The effect of anthropogenic aerosols on cloud droplet concentrations and radiative properties is the source of one of the largest uncertainties in the radiative forcing of climate over the industrial ...period. This uncertainty affects our ability to estimate how sensitive the climate is to greenhouse gas emissions. Here we perform a sensitivity analysis on a global model to quantify the uncertainty in cloud radiative forcing over the industrial period caused by uncertainties in aerosol emissions and processes. Our results show that 45 per cent of the variance of aerosol forcing since about 1750 arises from uncertainties in natural emissions of volcanic sulphur dioxide, marine dimethylsulphide, biogenic volatile organic carbon, biomass burning and sea spray. Only 34 per cent of the variance is associated with anthropogenic emissions. The results point to the importance of understanding pristine pre-industrial-like environments, with natural aerosols only, and suggest that improved measurements and evaluation of simulated aerosols in polluted present-day conditions will not necessarily result in commensurate reductions in the uncertainty of forcing estimates.
Oxidation of sulfur dioxide (SO2) in cloud water by reaction with ozone is an important sulfate aerosol formation mechanism and strongly dependent on the acidity of cloud water. Decadal reductions in ...Northern Hemisphere sulfur emissions have contributed to higher cloud water pH, thereby altering sulfate formation rates. Here we use a global composition‐climate model to show that changes in cloud water pH over the 1970–2009 period strongly affects the aerosol particle size distribution, cloud condensation nuclei concentrations, and the magnitude of aerosol radiative forcing. The simulated all‐sky aerosol radiative forcing (1970–2009) over the North Atlantic is +1.2 W m−2 if pH remains constant at 5.0, as in many climate models. However, the forcing increases to +5.2 W m−2 if pH is assumed to increase by 1.0 unit over this period. Global composition climate models need to account for variations in cloud water pH to improve the representation of sulfate aerosol formation and aerosol radiative effects.
Plain Language Summary
Particles in the atmosphere (aerosols) can be harmful to human health at the surface and also affect the Earth's climate. Sulfate is a major component of atmospheric aerosols. It is formed from chemical reactions involving sulfur dioxide, which has large man‐made sources in the energy and industrial sectors. This study investigates the sensitivity of sulfate aerosol formation in clouds to the assumed acidity of the cloud water. Large changes in man‐made emissions over the Northern Hemisphere in the last 30 years have altered the acidity of cloud water. Here we used climate model simulations that include a representation of aerosols and chemistry to investigate the effect of changes in cloud water acidity on sulfate aerosol formation between 1970 and 2009. Our analysis shows that changes in the acidity of cloud water has a strong effect on the properties of aerosols and how they interact with the Earth's radiative balance. The impact is shown to be particularly important for cloudy regions like the North Atlantic that are sensitive to changes in aerosols. The results highlight that the impact of changes in cloud water acidity on aerosols should be included within climate models to improve our representation of aerosols and their interaction with climate.
Key Points
Increases in assumed cloud water pH can improve simulation of sulfate aerosols
Aerosol radiative forcing over the last 40 years is sensitive to the value of cloud water pH
Global composition climate models assuming a fixed value of cloud water pH do not fully account for recent aerosol radiative forcing
This prospective 3-arm parallel-group randomized clinical trial investigated the effect of supplemental vibrational force on rate of orthodontic tooth alignment with fixed appliances. Eighty-one ...subjects (40 males, 41 females; mean age, 14.1 y) undergoing first premolar extraction-based fixed appliance treatment were randomly allocated to treatment supplemented with daily use (20 min) of a removable intraoral vibrational device (AcceleDent; OrthoAccel Technologies Inc.; n = 29), an identical nonfunctional (sham) device (n = 25), or fixed appliances only (n = 27). Mandibular study casts were taken at baseline (treatment start: placement of 0.014-in. nickel-titanium arch wire), initial alignment (0.018-in. nickel-titanium arch wire), and final alignment (0.019 x 0.025–in. stainless steel arch wire). Overall mean irregularity index in the mandibular arch at baseline was 8.5 ± 3.8 mm (95% CI, 7.6 to 9.3) with no significant difference between groups (P = 0.73). For the total sample, mean irregularity index at initial alignment was 2.7 ± 2.8 mm (95% CI, 2.2 to 3.4) with no significant difference between groups (P = 0.40). Mean time from baseline to initial alignment was 59 ± 25 d (95% CI, 54.5 to 65.6); from initial to final alignment, 150 ± 62.5 d (95% CI, 136 to 165); and baseline to final alignment, 209 ± 65 d (95% CI, 195 to 224). Kaplan-Meier analysis demonstrated that patterns of alignment were not significantly different among the 3 groups (P = 0.66). Multivariate linear regression for initial and overall alignment rates using initial irregularity index as the covariate showed no significant differences among groups. The most important influence on both initial and overall rates of alignment was initial irregularity (P = 0.1 × 10−4). This prospective randomized clinical trial found no evidence that supplemental vibrational force can significantly increase the rate of initial tooth movement or reduce the amount of time required to achieve final alignment when used in conjunction with a preadjusted edgewise fixed appliance (ClinicalTrials.gov NCT02314975).
The goal of the Tropospheric Ozone Assessment Report (TOAR) is to provide the research community with an up-to-date scientific assessment of tropospheric ozone, from the surface to the tropopause. ...While a suite of observations provides significant information on the spatial and temporal distribution of tropospheric ozone, observational gaps make it necessary to use global atmospheric chemistry models to synthesize our understanding of the processes and variables that control tropospheric ozone abundance and its variability. Models facilitate the interpretation of the observations and allow us to make projections of future tropospheric ozone and trace gas distributions for different anthropogenic or natural perturbations. This paper assesses the skill of current-generation global atmospheric chemistry models in simulating the observed present-day tropospheric ozone distribution, variability, and trends. Drawing upon the results of recent international multi-model intercomparisons and using a range of model evaluation techniques, we demonstrate that global chemistry models are broadly skillful in capturing the spatio-temporal variations of tropospheric ozone over the seasonal cycle, for extreme pollution episodes, and changes over interannual to decadal periods. However, models are consistently biased high in the northern hemisphere and biased low in the southern hemisphere, throughout the depth of the troposphere, and are unable to replicate particular metrics that define the longer term trends in tropospheric ozone as derived from some background sites. When the models compare unfavorably against observations, we discuss the potential causes of model biases and propose directions for future developments, including improved evaluations that may be able to better diagnose the root cause of the model-observation disparity. Overall, model results should be approached critically, including determining whether the model performance is acceptable for the problem being addressed, whether biases can be tolerated or corrected, whether the model is appropriately constituted, and whether there is a way to satisfactorily quantify the uncertainty.
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