Incorporating the direct effect of tropospheric aerosol on climate into global climate models involves coupling the optical properties of the aerosol with its physical and chemical properties. This ...coupling is strengthened if the optical, physical, and chemical properties of the individual aerosol components are known as well as how these properties depend on the air mass source and synoptic scale meteorology. To relate properties of the aerosol components to air mass sources over a wide range of meteorological conditions, two long latitudinal cruises were conducted in the central Pacific Ocean from 55°N to 70°S. Submicron non‐sea‐salt (nss) SO4= aerosol averaged about 35 to 40% of the submicron ionic mass as analyzed by ion chromatography and 6% of the total ionic mass, while supermicron nss SO4= aerosol contributed about 1% to the total ionic mass. About 1% of the remaining total ionic mass was composed of methanesulfonate and 90% was sea salt. Ionic mass fractions of nss SO4= aerosol were highest in regions having the longest marine boundary layer residence times or the largest source of marine or continental gas phase precursors. The calculated scattering by nss SO4= aerosol was highest in these same regions due to the dependence of scattering on particle size and the concentration of nss SO4= in the submicron size range. The calculated scattering by submicron sea salt was similar to that of the nss SO4= aerosol, indicating that its contribution to scattering in the marine boundary layer can be significant or even dominant depending on its mass concentration. Mass scattering efficiencies for nss SO4= at 30% RH ranged from 4.3 to 7.5 m2 g−1 and for submicron sea salt from 3.5 to 7.7 m2 g−1. Mass backscattering efficiencies for nss SO4= ranged from 0.41 to 0.58 m2 g−1 and for submicron sea salt from 0.33 to 0.63 m2 g−1. These values fall within the same range as others reported previously for the marine atmosphere.
Measurements of the relative humidity dependence of aerosol light scattering are reported from three experimental venues over the Pacific Ocean. The measurement platform utilized was the CIRPAS Twin ...Otter aircraft. Results are compared with previous measurements at other locales and with theoretical models. The relatively low values of hygroscopicity obtained in marine air are consistent with a substantial organic component to the aerosol.
Measurements of submicron aerosol mass and the mass of major ionic components have been made over the past 5 years on cruises in the Pacific and Southern Oceans and at monitoring stations across ...North America (Barrow, Alaska; Cheeka Peak, Washington; Bondville, Illinois; and Sable Island, Nova Scotia). Reported here are submicron concentrations of aerosol mass, nonsea salt (nss) sulfate, sea salt, methanesulfonate, other nss inorganic ions, and residual, or chemically unanalyzed, mass. Residual mass concentrations are based on the difference between simultaneously measured aerosol mass and the mass of the major ionic components. A standardized sampling protocol was used for all measurements making the data from each location directly comparable. For the Pacific and Southern Oceans, concentrations of the chemical components are presented in zonally averaged 20° latitude bins. For the monitoring stations, mean concentrations are presented for distinct air mass types (marine, clean continental, and polluted based on air mass back trajectories). In addition, percentile information for each chemical component is given to indicate the variability in the measured concentrations. Mean nss sulfate submicron aerosol mass fractions for the different latitude bins of the Pacific ranged from 0.14±0.01 to 0.34±0.03 (arithmetic mean±absolute uncertainty at the 95% confidence level). The lowest average value occurred in the 40°–60°S latitude band where nss sulfate concentrations were low due to the remoteness from continental sources and sea salt concentrations were relatively high. Mean nss sulfate aerosol mass fractions were more variable at the monitoring stations ranging from 0.13±0.004 to 0.65±0.02. Highest values occurred in polluted air masses at Bondville and Sable Island. Sea salt mean mass fractions ranged between 0.20±0.02 and 0.53±0.03 at all latitude bands of the Pacific (except 20°–40°N where the residual mass fraction was relatively high) and at Barrow. The concentration of residual mass was significant at the 95% confidence level at all stations and all Pacific latitude bands (assuming that all errors were random and normally distributed and contamination of the samples did not occur beyond that accounted for by storage and transport uncertainties). Mean residual mass fractions ranged from 0.09±0.07 to 0.74±0.04.
