Free Tropospheric Aerosol Research Articles

A STUDY OF PROCESSES THAT GOVERN THE MAINTENANCE OF AEROSOLS IN THE MARINE BOUNDARY LAYER

We systematically evaluate the relative influence of sources and sinks of particles in the remote marine boundary layer (MBL) to elucidate the principal factors that govern MBL aerosol behavior. Processes considered include: (1) surface flux of dimethyl sulfide (DMS); (2) gas-phase oxidation of DMS to SO 2; (3) gas-phase oxidation of SO 2 to H 2SO 4; (4) mass transfer of SO 2 and H 2SO 4 to pre-existing particles; (5) homogenous nucleation of H 2SO 4/H 2O; (6) entrainment of air from the free troposphere; (7) deposition to the sea surface; (8) cycling of air through clouds and rain scavenging; (9) oxidation of SO 2 in sea salt aerosols and cloud droplets; and (10) sea-salt particle production at sea surface. The average aerosol number concentration is found to be quite sensitive to the rate of entrainment of aerosol-containing air from the free troposphere. The path that leads to the greatest accumulation of non-sea-salt (nss) sulfate involves SO 2 (rather than H 2SO 4) absorption into existing particles. Because of scavenging of SO 2 and H 2SO 4 by sea-salt aerosol, a considerable fraction of nss-sulfate is internally mixed with sea-salt aerosol. Under the conditions assumed in this study, MBL aerosol number concentration is dominated by free tropospheric aerosol under virtually all conditions, 89% in the base case, and even 69% at a 17 m s -1 wind speed. Aerosol mass, on the other hand, is dominated by sea-salt particles, 62% in the base case and 98% at a wind speed of 17 m s -1. Evaporation of cloud droplets provides 4.6% of the particle number in the base case, but 28% of the particle mass. At the high nucleation rate case considered here, there is notably little change in the overall contributions to aerosol number and mass from the base case; only about 5% of the total particle number is provided by nucleation events. Variation in precipitation frequency also has only a minor effect on the overall contributions. One concludes that the MBL aerosol is remarkably robust in the face of ever-changing conditions. Free tropospheric aerosol entrainment tends to sustain particle number concentrations, and sea-salt emissions maintain most of the aerosol mass. Cloud processing, while not a major contributor to aerosol number, does provide, except under high wind conditions, the order of 20% of the aerosol mass. Although nucleation occurs only infrequently and does not contribute appreciably to long-term average aerosol number or mass, nucleation is, nonetheless, the mechanism that replenishes aerosol number in brief, intense episodes when aerosol surface area levels are substantially reduced by precipitation.

Chemical characteristics of free tropospheric aerosols over the Japan Sea coast: aircraft-borne measurements

Atmospheric aerosol particulate matter was directly collected in the free troposphere over the Japan Sea coast between 1992 and 1994 using an aircraft-borne nine-stage cascade impactor (particle size range: 0.1–8 μm). The water-soluble components in the aerosol particulate matter were analyzed by ion chromatography. Particulate sulfate and ammonium were detected in most of the samples and their size distributions showed noticeable peaks below the 1 μm particle size range. Water-soluble calcium (Ca 2+) was detected in half of the samples; the size distribution showed that the maximum particle size was larger than 1 μm. Highly concentrated Ca 2+ in larger particles was possibly due to transport of Kosa aerosols from the Asian continent in the free troposphere. The concentration of fine particulate sulfate and ammonium tended to increase whenever Ca 2+ was detected, which suggests possible mixing of Kosa aerosols and non-Kosa aerosols during long-range transport of air masses containing Kosa particles.

