Mesoscale variations of tropospheric aerosols ANDERSON, Theodore L; CHARLSON, Robert J; WINKER, David M ...
Journal of the atmospheric sciences,
2003, 2003-01-00, 20030101, Letnik:
60, Številka:
1
Journal Article
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Tropospheric aerosols are calculated to cause global-scale changes in the earth's heat balance, but these forcings are space/time integrals over highly variable quantities. Accurate quantification of ...these forcings will require an unprecedented synergy among satellite, airborne, and surface-based observations, as well as models. This study considers one aspect of achieving this synergy-the need to treat aerosol variability in a consistent and realistic way. This need creates a requirement to rationalize the differences in spatiotemporal resolution and coverage among the various observational and modeling approaches. It is shown, based on aerosol optical data from diverse regions, that mesoscale variability (specifically, for horizontal scales of 40-400 km and temporal scales of 2-48 h) is a common and perhaps universal feature of lower-tropospheric aerosol light extinction. Such variation is below the traditional synoptic or "airmass" scale (where the aerosol is often assumed to be essentially homogeneous except for plumes from point sources) and below the scales that are readily resolved by chemical transport models. The present study focuses on documenting this variability. Possible physical causes and practical implications for coordinated observational strategies are also discussed.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
New particle formation contributes significantly to the number concentration of condensation nuclei (CN) as well as cloud CN (CCN), a key factor determining aerosol indirect radiative forcing of the ...climate system. Using a physics-based nucleation mechanism that is consistent with a range of field observations of aerosol formation, it is shown that projected increases in global temperatures could significantly inhibit new particle, and CCN, formation rates worldwide. An analysis of CN concentrations observed at four NOAA ESRL/GMD baseline stations since the 1970s and two other sites since 1990s reveals long-term decreasing trends that are consistent in sign with, but are larger in magnitude than, the predicted temperature effects. The possible reasons for larger observed long-term CN reductions at remote sites are discussed. The combined effects of rising temperatures on aerosol nucleation rates and other chemical and microphysical processes may imply substantial decreases in future tropospheric particle abundances associated with global warming, delineating a potentially significant feedback mechanism that increases Earth's climate sensitivity to greenhouse gas emissions. Further research is needed to quantify the magnitude of such a feedback process.
The Plantower PMS5003 sensors (PMS) used in the PurpleAir monitor PA-II-SD configuration (PA-PMS) are equivalent to cell-reciprocal nephelometers using a 657 nm perpendicularly polarized light source ...that integrates light
scattering from 18 to 166∘. Yearlong field data at the National
Oceanic and Atmospheric Administration's (NOAA) Mauna Loa Observatory (MLO)
and Boulder Table Mountain (BOS) sites show that the 1 h average of the
PA-PMS first size channel, labeled “> 0.3 µm” (“CH1”), is
highly correlated with submicrometer aerosol scattering coefficients at the
550 and 700 nm wavelengths measured by the TSI 3563 integrating
nephelometer, from 0.4 to 500 Mm−1. This corresponds to an
hourly average submicrometer aerosol mass concentration of approximately 0.2 to 200 µg m−3. A physical–optical model of the PMS is developed to estimate light intensity on the photodiode, accounting for angular truncation of the volume scattering function as a function of particle size. The model predicts that the PMS response to particles > 0.3 µm decreases relative to an ideal nephelometer by about 75 % for particle diameters ≥ 1.0 µm. This is a result of using a laser that is polarized, the angular truncation of the scattered light, and particle losses (e.g., due to aspiration) before reaching the laser. It is shown that
CH1 is linearly proportional to the model-predicted intensity of the light
scattered by particles in the PMS laser to its photodiode over 4 orders
of magnitude. This is consistent with CH1 being a measure of the scattering
coefficient and not the particle number concentration or particulate matter
concentration. The model predictions are consistent with data from published laboratory studies which evaluated the PMS against a variety of aerosols. Predictions are then compared with yearlong fine aerosol size distribution
and scattering coefficient field data at the BOS site. Field data at BOS
confirm the model prediction that the ratio of CH1 to the scattering
coefficient would be highest for aerosols with median scattering diameters
< 0.3 µm. The PMS detects aerosols smaller than 0.3 µm
diameter in proportion to their contribution to the scattering coefficient.
