[Show abstract][Hide abstract] ABSTRACT: We use a global aerosol microphysics model in combination with an offline radiative transfer model to
quantify the radiative effect of biogenic secondary organic aerosol (SOA) in the present-day atmosphere. Through its role in particle growth and ageing, the presence of biogenic SOA increases the global annual mean concentration of cloud condensation nuclei (CCN; at 0.2% supersaturation) by 3.6–21.1 %, depending upon the yield of SOA production from biogenic volatile organic compounds (BVOCs),
and the nature and treatment of concurrent primary carbonaceous emissions. This increase in CCN causes a rise in global annual mean cloud droplet number concentration (CDNC) of 1.9–5.2%, and a global mean first aerosol indirect effect (AIE) of between +0.01Wm−2 and −0.12Wm−2. The radiative
impact of biogenic SOA is far greater when biogenic oxidation products also contribute to the very early stages of new particle formation; using two organically mediated mechanisms for new particle formation, we simulate global annual mean first AIEs of −0.22Wm−2 and −0.77Wm−2. The inclusion of biogenic SOA substantially improves the simulated seasonal cycle in the concentration of CCN-sized particles observed at three forested sites. The best correlation is found when the organically mediated nucleation mechanisms are applied, suggesting that the first AIE of biogenic SOA could be as large as −0.77Wm−2. The radiative impact of SOA is sensitive to the presence of anthropogenic emissions. Lower background aerosol concentrations simulated with anthropogenic emissions from 1750 give rise to a greater fractional CCN increase and a more substantial first AIE from biogenic SOA. Consequently, the anthropogenic
indirect radiative forcing between 1750 and the present day is sensitive to assumptions about the amount and role of biogenic SOA. We also calculate an annual global mean direct radiative effect of between −0.08Wm−2 and −0.78Wm−2 in the present day, with uncertainty in the amount of SOA
produced from the oxidation of BVOCs accounting for most of this range.
ATMOSPHERIC CHEMISTRY AND PHYSICS 01/2014; 14:447-470. · 5.51 Impact Factor
[Show abstract][Hide abstract] ABSTRACT:  The increase in atmospheric concentrations of water vapor with global warming is a large positive feedback in the climate system. Thus, even relatively small errors in its magnitude can lead to large uncertainties in predicting climate response to anthropogenic forcing. This study incorporates observed variability of water vapor over 2002–2009 from the Atmospheric Infrared Sounder instrument into a radiative transfer scheme to provide constraints on this feedback. We derive a short-term water vapor feedback of 2.2 ± 0.4 Wm−2K−1. Based on the relationship between feedback derived over short and long timescales in twentieth century simulations of 14 climate models, we estimate a range of likely values for the long-term twentieth century water vapor feedback of 1.9 to 2.8 Wm−2K−1. We use the twentieth century simulations to determine the record length necessary for the short-term feedback to approach the long-term value. In most of the climate models we analyze, the short-term feedback converges to within 15% of its long-term value after 25 years, implying that a longer observational record is necessary to accurately estimate the water vapor feedback.
Journal of Geophysical Research: Atmospheres. 11/2013; 118(22).
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: The intentional enhancement of cloud albedo via controlled sea-spray
injection from ships (marine cloud brightening) has been proposed as a
possible method to control anthropogenic global warming; however, there
remains significant uncertainty in the efficacy of this method due to,
amongst other factors, uncertainties in aerosol and cloud microphysics.
A major assumption used in recent cloud- and climate-modeling studies is
that all sea spray was emitted uniformly into some oceanic grid boxes,
and thus these studies did not account for subgrid aerosol coagulation
within the sea-spray plumes. We explore the evolution of these sea-salt
plumes using a multi-shelled Gaussian plume model with size-resolved
aerosol coagulation. We determine how the final number of particles
depends on meteorological conditions, including wind speed and
boundary-layer stability, as well as the emission rate and size
distribution of aerosol emitted. Under previously proposed injection
rates and typical marine conditions, we find that the number of aerosol
particles is reduced by over 50%, but this reduction varies from under
10% to over 90% depending on the conditions. We provide a
computationally efficient parameterization for cloud-resolving and
global-scale models to account for subgrid-scale coagulation, and we
implement this parameterization in a global-scale aerosol-climate model.