An empirical model study of the tropospheric meridional circulation based on SAGE II observations

This study investigates the tropospheric mean meridional circulation important to the development of opaque clouds and the measurement opportunity of the 1.02‐μm channel of the Stratospheric Aerosol and Gas Experiment (SAGE) II in the troposphere. A simple empirical model is formulated to derive the mean meridional circulation from the 6‐year (1985–1990) statistics of the SAGE II tropospheric measurement frequency. The vertical circulation of the model is assumed to be related to the departure field of the zonally averaged SAGE II measurement frequency from the corresponding global mean in a linear fashion. The proportional constant is calibrated with the observed upwelling circulation statistics in the tropics. The obtained model vertical circulation is then used to determine the distribution of meridional velocity according to the continuity equation. The derived model mean circulation features the influence from both the diabatic circulation and the eddy quasi‐isentropic transport, with a distinct pattern of material advection into the upper troposphere from both the lower troposphere and the stratosphere. Most significantly, the model circulation is shown to be highly consistent with the observed free tropospheric aerosol and ozone distributions, particularly with their seasonal variations given the aerosol and ozone source regions. This high degree of consistency illustrates the intimate relationship between the large‐scale circulation, cloudiness, and the SAGE II tropospheric measurement frequency, and the robust nature of the empirical model despite the model's simplicity. The discussion in relating the model circulation to the conventional Eulerian circulation and the Lagrangian transport, based on isentropic consideration, is also provided.

The Jungfraujoch high‐alpine research station (3454 m) as a background clean continental site for the measurement of aerosol parameters

The first annual data set of climatically important aerosol parameters, measured at the Jungfraujoch (JFJ) high‐alpine research station (3454 m, Switzerland) from an ongoing field campaign since July 1995, is presented. Analysis of diurnal variations in continuous measurements of the total and backward hemispheric scattering coefficients (σSP, σBSP), the absorption coefficient (σAP, from aethalometer data), condensation nuclei (CN) concentration, and epiphaniometer signal (related to surface area (S) concentration) established the diurnal period 0300 – 0900 as being representative of the free tropospheric background aerosol. The annual data set was then edited to omit (1) the period 0900–0300 (i.e., 18 hours), taken to represent possible planetary boundary layer influenced conditions and (2) in‐cloud conditions using a cloud liquid‐water monitor. The seasonal aerosol cycle exhibited a July maximum and a December minimum in most aerosol parameters. Typical monthly median values for the free troposphere exhibit the following seasonal maxima and minima, respectively: σSP (550 nm) ∼ 16.1 and 0.43 × 10−6 m−1, σBSP (550 nm) ∼ 2.10 and 0.09 × 10−6 m−1, σAP (550 nm) ∼ 10.4 and 0.76 × 10−7 m−1 (≈ 104 and 7.6 ng m−3 black carbon), CN concentration ∼ 670 and 280 cm−3, and epiphaniometer signal ∼ 9.26 and 0.67 counts s−1 (S concentration ≈24.1 and 1.7 μm2 cm−3). Aerosol parameters were found to be comparable in magnitude to other NOAA baseline and regional stations and suggest that a clean continental designation for the JFJ site is applicable, when removing the planetary boundary layer influenced period.

On the impact of Pacific Ocean free tropospheric background aerosols at the surface of the Earth-atmosphere system

On the impact of Pacific Ocean free tropospheric background aerosols at the surface of the Earth-atmosphere system

The vertical structure of aerosol and its relationship to boundary‐layer thermodynamics over the rural UK

AbstractThe vertical structure of aerosol, covering sizes from 0.05‐4 μm radius, was examined under conditions of subsidence during winter and summer over the rural UK. Under well‐mixed boundary layer conditions, dry accumulation mode aerosol was found to be well mixed with height. During the winter campaigns, nocturnal cooling resulted in the development of a stable surface layer, typically 100 m in depth, within which the surface emitted pollutants became trapped leading to concentrations significantly greater than that observed in the mixed boundary layer above it. Under stable boundary‐layer conditions, the aerosol and water vapour vertical profiles exhibited strong negative gradients with height and were indicative of suppressed turbulence associated with stable boundary‐layer conditions. During summer, the boundary layer was normally decoupled and possessed two cloud layers: cumuli forming just below, and penetrating the surface‐layer inversion; and stratocumulus occupying the region under the boundary‐layer capping inversion. Aerosol profiles under decoupled conditions exhibited considerable variability with peak concentrations being observed in the vicinity of cloud edges. Average aerosol concentrations in the main boundary‐layer ranged from 209–651 cm−3 and 0.89–4.3 μm−3 cm−3 for dry number and volume respectively, whilst concentrations and volumetric loadings of 239–2430 cm−3 and 1.1–13.5 μm−3 cm−3 were encountered in surface layers. The majority of the aerosol number and mass concentrations were almost exclusively derived from the size range 0.05‐0.2 μm radius with mode radii often occurring at 0.1 μm or larger. By comparison, free tropospheric aerosol possessed typically an order of magnitude lower concentrations and mass with an associated mode radius of 0.05–0.06 μm or less.