The results of this study indicate that the PMS is not an optical
particle counter and that its six size fractions are not a meaningful
representation of particle size distribution. The relationship between the
PMS 1 h average CH1 and bsp1, the scattering coefficient in
Mm−1 due to particles below 1 µm aerodynamic diameter, at
wavelength 550 nm, is found to be
bsp1 = 0.015 ± 2.07 × 10−5 × CH1, for relative humidity below 40 %. The
coefficient of determination r2 is 0.97. This suggests that the
low-cost and widely used PA monitors can be used to measure and predict the
submicron aerosol light scattering coefficient in the mid-visible nearly as
well as integrating nephelometers. The effectiveness of the PA-PMS to serve
as a PM2.5 mass concentration monitor is due to both the sensor
behaving like an imperfect integrating nephelometer and the mass
scattering efficiency of ambient PM2.5 aerosols being roughly constant.
CONTEXT Prediction models to identify healthy individuals at high risk of cardiovascular disease have limited accuracy. A low ankle brachial index (ABI) is an indicator of atherosclerosis and has the ...potential to improve prediction. OBJECTIVE To determine if the ABI provides information on the risk of cardiovascular events and mortality independently of the Framingham risk score (FRS) and can improve risk prediction. DATA SOURCES Relevant studies were identified. A search of MEDLINE (1950 to February 2008) and EMBASE (1980 to February 2008) was conducted using common text words for the term ankle brachial index combined with text words and Medical Subject Headings to capture prospective cohort designs. Review of reference lists and conference proceedings, and correspondence with experts was conducted to identify additional published and unpublished studies. STUDY SELECTION Studies were included if participants were derived from a general population, ABI was measured at baseline, and individuals were followed up to detect total and cardiovascular mortality. DATA EXTRACTION Prespecified data on individuals in each selected study were extracted into a combined data set and an individual participant data meta-analysis was conducted on individuals who had no previous history of coronary heart disease. RESULTS Sixteen population cohort studies fulfilling the inclusion criteria were included. During 480 325 person-years of follow-up of 24 955 men and 23 339 women, the risk of death by ABI had a reverse J-shaped distribution with a normal (low risk) ABI of 1.11 to 1.40. The 10-year cardiovascular mortality in men with a low ABI (≤0.90) was 18.7% (95% confidence interval CI, 13.3%-24.1%) and with normal ABI (1.11-1.40) was 4.4% (95% CI, 3.2%-5.7%) (hazard ratio HR, 4.2; 95% CI, 3.3-5.4). Corresponding mortalities in women were 12.6% (95% CI, 6.2%-19.0%) and 4.1% (95% CI, 2.2%-6.1%) (HR, 3.5; 95% CI, 2.4-5.1). The HRs remained elevated after adjusting for FRS (2.9 95% CI, 2.3-3.7 for men vs 3.0 95% CI, 2.0-4.4 for women). A low ABI (≤0.90) was associated with approximately twice the 10-year total mortality, cardiovascular mortality, and major coronary event rate compared with the overall rate in each FRS category. Inclusion of the ABI in cardiovascular risk stratification using the FRS would result in reclassification of the risk category and modification of treatment recommendations in approximately 19% of men and 36% of women. CONCLUSION Measurement of the ABI may improve the accuracy of cardiovascular risk prediction beyond the FRS.