While designed to address subgrid-scale coagulation of sea-salt
particles, the parameterization is generally applicable for coagulation
of subgrid-scale aerosol from point sources. We find that accounting for
this subgrid-scale coagulation reduces cloud droplet number
concentrations by 46% over emission regions, and reduces the global mean
radiative flux perturbation from -1.5 W m-2 to -0.8 W
ATMOSPHERIC CHEMISTRY AND PHYSICS 10/2013; 13(20):10385-10396. · 5.51 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Sea-salt aerosols proposed for injection in marine cloud brightening geoengineering would likely result from evaporation of sea-water droplets. Previous simulations have omitted this mechanism. Using the WRF/Chem model (Weather Research and Forecasting model coupled with Chemistry) in large-eddy simulation mode, we find that droplet evaporation creates cold pools, suppressing initial aerosol plume heights by up to 30% (40 m). This lessens cloud albedo increases from 94.1 to 88.5% in our weakly-precipitating case and from 4.3 to 1.4% for daytime injection into our nonprecipitating case (cloud albedo differences of 0.012 and 0.009, respectively). Inclusion of this effect in future modelling would allow increasingly realistic effectiveness estimates.
[Show abstract][Hide abstract] ABSTRACT: stratospheric ozone depletion has acted to cool the Earth's surface. As
the result of the phase-out of anthropogenic halogenated compounds
emissions, stratospheric ozone is projected to recover and its radiative
forcing (RF-O3 ~ -0.05 W/m2 presently) might
therefore be expected to decay in line with ozone recovery itself. Using
results from chemistry-climate models, we find that, although model
projections using a standard greenhouse gas scenario broadly agree on
the future evolution of global ozone, they strongly disagree on
RF-O3 because of a large model spread in ozone changes in a
narrow (several km thick) layer, in the northern lowermost stratosphere.
Clearly, future changes in global stratospheric ozone cannot be
considered an indicator of its overall RF. The multi-model mean
RF-O3 estimate for 2100 is +0.06 W/m2 but with a
range such that it could remain negative throughout this century or
change sign and reach up to ~0.25 W/m2.
Geophysical Research Letters 06/2013; 40(11):2796-2800. · 3.98 Impact Factor
[Show abstract][Hide abstract] ABSTRACT:  We utilize energy budget diagnostics from the Coupled Model Intercomparison Project phase 5 (CMIP5) to evaluate the models' climate forcing since preindustrial times employing an established regression technique. The climate forcing evaluated this way, termed the adjusted forcing (AF), includes a rapid adjustment term associated with cloud changes and other tropospheric and land-surface changes. We estimate a 2010 total anthropogenic and natural AF from CMIP5 models of 1.9 ± 0.9 W m−2 (5–95% range). The projected AF of the Representative Concentration Pathway simulations are lower than their expected radiative forcing (RF) in 2095 but agree well with efficacy weighted forcings from integrated assessment models. The smaller AF, compared to RF, is likely due to cloud adjustment. Multimodel time series of temperature change and AF from 1850 to 2100 have large intermodel spreads throughout the period. The intermodel spread of temperature change is principally driven by forcing differences in the present day and climate feedback differences in 2095, although forcing differences are still important for model spread at 2095. We find no significant relationship between the equilibrium climate sensitivity (ECS) of a model and its 2003 AF, in contrast to that found in older models where higher ECS models generally had less forcing. Given the large present-day model spread, there is no indication of any tendency by modelling groups to adjust their aerosol forcing in order to produce observed trends. Instead, some CMIP5 models have a relatively large positive forcing and overestimate the observed temperature change.
Journal of Geophysical Research-Atmospheres. 01/2013; 118.
