3.90 ± 2.93 in about 4 h, but accounted for just ~36% of the gaseous ammonia lost on a molar basis. Some of the gas phase NH_3 loss may have been due to condensation on, or formation of, particles below the AMS detection range. NO_x was converted to PAN and particle nitrate with PAN production being about two times greater than production of observable nitrate in the first ~4 h following emission. The excess aerosol light scattering in the plume (normalized to excess CO_2) increased by a factor of 2.50 ± 0.74 over 4 h. The increase in light scattering was similar to that observed in an earlier study of a biomass burning plume in Mexico where significant secondary formation of OA closely tracked the increase in scattering. In the California plume, however, ΔOA/ΔCO_2 decreased sharply for the first hour and then increased slowly with a net decrease of ~20% over 4 h. The fraction of thickly coated rBC particles increased up to ~85% over the 4 h aging period. Decreasing OA accompanied by increased scattering/particle coating in initial aging may be due to a combination of particle coagulation and evaporation processes. Recondensation of species initially evaporated from the particles may have contributed to the subsequent slow rise in OA. We compare our results to observations from other plume aging studies and suggest that differences in environmental factors such as smoke concentration, oxidant concentration, actinic flux, and RH contribute significantly to the variation in plume evolution observations
In situ measurements of trace gases, PM, and aerosol optical properties during the 2017 NW US wildfire smoke event
In mid-August through mid-September of 2017 a major wildfire smoke and haze
episode strongly impacted most of the NW US and SW Canada. During this period
our ground-based site in Missoula, Montana, experienced heavy smoke impacts
for ∼ 500 h (up to 471 µg m−3 hourly average
PM2.5). We measured wildfire trace gases, PM2.5 (particulate matter
≤2.5 µm in diameter), and black carbon and submicron aerosol
scattering and absorption at 870 and 401 nm. This may be the most extensive
real-time data for these wildfire smoke properties to date. Our range of
trace gas ratios for ΔNH3∕ΔCO and ΔC2H4∕ΔCO confirmed that the smoke from mixed, multiple sources
varied in age from ∼ 2–3 h to ∼ 1–2 days. Our study-average
ΔCH4∕ΔCO ratio (0.166±0.088) indicated a large
contribution to the regional burden from inefficient smoldering combustion.
Our ΔBC∕ΔCO ratio (0.0012±0.0005) for our ground
site was moderately lower than observed in aircraft studies (∼ 0.0015)
to date, also consistent with a relatively larger contribution from
smoldering combustion. Our ΔBC∕ΔPM2.5 ratio (0.0095±0.0003) was consistent with the overwhelmingly non-BC (black carbon),
mostly organic nature of the smoke observed in airborne studies of wildfire
smoke to date. Smoldering combustion is usually associated with enhanced PM
emissions, but our ΔPM2.5∕ΔCO ratio (0.126±0.002)
was about half the ΔPM1.0∕ΔCO measured in fresh
wildfire smoke from aircraft (∼ 0.266). Assuming PM2.5 is
dominated by PM1, this suggests that aerosol evaporation, at least near
the surface, can often reduce PM loading and its atmospheric/air-quality
impacts on the timescale of several days. Much of the smoke was emitted late
in the day, suggesting that nighttime processing would be important in the
early evolution of smoke. The diurnal trends show brown carbon (BrC),
PM2.5, and CO peaking in the early morning and BC peaking in the early
evening. Over the course of 1 month, the average single scattering albedo for
individual smoke peaks at 870 nm increased from ∼ 0.9 to ∼ 0.96.