Results are presented from 3 years of simultaneous measurements of aerosol chemical composition and light scattering and absorption at Barrow, Alaska. All results are reported at the measurement ...relative humidity of ≤ 40%. Reported are the annual cycles of the concentration of aerosol mass, sea salt, non‐sea‐salt (nss) sulfate, methanesulfonate or MSA−, NH4+, and nss K+, Mg+2, and Ca+2 for the submicron and supermicron size ranges. Submicron nss SO4=, NH4+, and nss K+, Mg+2, and Ca+2 peak in winter and early spring corresponding to the arrival and persistence of Arctic Haze. Submicron sea salt displays a similar annual cycle presumably due to long‐range transport from the northern Pacific Ocean. Supermicron sea salt peaks in summer corresponding to a decrease in sea ice extent. Submicron and supermicron MSA− peak in the summer due to a seasonal increase in the flux of dimethylsulfide from the ocean to the atmosphere. A correlation of MSA− and particle number concentrations suggests that summertime particle production is associated with this biogenic sulfur. Mass fractions of the dominant chemical species were calculated from the concentrations of aerosol mass and chemical species. For the submicron size range the ionic mass and associated water make up 80 to 90% of the aerosol mass from November to May. Of this ionic mass, sea salt and nss SO4= are the dominant species. The residual mass fraction, or fraction of mass that is chemically unanalyzed, is equivalent to the ionic mass fraction in June through October. For the supermicron size range the ionic mass and associated water make up 60 to 80% of the aerosol mass throughout the year with sea salt being the dominant species. Also reported for the submicron size range are the annual cycles of aerosol light scattering and absorption at 550 nm, Ångström exponent for the 450 and 700 nm wavelength pair, and single scattering albedo at 550 nm. On the basis of linear regressions between the concentrations of sea salt and nss SO4= and the light scattering coefficient, sea salt has a dominant role in controlling light scattering during the winter, nss SO4= is dominant in the spring, and both components contribute to scattering in the summer. Submicron mass scattering efficiencies of the dominant aerosol chemical components (nss SO4=, sea salt, and residual mass) were calculated from a multiple linear regression of the measured light scattering versus the component concentrations. Submicron nss SO4= mass scattering efficiencies were relatively constant throughout the year with seasonal averages ranging from 4.1 ± 2.9 to 5.8 ± 1.0 m2 g−1. Seasonal averages for submicron sea salt ranged from 1.8 ± 0.37 to 5.1 ± 0.97 m2 g−1 and for the residual mass ranged from 0.21 ± 0.31 to 1.5 ± 1.0 m2 g−1. Finally, concentrations of nss SO4= measured at Barrow were compared to those measured at Poker Flat Rocket Range, Denali National Park, and Homer for the 1997/1998 and 1998/1999 Arctic Haze seasons. Concentrations were highest at Barrow and decreased with latitude from Poker Flat to Denali to Homer revealing a north to south gradient in the extent of the haze.
Several types of filter-based instruments are used to estimate aerosol light absorption coefficients. Two significant results are presented based on Aethalometer measurements at six Arctic stations ...from 2012 to 2014. First, an alternative method of post-processing the Aethalometer data is presented, which reduces measurement noise and lowers the detection limit of the instrument more effectively than boxcar averaging. The biggest benefit of this approach can be achieved if instrument drift is minimised. Moreover, by using an attenuation threshold criterion for data post-processing, the relative uncertainty from the electronic noise of the instrument is kept constant. This approach results in a time series with a variable collection time (Δt) but with a constant relative uncertainty with regard to electronic noise in the instrument. An additional advantage of this method is that the detection limit of the instrument will be lowered at small aerosol concentrations at the expense of temporal resolution, whereas there is little to no loss in temporal resolution at high aerosol concentrations ( > 2.1–6.7 Mm−1 as measured by the Aethalometers). At high aerosol concentrations, minimising the detection limit of the instrument is less critical. Additionally, utilising co-located filter-based absorption photometers, a correction factor is presented for the Arctic that can be used in Aethalometer corrections available in literature. The correction factor of 3.45 was calculated for low-elevation Arctic stations. This correction factor harmonises Aethalometer attenuation coefficients with light absorption coefficients as measured by the co-located light absorption photometers. Using one correction factor for Arctic Aethalometers has the advantage that measurements between stations become more inter-comparable.