[Show abstract][Hide abstract] ABSTRACT: The marine-cloud brightening geoengineering technique has been suggested
as a~possible means of counteracting the positive radiative forcing
associated with anthropogenic atmospheric CO2 increases. The
focus of this study is to quantify the albedo response to aerosols
injected into marine stratocumulus cloud from a point source at
different times of day. We use a cloud-resolving model to investigate
both weakly precipitating and non-precipitating regimes. Injection into
both regimes induces a first indirect aerosol effect. Additionally, the
weakly precipitating regime shows evidence of the second indirect
aerosol effect and leads to cloud changes indicative of a regime change
to more persistent cloud. This results in a cloud albedo increase up to
six times larger than in the non-precipitating case. These indirect
effects show considerable variation with injection at different times in
the diurnal cycle. For the weakly precipitating case, aerosol injection
results in average increases in cloud albedo of 0.28 and 0.17 in the
early and mid morning (03:00:00 local time (LT) and 08:00:00 LT,
respectively) and 0.01 in the evening (18:00:00 LT). No cloud develops
when injecting into the cloud-free day (13:00:00 LT). However, the
all-sky albedo increases (which include both the indirect and direct
aerosol effects) are highest for early morning injection (0.11).
Mid-morning and daytime injections produce increases of 0.06, with the
direct aerosol effect compensating for the lack of cloud albedo
perturbation during the cloud-free day. Evening injection results in an
increase of 0.04. Penetration and accumulation of aerosols above the
cloud top may lead to a reduction of all-sky albedo that tempers the
cloud albedo increases. The apparent direct aerosol tempering effect
increases with injection rate, although not enough to overcome the
increase in all-sky planetary albedo resulting from increases in cloud
albedo. For the weakly precipitating case considered, the optimal
injection time for planetary albedo response is the early morning. Here,
the cloud has more opportunity develop into a more persistent
non-precipitating regime prior to the dissipative effects of solar
heating. The effectiveness of the sea-spray injection method is highly
sensitive to diurnal injection time and the direct aerosol effect of an
intense aerosol point source. Studies which ignore these factors could
overstate the effectiveness of the marine cloud brightening technique.
Atmospheric Chemistry and Physics 09/2012; 12(9):24205-24241. · 4.88 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Observations and models have shown that con-tinuously degassing volcanoes have a potentially large ef-fect on the natural background aerosol loading and the ra-diative state of the atmosphere. We use a global aerosol microphysics model to quantify the impact of these vol-canic emissions on the cloud albedo radiative forcing under pre-industrial (PI) and present-day (PD) conditions. We find that volcanic degassing increases global annual mean cloud droplet number concentrations by 40 % under PI conditions, but by only 10 % under PD conditions. Consequently, vol-canic degassing causes a global annual mean cloud albedo effect of −1.06 W m −2 in the PI era but only −0.56 W m −2 in the PD era. This non-equal effect is explained partly by the lower background aerosol concentrations in the PI era, but also because more aerosol particles are produced per unit of volcanic sulphur emission in the PI atmosphere. The higher sensitivity of the PI atmosphere to volcanic emissions has an important consequence for the anthropogenic cloud radiative forcing because the large uncertainty in volcanic emissions translates into an uncertainty in the PI baseline cloud radia-tive state. Assuming a −50/+100 % uncertainty range in the volcanic sulphur flux, we estimate the annual mean anthro-pogenic cloud albedo forcing to lie between −1.16 W m −2 and −0.86 W m −2 . Therefore, the volcanically induced un-certainty in the PI baseline cloud radiative state substantially adds to the already large uncertainty in the magnitude of the indirect radiative forcing of climate.