Size Distributions of Aerosol Particles in the Free Troposphere:Aircraft Measurements in the Spring of 1991-1994 Over Japan

Measurements of number-, mass-, and volume-size distributions of free tropospheric aerosols were made over Japan in the spring of 1991-1994. Number-size distribution frequently shows a peak in the area of a diameter of D≥1μm in the free troposphere during observational periods. A few peaks are identified in the volume-size distribution as estimated on the basis of number-size distribution having single mode in a coarse range. Mass-size distribution frequently indicate enhancement in the coarse size range. This feature of the distribution is more frequent in those measurements made at 4.4(±0.3) km than those at 2.3(±0.3) km. On the basis of a backward trajectory analysis of an air mass containing those particles, soil particles originating in the Asian continent affect the features found in the size distributions as well as vertical change in those size distributions.

Entrainment of free tropospheric aerosols as a regulating mechanism for cloud condensation nuclei in the remote marine boundary layer

Entrainment of free tropospheric aerosols as a regulating mechanism for cloud condensation nuclei in the remote marine boundary layer

Size distributions of sub-micron aerosols in the free troposphere and marine boundary layer of the sub-tropical North Atlantic

Size distributions of sub-micron aerosols in the free troposphere and marine boundary layer of the sub-tropical North Atlantic

Entrainment of free tropospheric aerosols as a regulating mechanism for cloud condensation nuclei in the remote marine boundary layer

A box model has been developed to study the formation of cloud condensation nuclei from DMS, in the unpolluted marine boundary layer (MBL). Chemical and microphysical processes include gas phase chemistry of DMS and SO2, formation and growth of H2SO4‐H2O aerosols, in‐cloud SO2 oxidation, and scavenging of SO2 and H2SO4 molecules by sea salt, cloud drops, and precipitation. Meteorological processes include exchange between the MBL and the overlying free troposphere (FT) and horizontal mixing after particle removal by precipitation. The description of the FT aerosol is based on the notion of new particle production in the upper troposphere (for which there is experimental evidence) and the notion that these particles evolve into a self‐preserving aerosol (for which there is no clear experimental evidence yet). We argue that FT aerosols are expected to be self‐preserving in areas of large‐scale subsidence such as the subtropics. The model results are in reasonable agreement with summer observations of DMS, SO2, non‐sea‐salt (nss) SO4, CN, and CCN at Cape Grim. They show that entrainment of FT aerosols in the MBL quenches new particle formation within the MBL. A sensitivity/uncertainty analysis shows that most of the variation in the number concentration of MBL aerosols is due to the variation in parameters linked to the FT aerosol, whereas most of the variations in the MBL nss‐sulphate mass is due to variations in the DMS flux. We conclude that FT‐MBL exchange is likely to be an important mechanism that can explain both the observed levels of CN and CCN (active in stratiform clouds) in the MBL and their lack of short‐term variability.

Aircraft observations of aerosols in the free marine troposphere over the North Pacific Ocean: Particle chemistry in relation to air mass origin