Methods for reducing and quantifying the uncertainties in aerosol optical properties measured with the TSI 3563 integrating nephelometer are presented. For nearly all applications, the recommended ...calibration gases are air and CO
2
. By routinely characterizing the instrumental response to these gases, a diagnostic record of instrument performance can be created. This record can be used to improve measurement accuracy and quantify uncertainties due to instrumental noise and calibration drift. When measuring scattering by particles, size segregation upstream of the nephelometer at about 1 μm aerodynamic diameter greatly increases the information content of the data for two reasons: one stemming from the independence of coarse and fine particles in the atmosphere, and the second stemming from the size dependence of the nephelometer response. For many applications (e.g., extinction budget studies) it is important to correct nephelometer data for the effects of angular nonidealities. Correction factors appropriate to a broad range of sampling conditions are given herein and are shown to be constrained by the wavelength dependence of light scattering, as measured by the nephelometer. Finally, the nephelometer measurement is nondestructive, such that the sampled aerosol can be further analyzed downstream. Data from two nephelometers operated in series are used to evaluate this procedure. A small loss of super-μm particles (5-10%) is found, while the sub-μm data demonstrates measurement reproducibility within ± 1%.
The aerosol monitoring network of the NOAA Earth System Research Laboratory routinely provides data on the aerosol absorption and spectral scattering and backscattering coefficients. An algorithm is ...introduced that retrieves from the primary measurements the aerosol asymmetry parameter, the optical property needed in addition to those mentioned above to completely characterize aerosol with respect to its direct radiative effect on climate. The algorithm is tested on synthetic data and shown to retrieve the spectral aerosol asymmetry parameter for aerodynamic particle diameters Dp,aer < 1.0 μm with ∼0.8% systematic uncertainty and ∼1.1% systematic uncertainty for Dp,aer < 10.0 μm. The algorithm is applied to data from six ESRL stations representing aerosol in the dried state of continental, marine, arctic, and free tropospheric origin. The retrieved asymmetry parameters show a significant dependence on wavelength. Both wavelength dependence and absolute value of the asymmetry parameter depend on location or air mass type. The median values range from 0.57 for free tropospheric aerosol to 0.65 for marine aerosol at 550 nm wavelength and Dp,aer < 10.0 μm. It is shown that the accuracy of the asymmetry parameters retrieved with the algorithm is significantly larger than that of asymmetry parameters derived from backscatter fractions assuming a Henyey‐Greenstein phase function.
•The functional architecture among regulatory structures, and the whole brain, is less modular in confirmed cases of SUDEP and those at high-risk.•Altered functional organisation may mean potential ...impairment of communication among key regulatory circuits.•SUDEP is associated with regional connectivity disruptions among cortical and sub-cortical regulatory sites.•Medial thalamic connectivity was significantly altered in SUDEP compared with all control groups, including those at high-risk.•Increases in the number, and a shift in organisation, of hubs appears to relate to lower mortality risk.
The circumstances surrounding SUDEP suggest autonomic or respiratory collapse, implying central failure of regulation or recovery. Characterisation of the communication among brain areas mediating such processes may shed light on mechanisms and noninvasively indicate risk.
We used rs-fMRI to examine network properties among brain structures in people with epilepsy who suffered SUDEP (n = 8) over an 8-year follow-up period, compared with matched high- and low-risk subjects (n = 16/group) who did not suffer SUDEP during that period, and a group of healthy controls (n = 16). Network analysis was employed to explore connectivity within a ‘regulatory-subnetwork’ of brain regions involved in autonomic and respiratory regulation, and over the whole-brain.