Correlated measurements of dimethylsulfide (DMS), gas phase dimethylsulfoxide (DMSO), methane‐sulfonic acid (MSA)(g), sulfuric acid (H2SO4), and cloud condensation nuclei (CCN) were conducted in ...April 1991 at a Pacific coastal site in northern Washington. Measurements of SO2, aerosol methanesulfonate (MSA)(p), and non‐sea‐salt sulfate (nss‐SO4) concentrations were also included. Maximum DMS concentrations between 100 and 240 pptv were observed when the measurement site (480 m above sea level) was embedded in clouds and air from the marine boundary layer was flowing upslope to the site. DMS levels measured in continental air and/or above the mixed layer were typically less than 20 pptv. The sulfur gases DMSO, H2SO4, and MSA(g) were measured in real time on a continuous basis (once every 60–150 s) using selected ion chemical ionization mass spectrometry. Corresponding concentrations ranged between <0.5–3.2 pptv, 0.001–1.19 pptv, and 0.002–0.19 pptv, respectively. All three sulfur gases showed significant diel variations mostly in phase with each other. Their corresponding lifetimes in the marine atmosphere are estimated to be of the order of a few hours. The results in connection with recent laboratory studies and model calculations suggest that dimethylsulfone (DMSO2) was the dominant end product of DMS oxidation under the present conditions. CCN concentrations measured in marine air ranged roughly between 10–200 cm−3 and 200–400 cm−3 at 0.3% and 0.9% supersaturation, respectively. A statistical analysis using only data obtained in predominantly marine air and during non‐fog/non‐precipitation periods showed significant correlations between individual sulfur species and between CCN (0.3% ss) and H2SO4, and CCN (0.3% ss) and nss‐SO4. The results indirectly support a relationship between DMS and CCN concentrations. However, other observations also suggest that at higher supersaturations (0.9%), compounds less soluble than sulfate may become important in marine CCN formation.
ABSTRACT
An experiment was conducted to measure aerosol turbulent fluxes to a grass field. A new high‐flow‐rate aerosol sensor was deployed from a tower to make eddy correlation (EC) measurements of ...aerosol turbulent flux and deposition velocity. The EC data were screened and analysed for uncertainties associated with advection, boundary layer growth, instrument separation and counting particles. An apparent bias in the aerosol flux due to particle hygroscopic growth was evaluated from chemical and microphysical measurements and removed from the results based on derived corrections. The resulting aerosol deposition velocity for 0.52 μm diameter particles depended on atmospheric stability with values of 0.3 cm s−1 during near‐neutral stability, 0.44 cm s−1 during unstable periods and 0.16 cm s−1 during stable periods with an estimated uncertainty of ±0.07 cm s−1 due to chemical composition and particle counting.
Shipboard measurements of aerosol chemical composition and optical properties were made during both ACE‐1 and ACE‐2. ACE‐1 focused on remote marine aerosol minimally perturbed by continental sources. ...ACE‐2 studied the outflow of European aerosol into the NE Atlantic atmosphere. A variety of air masses were sampled during ACE‐2 including Atlantic, polar, Iberian Peninsula, Mediterranean, and Western European. Reported here are mass size distributions of non‐sea salt (nss) sulfate, sea salt, and methanesulfonate and submicron and supermicron concentrations of black and organic carbon. Optical parameters include submicron and supermicron aerosol scattering and backscattering coefficients at 550 nm, the absorption coefficient at 550±20 nm, the Ångström exponent for the 550 and 700 nm wavelength pair, and single scattering albedo at 550 nm. All data are reported at the measurement relative humidity of 55%. Measured concentrations of nss sulfate aerosol indicate that, relative to ACE‐1, ACE‐2 aerosol during both marine and continental flow was impacted by continental sources. Thus, while sea salt controlled the aerosol chemical composition and optical properties of both the submicron and supermicron aerosol during ACE‐1, it played a relatively smaller role in ACE‐2. This is confirmed by the larger average Ångström exponent for ACE‐2 continental aerosol of 1.2±0.26 compared to the ACE‐1 average of −0.03±0.38. The depletion of chloride from sea salt aerosol in ACE‐2 continental air masses averaged 55±25% over all particle sizes. This compares to the ACE‐2 marine average of 4.8±18% and indicates the enhanced interaction of anthropogenic acids with sea salt as continental air masses are transported into the marine atmosphere. Single scattering albedos averaged 0.95±0.03 for ACE‐2 continental air masses. Averages for ACE‐2 and ACE‐1 marine air masses were 0.98±0.01 and 0.99±0.01, respectively.