Aircraft measurements of atmospheric aerosol particles were performed in the free troposphere up to 5.0 km altitude over the North Pacific Ocean in August 1983 and 1984. Particles were analyzed for a single‐particle base with the regent thin‐film technique in conjunction with a transmission electron microscope. The calcium thin‐film method was used for the specific determination of the molecular form of sulfate particles. Nitrate and sulfate in particles was identified by the nitron and barium chloride thin‐film method, respectively. Ammonium was examined by the thin‐film method of sodium tetraphenyl boron. Collected aerosols were related to the origin of air parcels. Sulfuric acid particles in submicron size range formed by the photochemical oxidation of sulfur‐bearing gases and subsequent gas‐to‐particle conversion are the basic aerosols in the unpolluted free marine troposphere. They contain a small amount of NH4+ and more acidic than NH4HSO4. The chemical composition of the predominant aerosol changed drastically according to the origin of air parcels. Sulfuric acid aerosols were predominant in the air parcels originating from the midlatitude maritime atmosphere. On the other hand, for air parcels that have passed over the continent within several days, sulfate aerosols neutralized fully by ammonia are predominant. The extension of the continental air mass supplies ammoniated sulfate aerosols to remote marine atmosphere about 1200 km from land. Mineral dust particles are transported from Asian continent over the northwestern Pacific Ocean via the free troposphere. Nitrate was heterogeneously formed on the mineral dust particles and they were present in the air parcels passed over mainland of Japan, whereas few nitrate particles were found in the midlatitude marine atmosphere. Ammoniated sulfate particles, nitrate‐containing particles, and mineral dust particles are the tracers of the continental air mass.

A seasonal cycle in the southwest Pacific free tropospheric aerosol concentration

Measurements made of the concentration of aerosol particles in the diameter range 0.15 − 3μm in the free troposphere over the southwest Pacific show evidence of a seasonal cycle. The maximum concentration occurs in spring and the minimum in autumn. The amplitude of the cycle is greatest at 30°S where, in early spring, the mean aerosol concentration is 18 cm−3, more than 10 times the mean autumn value. In the vicinity of the equator the seasonality disappears and concentrations throughout the year apparently remain close to the autumn levels at other latitudes. A number of different mechanisms could account for the observed seasonality but it is probable that the peak in early spring is due to biomass burning through the tropical southern hemisphere dry season. Continued high values through late spring and early summer are thought to be the result of the seasonal production of aerosol precursors, such as dimethyl sulfide (DMS), at middle and high latitudes.

Measured and calculated optical property profiles in the mixed layer and free troposphere

Nearly simultaneous measurements of the physical and optical properties of mixed layer and free tropospheric aerosols near Boulder, Colorado, were made on several occasions using aircraft, balloon, and ground‐based sensors. This effort (Front Range Lidar, Aircraft, and Balloon experiment (FRLAB)) was conducted with the purpose of obtaining a diverse, self‐consistent data set that could be used for testing optical model calculations based on measured physical characteristics such as apparent size distribution, composition, and shape. It was found that even with the uncertainties involved, the model predictions are in good agreement with the measurements in the visible and near infrared wavelength regions. At CO2 lidar wavelengths there is considerably more uncertainty in both the calculated and measured values; however, within the estimated errors there appears to be satisfactory agreement except for the highest free tropospheric layer studied. The results also indicate that during FRLAB the aerosol in the boundary layer and free troposphere behaved as spherical particles for optical modeling purposes. The utility of the observations for determining the extinction‐to‐backscatter ratio relevant to aerosols in the boundary layer and free troposphere is described with typical measured values being in the 20 to 30 sr range.

Aerosol size distributions in the southwest Pacific

Measurements of the size distribution and concentration of the atmospheric aerosol were made from an instrumented aircraft at altitudes of up to 6 km around New Zealand (37°S to 46°S) and during flights to other southwest Pacific islands (to 9°S). Measurements were also made at the surface in Antarctica (78°S). Single‐particle optical counters were used to cover the size range 0.06 μm to 8.0 μm radius. The results show that the free troposphere aerosol concentration ranged from 1.8 to 46 cm−3 and the size distribution was characterized by a “Junge” slope around −3.5. In the planetary boundary layer, including Antarctica, the concentrations were generally greater and more variable, and the size distributions often bimodal.

SAGE I and SAM II Measurements of 1 μm Aerosol Extinction in the Free Troposphere

The SAGE 1 and SAM 2 satellite sensors were designed to measure, with global coverage, the 1 micron extinction produced by the stratospheric aerosol. In the absence of high altitude cloud, similar measurements may be made for the free tropospheric aerosol. Median extinction values in the Northern Hemisphere, for altitudes between 5 and 10 km, are found to be one-half to one order of magnitude greater than values at corresponding latitudes in the Southern Hemisphere. In addition, a seasonal increase by a factor of 1.5 yields 2 is observed in both hemispheres in local spring and summer. Following major volcanic eruptions, a long-lived enhancement of the aerosol extinction is observed for altitudes above 5 km.