Modularity, the extent of network organization into separate modules, was significantly reduced in the regulatory-subnetwork, and the whole-brain, in SUDEP and high-risk. Increased participation, a local measure of inter-modular belonging, was evident in SUDEP and high-risk groups, particularly among thalamic structures. The medial prefrontal thalamus was increased in SUDEP compared with all other control groups, including high-risk. Patterns of hub topology were similar in SUDEP and high-risk, but were more extensive in low-risk patients, who displayed greater hub prevalence and a radical reorganization of hubs in the subnetwork.
SUDEP is associated with reduced functional organization among cortical and sub-cortical brain regions mediating autonomic and respiratory regulation. Living high-risk subjects demonstrated similar patterns, suggesting such network measures may provide prospective risk-indicating value, though a crucial difference between SUDEP and high-risk was altered connectivity of the medial thalamus in SUDEP, which was also elevated compared with all sub-groups. Disturbed thalamic connectivity may reflect a potential non-invasive marker of elevated SUDEP risk.
A detailed analysis of optical and microphysical properties of aerosol particles during the dry winter monsoon season above the northern Indian Ocean is presented. The Cloud Aerosol Radiative Forcing ...Experiment (CARDEX), conducted from 16 February to 30 March 2012 at the Maldives Climate Observatory on Hanimaadhoo island (MCOH) in the Republic of the Maldives, used autonomous unmanned aerial vehicles (AUAV) to perform vertical in situ measurements of particle number concentration, particle number size distribution as well as particle absorption coefficients. These measurements were used together with surface- based Mini Micro Pulse Lidar (MiniMPL) observations and aerosol in situ and off-line measurements to investigate the vertical distribution of aerosol particles.Air masses were mainly advected over the Indian subcontinent and the Arabian Peninsula. The mean surface aerosol number concentration was 1717 ± 604 cm−3 and the highest values were found in air masses from the Bay of Bengal and Indo-Gangetic Plain (2247 ± 370 cm−3). Investigations of the free tropospheric air showed that elevated aerosol layers with up to 3 times higher aerosol number concentrations than at the surface occurred mainly during periods with air masses originating from the Bay of Bengal and the Indo-Gangetic Plain. This feature is different compared to what was observed during the Indian Ocean Experiment (INDOEX) conducted in winter 1999, where aerosol number concentrations generally decreased with height. In contrast, lower particle absorption at the surface (σabs(520 nm) = 8.5 ± 4.2 Wm−1) was found during CARDEX compared to INDOEX 1999.Layers with source region specific single-scattering albedo (SSA) values were derived by combining vertical in situ particle absorption coefficients and scattering coefficients calculated with Mie theory. These SSA layers were utilized to calculate vertical particle absorption profiles from MiniMPL profiles. SSA surface values for 550 nm for dry conditions were found to be 0.94 ± 0.02 and 0.91 ± 0.02 for air masses from the Arabian Sea (and Middle East countries) and India (and Bay of Bengal), respectively. Lidar-derived particle absorption coefficient profiles showed both a similar magnitude and structure as the in situ profiles measured with the AUAV. However, primarily due to insufficient accuracy in the SSA estimates, the lidar-derived absorption coefficient profiles have large uncertainties and are generally weakly correlated to vertically in situ measured particle absorption coefficients.Furthermore, the mass absorption efficiency (MAE) for the northern Indian Ocean during the dry monsoon season was calculated to determine equivalent black carbon (EBC) concentrations from particle absorption coefficient measurements. A mean MAE of 11.6 and 6.9 m2 g−1 for 520 and 880 nm, respectively, was found, likely representing internally mixed BC containing particles. Lower MAE values for 880 and 520 nm were found for air masses originating from dust regions such as the Arabian Peninsula and western Asia (MAE(880 nm) = 5.6 m2 g−1, MAE(520 nm) = 9.5 m2 g−1) or from closer source regions as southern India (MAE(880 nm) = 4.3 m2 g−1, MAE(520 nm) = 7.3 m2 g−1).
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