ABSTRACT
In this work we propose and test a method to calculate cloud condensation nuclei (CCN) spectra based on aerosol number size distributions and hygroscopic growth factors. Sensitivity studies ...show that this method can be used in a wide variety of conditions except when the aerosol consist mainly of organic compounds. One crucial step in the calculations, estimating soluble ions in an aerosol particle based on hygroscopic growth factors, is tested in an internal hygroscopic consistency study. The results show that during the second Aerosol Characterization Experiment (ACE‐2) the number concentration of inorganic ions analyzed in impactor samples could be reproduced from measured growth factors within the measurement uncertainties at the measurement site in Sagres, Portugal.
CCN spectra were calculated based on data from the ACE‐2 field experiment at the Sagres site. The calculations overestimate measured CCN spectra on average by approximately 30%, which is comparable to the uncertainties in measurements and calculations at supersaturations below 0.5%. The calculated CCN spectra were averaged over time periods when Sagres received clean air masses and air masses influenced by aged and recent pollution. Pollution outbreaks enhance the CCN concentrations at supersaturations near 0.2% by a factor of 3 (aged pollution) to 5 (recent pollution) compared to the clean marine background concentrations. In polluted air masses, the shape of the CCN spectra changes. The clean spectra can be approximated by a power function, whereas the polluted spectra are better approximated by an error function.
ABSTRACT
Airborne data are presented on the impact of cloud processing on the aerosol mass light‐scattering efficiency. The measurements, on marine stratocumulus, suggest that cloud processing ...significantly enhanced the mass light‐scattering efficiency in three of the five cases analysed. Enhancements were of the order of 10% for air detraining from the cloud deck relative to non‐detraining air. A diagnostic modelling analysis suggested that the observed enhancements were consistent with the previously proposed explanation of in‐cloud sulfate production in the particle size range for efficient light scattering.
Abstract
Although the importance of the aerosol contribution to the global radiative budget has been recognized, the forcings of aerosols in general, and specifically the role of the organic ...component in these forcings, still contain large uncertainties. In an attempt to better understand the relationship between the background forcings of aerosols and their chemical speciation, marine air samples were collected off the windward coast of Oahu, Hawaii, during the Rough Evaporation Duct project (RED) using filters mounted on both the Twin Otter aircraft and the Floating Instrument Platform (FLIP) research platform. Laboratory analysis revealed a total of 17 species, including 4 carboxylic acids and 2 carbohydrates that accounted for 74% ± 20% of the mass gain observed on the shipboard filters, suggesting a possible significant unresolved organic component. The results were correlated with in situ measurements of particle light scattering (σsp) at 550 nm and with aerosol hygroscopicities. Principal component analysis revealed a small but ubiquitous pollution component affecting the σsp and aerosol hygroscopicity of the remote marine air. The Princeton Organic-Electrolyte Model (POEM) was used to predict the growth factor of the aerosols based upon the chemical composition. This output, coupled with measured aerosol size distributions, was used to attempt to reproduce the observed σsp. It was found that while the POEM model was able to reproduce the expected trends when the organic component of the aerosol was varied, due to large uncertainties especially in the aerosol sizing measurements, the σsp predicted by the POEM model was consistently higher than observed.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A filter-based single-wavelength photometer (Particle Soot Absorption Photometer, PSAP) for measuring light absorption by aerosols was modified to measure at three wavelengths, 467 nm, 530 nm, and ...660 nm. The modified and an unmodified photometer were calibrated during the Reno Aerosol Optics Study (RAOS) 2002 against two absorption standards: a photoacoustic instrument and the difference between the extinction and scattering coefficient. This filter-based absorption method has to be corrected for scattering aerosol and transmission changes. A simple function for this was derived from the calibration experiment as a function of transmission and single-scattering albedo. For an unmodified PSAP at typical atmospheric absorption coefficients the algorithm yields about 5-7% lower absorption coefficients than does the usually used method. The three-wavelength PSAP was used for atmospheric measurements both during RAOS and during the New England Air Quality Study (NEAQS).
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