Global measurements of aerosols in remote continental and marine regions: Concentrations, size distributions, and optical properties

The phase I Gametag (Global Atmospheric Measurements Experiment of Tropospheric Aerosols and Gases) aerosol measurements were designed to provide an initial assessment of the levels, types, and optical effects of tropospheric aerosols in remote marine and continental regions and to examine the possible causal relationships between the observed distributions and the dominant factors controlling aerosol population: chemical and physical transformations, source and sink strengths, and transport. We used size‐number data to determine mass concentrations and to estimate extinction, using nominal optical properties. Filter and impactor data have been used to determine aerosol composition, and correlative aircraft measurements have been used to aid in our data interpretation. Our data have been used to generate latitudinal profiles along our Pacific flight tracks. Our continental measurements, in general, show bimodal aerosol size distributions that reflect different source for each mode. The aerosol population consists primarily of crustal aerosols with r ≥ 0.5 μm and sulfate and combustion aerosols with r < 0.5 μm, with only a minor sea salt component. Owing to vertical mixing, there are no qualitative differences between the boundary layer and the free troposphere. Our data indicate that crustal aerosols represent a significant component of a background tropospheric aerosol in western North America and suggest that the possible contribution of the crustal aerosol to extinction should not be ignored. Pacific marine measurements show a qualitative difference between the boundary layer and the free troposphere. The boundary‐layer aerosol population is dominated by a bimodal sea spray aerosol; optical effects and mass concentration are dominated by a mode with a volume mean radius of ∼1 μm. Our measurements show only a small crustal component of the marine boundary‐layer aerosol. Our data indicate a loss of Cl from the sea spray aerosol, with the greatest loss in the small particles. We have inferred a background concentration of 0.2 ppbm for our measured particles that does not appear to be directly related to the sea spray aerosol. We have identified some of these particles as locally produced secondary aerosols; simultaneous measurements of gaseous species support this interpretation. Our Pacific free tropospheric aerosol measurements show a highly variable aerosol component, with local variations in concentration by 1 order of magnitude within a few kilometers. Our measured total aerosol and crustal component concentrations show a general decrease from north to south. Our lowest mean mid tropospheric concentration was seen south of 20°S; we have identified this mean concentration of 0.08 ppbm as a midtropospheric background aerosol.

Aerosol Number Size Distribution Measurements at Hanle, a Free Tropospheric High-Altitude Site in Western Himalayas

Aerosol characteristics over the high altitude stations, akin to pristine and free conditions are important to understand the background aerosol features against which polluted and urban environments could be compared. In addition it is also important to monitor the changes brought about by the large-scale processes, which result in lofting and transporting of aerosols from different source regions to higher altitude levels. Moreover, these aerosols have a significant role in modifying clouds especially cold clouds. The availability of significant amount of solar UV radiation at these altitudes, along with water vapour and OH, if present, makes these regions conducive for formation of new particles from gas phase reaction products involving precursors of natural or anthropogenic origin. Such processes play a significant role in modulating the size distribution of free tropospheric aerosols and hence their radiative impacts. In this work, aerosol number size distribution measurements carried out from a high altitude free tropospheric Himalayan location, Hanle (32.78°N, 78.96°E, 4530 m amsl) during the period from May through December 2010 are examined. The monthly mean total number concentration (Nt) increased from May (614 ± 188 cm-3) to October (1498 ± 792 cm-3) and decreased slightly through December (1158 ± 470 cm-3). Fine mode aerosols (size < 100nm) contributed mostly to the total number concentration. The fractional contribution of the fine mode aerosols to the total number concentration, showed a clear increasing trend from May (~ 0.57) to December (~ 0.81). The number size distribution, which remained unimodal in May, June months with a mode in accumulation size range (around 100nm), changed to a bimodal distribution subsequently with a mode in the nucleation size range (< 25nm), indicating the possibility of new particle formation. The results are discussed in light of the possible association between the variation of the aerosol number concentration to long-range transport and thermally driven mesoscale processes such as mountain/valley winds. DOI: http://dx.doi.org/10.3126/jie.v8i3.5940 JIE 2011; 8(3): 140-146