ATMOSPHERIC CHEMISTRY AND PHYSICS 08/2012; 12:7321-7339. · 5.51 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Precipitation changes are a key driver of climate change impacts. On average, global precipitation is expected to increase with warming. However, model projections show that precipitation does not scale linearly with surface air temperature. Instead, global hydrological sensitivity, the relative change of global-mean precipitation per degree of global warming, seems to vary across different scenarios and even with time. Based on output from 20 coupled Atmosphere-Ocean-General-Circulation-Models for up to 7 different scenarios, we discuss to what extent these variations can be explained by changes in the tropospheric energy budget. Our analysis supports earlier findings that long-and shortwave absorbers initially decrease global-mean precipitation. Including these absorbers into a multivariate scaling approach allows to closely reproduce the simulated global-mean precipitation changes. We find a sensitivity of global-mean precipitation to tropospheric greenhouse gas forcing of -0.42 +/- 0.23%/(W/m(2)) ( uncertainty given as one std of inter-model variability) and to black carbon emissions of -0.07 +/- 0.02%/(Mt/yr). In combination with these two predictors the dominant longer-term effect of surface air temperatures on precipitation is estimated to be 2.2 +/- 0.52%/K - much lower than the 6.5%/K that may be expected from the Clausius-Clapeyron relationship. Citation: Frieler, K., M. Meinshausen, T. Schneider von Deimling, T. Andrews, and P. Forster ( 2011), Changes in global-mean precipitation in response to warming, greenhouse gas forcing and black carbon, Geophys. Res. Lett., 38, L04702, doi:10.1029/2010GL045953.
Geophysical Research Letters 02/2011; 38:L04702. · 3.98 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Black carbon in carbonaceous combustion aerosol warms the climate by absorbing solar radiation, meaning reductions in black carbon emissions are often perceived as an attractive global warming mitigation option. However, carbonaceous combustion aerosol can also act as cloud condensation nuclei (CCN) so they also cool the climate by increasing cloud albedo. The net radiative effect of carbonaceous combustion aerosol is uncertain because their contribution to CCN has not been evaluated on the global scale. By combining extensive observations of CCN concentrations with the GLOMAP global aerosol model, we find that the model is biased low (normalised mean bias = -77%) unless carbonaceous combustion aerosol act as CCN. We show that carbonaceous combustion aerosol accounts for more than half (52-64%) of global CCN with the range due to uncertainty in the emitted size distribution of carbonaceous combustion particles. The model predicts that wildfire and pollution (fossil fuel and biofuel) carbonaceous combustion aerosol causes a global mean cloud albedo aerosol indirect effect of -0.34 W m(-2), with stronger cooling if we assume smaller particle emission size. We calculate that carbonaceous combustion aerosol from pollution sources cause a global mean aerosol indirect effect of -0.23 W m(-2). The small size of carbonaceous combustion particles from fossil fuel sources means that whilst pollution sources account for only one-third of the emitted mass they cause two-thirds of the cloud albedo aerosol indirect effect that is due to carbonaceous combustion aerosol. This cooling effect must be accounted for, along with other cloud effects not studied here, to ensure that black carbon emissions controls that reduce the high number concentrations of fossil fuel particles have the desired net effect on climate.
ATMOSPHERIC CHEMISTRY AND PHYSICS 01/2011; 11(17):9067-9087. · 5.51 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Climate change is often understood in the context of radiative forcings and climate feedbacks. However, the climate system involves different responses that act on different timescales and so it is not always obvious how the two should be separated. Here a suite of climate model experiments and radiative transfer calculations are used to illustrate a useful distinction that allows a separation of fast tropospheric adjustments from slower responses that scale with global surface temperature change. The utility of evaluating fast/slow responses is discussed in the context of cloud feedback, the hydrological cycle, climate efficacies and how climate models deal with these processes. In particular, it is shown that changes in precipitation strongly depend on the radiative forcing and how it is partitioned between the atmosphere and surface. For example, climate change mechanisms that largely force climate through atmospheric heating, such as greenhouse gases and black carbon aerosol, induce tropospheric temperature adjustments that act to decrease precipitation. In the longer term, the precipitation response to global surface temperature change, and associated climate feedbacks that affect precipitation processes, is largely independent of the climate change mechanism. Most of the evaluated fast/slow responses are qualitatively robust across climate models, but there is some uncertainty in the magnitude of these responses. Hence identifying the key atmospheric processes and model parameterizations that control them, may help identify sources of uncertainty in model predicted cloud and hydrological cycle feedbacks.