Bscat401∕Bscat870 was used as a proxy for the size and
“photochemical age” of the smoke particles, with this interpretation being
supported by the simultaneously observed ratios of reactive trace gases to
CO. The size and age proxy implied that the Ångström absorption
exponent decreased significantly after about 10 h of daytime smoke aging,
consistent with the only airborne measurement of the BrC lifetime in an
isolated plume. However, our results clearly show that non-BC absorption can
be important in “typical” regional haze and moderately aged smoke, with BrC
ostensibly accounting for about half the absorption at 401 nm on average for
our entire data set.</p
Simulation of the contractile response of cells on an array of micro-posts
A bio-chemo-mechanical model has been used to predict the contractile responses of smooth cells on a bed of micro-posts. Predictions obtained for smooth muscle cells reveal that, by converging onto a single set of parameters, the model captures all of the following responses in a self-consistent manner: (i) the scaling of the force exerted by the cells with the number of posts; (ii) actin distributions within the cells, including the rings of actin around the micro-posts; (iii) the curvature of the cell boundaries between the posts; and (iv) the higher post forces towards the cell periphery. Similar correspondences between predictions and measurements have been demonstrated for fibroblasts and mesenchymal stem cells once the maximum stress exerted by the stress fibre bundles has been recalibrated. Consistent with measurements, the model predicts that the forces exerted by the cells will increase with both increasing post stiffness and cell area (or equivalently, post spacing). In conjunction with previous assessments, these findings suggest that this framework represents an important step towards a complete model for the coupled bio-chemo-mechanical responses of cells
Black carbon measurements in the boundary layer over western and northern Europe
Europe is a densely populated region that is a significant global source of black carbon (BC) aerosol, but there is a lack of information regarding the physical properties and spatial/vertical distribution of rBC in the region. We present the first aircraft observations of sub-micron refractory BC (rBC) aerosol concentrations and physical properties measured by a single particle soot photometer (SP2) in the lower troposphere over Europe. The observations spanned a region roughly bounded by 50° to 60° N and from 15° W to 30° E. The measurements, made between April and September 2008, showed that average rBC mass concentrations ranged from about 300 ng m−3 near urban areas to approximately 50 ng m−3 in remote continental regions, lower than previous surface-based measurements. rBC represented between 0.5 and 3% of the sub-micron aerosol mass. Black carbon mass size distributions were log-normally distributed and peaked at approximately 180 nm, but shifted to smaller diameters (~160 nm) near source regions. rBC was correlated with carbon monoxide (CO) but had different ratios to CO depending on location and air mass. Light absorption coefficients were measured by particle soot absorption photometers on two separate aircraft and showed similar geographic patterns to rBC mass measured by the SP2. We summarize the rBC and light absorption measurements as a function of longitude and air mass age and also provide profiles of rBC mass concentrations and size distribution statistics. Our results will help evaluate model-predicted regional rBC concentrations and properties and determine regional and global climate impacts from rBC due to atmospheric heating and surface dimming
Observations and analysis of organic aerosol evolution in some prescribed fire smoke plumes
Open biomass burning is a significant source of primary air pollutants such as particulate matter and non-methane organic gases. However, the physical and chemical atmospheric processing of these emissions during transport is poorly understood. Atmospheric 5 transformations of biomass burning emissions have been investigated in environmental chambers, but there have been limited opportunities to investigate these transformations in the atmosphere. In this study, we deployed a suite of real-time instrumentation on a Twin Otter aircraft to sample smoke from prescribed fires in South Carolina, conducting measurements at both the source and downwind to character10 ize smoke evolution with atmospheric aging. Organic aerosol (OA) within the smoke plumes was quantified using an Aerosol Mass Spectrometer (AMS), along with refractory black carbon (rBC) using a Single Particle Soot Photometer and carbon monoxide (CO) and carbon dioxide (CO2) using a Cavity Ring-Down Spectrometer. During the two fires for which we were able to obtain aerosol aging data, normalized excess mix15 ing ratios and “export factors” of conserved species (rBC, CO, CO2) were unchanged with increasing sample age. Investigation of AMS mass fragments indicated that the inplume fractional contribution (fm/z) to OA of the primary fragment (m/z 60) decreased downwind, while the fractional contribution of the secondary fragment (m/z 44) increased. Increases in f44 are typically interpreted as indicating chemical production 20 of secondary OA (SOA). Likewise, we observed an increase in the O:C elemental ratio downwind, which is usually associated with aerosol aging. However, the rapid mixing of these plumes into the background air suggests that these chemical transformations may be attributable to the dierent volatilities of the compounds that fragment to these m/z in the AMS. The gas-particle partitioning behavior of the bulk OA ob25 served during the study was consistent with the predictions from a parameterization developed for open biomass burning emissions in the laboratory. Furthermore, we observed no statistically-significant increase in total organic mass with atmospheric transport. Hence, our results suggest that dilution-driven evaporation likely dominated over chemical production of SOA within our smoke plumes, likely due to the fast dilution and limited aging times
Towards closing the gap between hygroscopic growth and activation for secondary organic aerosol: Part 1 – Evidence from measurements
Secondary Organic Aerosols (SOA) studied in previous laboratory experiments generally showed only slight hygroscopic growth, but a much better activity as a CCN (Cloud Condensation Nucleus) than indicated by the hygroscopic growth. This discrepancy was examined at LACIS (Leipzig Aerosol Cloud Interaction Simulator), using a portable generator that produced SOA particles from the ozonolysis of <i>α</i>-pinene, and adding butanol or butanol and water vapor during some of the experiments. The light scattering signal of dry SOA-particles was measured by the LACIS optical particle spectrometer and was used to derive a refractive index for SOA of 1.45. LACIS also measured the hygroscopic growth of SOA particles up to 99.6% relative humidity (RH), and a CCN counter was used to measure the particle activation. SOA-particles were CCN active with critical diameters of e.g. 100 nm and 55 nm at super-saturations of 0.4% and 1.1%, respectively. But only slight hygroscopic growth with hygroscopic growth factors &le;1.05 was observed at RH<98% RH. At RH>98%, the hygroscopic growth increased stronger than would be expected if a constant hygroscopicity parameter for the particle/droplet solution was assumed. An increase of the hygroscopicity parameter by a factor of 4–6 was observed in the RH-range from below 90% to 99.6%, and this increase continued for increasingly diluted particle solutions for activating particles. This explains an observation already made in the past: that the relation between critical super-saturation and dry diameter for activation is steeper than what would be expected for a constant value of the hygroscopicity. Combining measurements of hygroscopic growth and activation, it was found that the surface tension that has to be assumed to interpret the measurements consistently is greater than 55 mN/m, possibly close to that of pure water, depending on the different SOA-types produced, and therefore only in part accounts for the discrepancy between hygroscopic growth and CCN activity observed for SOA particles in the past
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Aerosol scattering and absorption during the EUCAARI-LONGREX flights of the Facility for Airborne Atmospheric Measurements (FAAM) BAe-146: can measurements and models agree?
Scattering and absorption by aerosol in anthropogenically
perturbed air masses over Europe has been measured
using instrumentation flown on the UK’s BAe-146-301
large Atmospheric Research Aircraft (ARA) operated by the
Facility for Airborne Atmospheric Measurements (FAAM)
on 14 flights during the EUCAARI-LONGREX campaign
in May 2008. The geographical and temporal variations of
the derived shortwave optical properties of aerosol are presented. Values of single scattering albedo of dry aerosol at 550 nm varied considerably from 0.86 to near unity, with a campaign average of 0.93 ± 0.03. Dry aerosol optical depths ranged from 0.030 ± 0.009 to 0.24 ± 0.07. An optical properties closure study comparing calculations from composition data and Mie scattering code with the measured properties is presented. Agreement to within measurement uncertainties of 30% can be achieved for both scattering and absorption,but the latter is shown to be sensitive to the refractive indices chosen for organic aerosols, and to a lesser extent black carbon, as well as being highly dependent on the accuracy of the absorption measurements. Agreement with the measured absorption can be achieved either if organic carbon is assumed to be weakly absorbing, or if the organic aerosol is purely scattering and the absorption measurement is an overestimate due to the presence of large amounts of organic carbon. Refractive
indices could not be inferred conclusively due to this
uncertainty, despite the enhancement in methodology compared
to previous studies that derived from the use of the
black carbon measurements. Hygroscopic growth curves derived
from the wet nephelometer indicate moderate water uptake
by the aerosol with a campaign mean f (RH) value (ratio
in scattering) of 1.5 (range from 1.23 to 1.63) at 80% relative humidity. This value is qualitatively consistent with the major chemical components of the aerosol measured by the aerosol mass spectrometer, which are primarily mixed organics and nitrate and some sulphate
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