[Show abstract][Hide abstract] ABSTRACT: Persistent contrails are believed to currently have a relatively small but significant positive radiative forcing on climate. With air travel predicted to continue its rapid growth over the coming years, the contrail warming effect on climate is expected to increase. Nevertheless, there remains a high level of uncertainty in the current estimates of contrail radiative forcing. Contrail formation depends mostly on the aircraft flying in cold and moist enough air masses. Most studies to date have relied on simple parameterizations using averaged meteorological conditions. In this paper we take into account the short-term variability in background cloudiness by developing an on-line contrail parameterization for the UK Met Office climate model. With this parameterization, we estimate that for the air traffic of year 2002 the global mean annual linear contrail coverage was approximately 0.11%. Assuming a global mean contrail optical depth of 0.2 or smaller and assuming hexagonal ice crystals, the corresponding contrail radiative forcing was calculated to be less than 10 mW m-2 in all-sky conditions. We find that the natural cloud masking effect on contrails may be significantly higher than previously believed. This new result is explained by the fact that contrails seem to preferentially form in cloudy conditions, which ameliorates their overall climate impact by approximately 40%.
Journal of Geophysical Research 01/2010; 115. · 3.17 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: With air travel predicted to increase over the coming century, the emissions associated with air traffic are expected to have a significant warming effect on climate. According to current best estimates, an important contribution comes from contrails. However, as reported by the IPCC fourth assessment report, these current best estimates still have a high uncertainty. The development and validation of contrail parameterizations in global climate models is therefore very important. This current study develops a contrail parameterization within the UK Met Office Climate Model. Using this new parameterization, we estimate that for the 2002 traffic, the global mean annual contrail coverage is approximately 0.11%, a value which in good agreement with several other estimates. The corresponding contrail radiative forcing (RF) is calculated to be approximately 4 and 6 mWm-2 in all-sky and clear-sky conditions, respectively. These values lie within the lower end of the RF range reported by the latest IPCC assessment. The relatively high cloud masking effect on contrails observed by our parameterization compared with other studies is investigated, and a possible cause for this difference is suggested. The effect of the diurnal variations of air traffic on both contrail coverage and contrail RF is also investigated. The new parameterization is also employed in thirty-year slab-ocean model runs in order to give one of the first insights into contrail effects on daily temperature range and the climate impact of contrails.
[Show abstract][Hide abstract] ABSTRACT: In order to improve the representation of ozone absorption in the stratosphere of the MAECHAM5 general circulation model, the spectral resolution of the shortwave radiation parameterization used in the model has been increased from 4 to 6 bands. Two 20-years simulations with the general circulation model have been performed, one with the standard and the other with the newly introduced parameterization respectively, to evaluate the temperature and dynamical changes arising from the two different representations of the shortwave radiative transfer. In the simulation with the increased spectral resolution in the radiation parameterization, a significant warming of almost the entire model domain is reported. At the summer stratopause the temperature increase is about 6 K and alleviates the cold bias present in the model when the standard radiation scheme is used. These general circulation model results are consistent both with previous validation of the radiation scheme and with the offline clear-sky comparison performed in the current work with a discrete ordinate 4 stream scattering line by line radiative transfer model. The offline validation shows a substantial reduction of the daily averaged shortwave heating rate bias (1–2 K/day cooling) that occurs for the standard radiation parameterization in the upper stratosphere, present under a range of atmospheric conditions. Therefore, the 6 band shortwave radiation parameterization is considered to be better suited for the representation of the ozone absorption in the stratosphere than the 4 band parameterization. Concerning the dynamical response in the general circulation model, it is found that the reported warming at the summer stratopause induces stronger zonal mean zonal winds in the middle atmosphere. These stronger zonal mean zonal winds thereafter appear to produce a dynamical feedback that results in a dynamical warming (cooling) of the polar winter (summer) mesosphere, caused by an increased downward (upward) circulation in the winter (summer) hemisphere. In addition, the comparison of the two simulations performed with the general circulation model shows that the increase in the spectral resolution of the shortwave radiation and the associated changes in the cloud optical properties result in a warming (0.5–1 K) and moistening (3%–12%) of the upper tropical troposphere. By comparing these modeled differences with previous works, it appears that the reported changes in the solar radiation scheme contribute to improve the model mean temperature also in the troposphere.
ATMOSPHERIC CHEMISTRY AND PHYSICS 01/2007; · 5.51 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: It has previously been suggested that a significant weekly cycle exists in the diurnal temperature range (DTR) across the USA. It has been proposed that this may be due to the indirect forcing by anthropogenic aerosols through the modification of cloud properties over a weekly timescale. If this is the case then it may be possible to use the weekly cycle to determine climate sensitivity to changing aerosol loading. It is found that the weekly cycle is significant, but to a lesser degree than previously suggested. When the same analysis is performed on the DTR data for cycles from 4 to 40 days in length several other cycles are found to be at least as significant as the weekly cycle. To determine a cause of the weekly cycle in DTR the significance of the other cycles must be explained. It is concluded that although a weekly signature in aerosol loading is measured, attributing the weekly cycle in DTR to the indirect effect is not yet possible
[Show abstract][Hide abstract] ABSTRACT: Atmosphere and Ocean General Circulation Model (AOGCM) experiments for the Intergovernmental Panel on Climate Change Fourth Assessment Report are analyzed using both 20th and 21st century model output to better understand model variability and assess the importance of various forcing mechanisms on stratospheric trends. While models represent the climatology of the stratosphere reasonably well in comparison with NCEP reanalysis, there are biases and large variability among models. In general, AOGCMs are cooler than NCEP throughout the stratosphere, with the largest differences in the tropics. Around half the AOGCMs have a top level beneath ~2 hPa and show a significant cold bias in their upper levels (~10 hPa) compared to NCEP, suggesting that these models may have compromised simulations near 10 hPa due to a low model top or insufficient stratospheric levels. In the lower stratosphere (50 hPa), the temperature variability associated with large volcanic eruptions is either absent (in about half of the models) or the warming is overestimated in the models that do include volcanic aerosols. There is general agreement on the vertical structure of temperature trends over the last few decades, differences between models are explained by the inclusion of different forcing mechanisms, such as stratospheric ozone depletion and volcanic aerosols. However, even when human and natural forcing agents are included in the simulations, significant differences remain between observations and model trends, particularly in the upper tropical troposphere (200 hPa?100 hPa), where, since 1979, models show a warming trend and the observations a cooling trend.
Atmospheric Chemistry and Physics 08/2006; · 4.88 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In order to improve the representation of the shortwave radiative transfer in the MAECHAM5 general circulation model, the spectral resolution of the shortwave radiation parameterization used in the model has been increased and extended in the UV-B and UV-C bands. The upgraded shortwave parameterization is first validated offline with a 4 stream discrete-ordinate line-by-line model. Thereafter, two 20-years simulations with the MAECHAM5 middle atmosphere general circulation model are performed to evaluate the temperature changes and the dynamical feedbacks arising from the newly introduced parameterization. The offline clear-sky comparison of the standard and upgraded parameterizations with the discrete ordinate model shows considerable improvement for the upgraded parameterization in terms of shortwave fluxes and heating rates. In the simulation with the upgraded ratiation parameterization, we report a significant warming of almost the entire atmosphere, largest at 1 hPa at the stratopause, and stronger zonal mean zonal winds in the middle atmosphere. The warming at the summer stratopause alleviates the cold bias present in the model when the standard radiation scheme is used. The stronger zonal mean zonal winds induce a dynamical feedback that results in a dynamical warming (cooling) of the polar winter (summer) mesosphere, caused by an increased downward (upward) circulation in the winter (summer) hemisphere. In the troposphere, the changes in the spectral resolution and the associated changes in the cloud optical parameters introduce a relatively small warming and, consistenly, a moisteneing. The warming occurs mostly in the upper troposphere and can contribute to a possible improvement of the model temperature climatology.
Atmospheric Chemistry and Physics 01/2006; · 4.88 Impact Factor