Effective radiative forcing of anthropogenic aerosols in E3SMv1: historical changes, causality, decomposition, and parameterization sensitivities

The effective radiative forcing of anthropogenic aerosols (ERFaer) is an important measure of the anthropogenic aerosol effects simulated by a global climate model. Here we analyze ERFaer simulated by the E3SMv1 atmosphere model using both century-long free-running atmosphere-land simulations and short nudged simulations. We relate the simulated ERFaer to characteristics of the aerosol composition and optical properties, and evaluate the relationships between key aerosol and cloud properties. In terms of historical changes from the year 1870 to 2014, our results show that the global mean anthropogenic aerosol burden and optical depth increase during the simulation period as expected, but the regional averages show large differences in the temporal evolution. The largest regional differences are found in the emission-induced evolution of the burden and optical depth of the sulfate aerosol: a strong decreasing trend is seen in the Northern Hemisphere high-latitude region after around 1970, while a continued increase is simulated in the tropics. Consequently, although the global mean anthropogenic aerosol burden and optical depth increase from 1870 to 2014, the ERFaer magnitude does not increase after around year 1970. The relationships between key aerosol and cloud properties (relative changes between preindustrial and present-day conditions) also show evident changes after 1970, diverging from the linear relationships exhibited for the period from 1870 to 2014. The ERFaer in E3SMv1 is relatively large compared to the recently published multi-model estimates; the primary reason is the large indirect aerosol effect (i.e., through aerosol-cloud interactions). Compared to other models, E3SMv1 features a stronger sensitivity of the cloud droplet effective radius to changes in the cloud droplet number concentration. Large sensitivity is also seen in the liquid cloud optical depth, which is determined by changes in both the effective radius and liquid water path. Aerosol-induced changes in liquid and ice cloud properties in E3SMv1 are found to have a strong correlation, as the evolution of anthropogenic sulfate aerosols affects both the liquid cloud formation and the homogeneous ice nucleation in cirrus clouds. The ERFaer estimates in E3SMv1 for the shortwave and longwave components are sensitive to the parameterization changes in both liquid and ice cloud processes. When the parameterization of ice cloud processes is modified, the top-of-atmosphere forcing changes in the shortwave and longwave components largely offset each other, so the net effect is negligible. This suggests that, to reduce the magnitude of the net ERFaer, it would be more effective to reduce the anthropogenic aerosol effect through liquid or mixed-phase clouds.

Annual Variation of Global Air Pollution: Initial Aerosol Effect or Climate Interaction?

The aerosol at the previous time (initial aerosol) and climate conditions control the next step annual variation of global air pollution through the complex aerosol-climate interaction. However, the individual influences remain unclear, leaving a great gap for understanding the mechanism of air pollution evolution and supporting the environment management. We estimate the annual variation using statistical methods and satellite observations at global scale from 2001 to 2016 Results show that significant variation of annual aerosol occurs over 13.6% of land areas, in which a perturbation of aerosol may cause 0.58 ± 0.45 times change in the next phase. Initial aerosol and climate influences contribute 48.4–51.6% of the total variation, respectively. Specifically, the influences of precipitation, air temperature and surface temperature represent 0.1, 18.3 and 33.2% of the total variation. Physically, the observed variation is strongly correlated with fine mode aerosols, radiative scattering and warm/hot summers in temperate and cold zones. The environmental management therefore should implement cause-oriented strategies for emission control or climatic adaption.

Hemisphere-asymmetric tropical cyclones response to anthropogenic aerosol forcing

How anthropogenic forcing could change tropical cyclones (TCs) is a keen societal concern owing to its significant socio-economic impacts. However, a global picture of the anthropogenic aerosol effect on TCs has not yet emerged. Here we show that anthropogenic aerosol emission can reduce northern hemisphere (NH) TCs but increase southern hemisphere (SH) TCs primarily through altering vertical wind shear and mid-tropospheric upward motion in the TC formation zones. These circulation changes are driven by anthropogenic aerosol-induced NH-cooler-than-SH and NH-increased versus SH-decreased meridional (equator to mid-latitudes) temperature gradients. The cooler NH produces a low-level southward cross-equatorial transport of moist static energy, weakening the NH ascent in the TC formation zones; meanwhile, the increased meridional temperature gradients strengthen vertical wind shear, reducing NH TC genesis. The opposite is true for the SH. The results may help to constrain the models’ uncertainty in the future TC projection. Reduction of anthropogenic aerosol emission may increase the NH TCs threat.

Testing the possibility of using desiccators to study the aerosol effect of liquid deicing material

Introduction. In many countries, there is an additional group of pollutants - deicing materials (DIM) in winter. Salt-containing DIM is one of the factors for increasing the content of PM2.5 and PM10 in the air. The purpose was to determine the possibility of using desiccators to study the aerosol effect of liquid deicing material, identify the chemical composition in the air at spreading liquid DIM in various ways, and establish the calculated doses for conducting a toxicological experiment to study the DIM aerosol effect on the organism of warm-blooded animals. Materials and methods. A model experiment was conducted in airtight containers (desiccators) using a liquid DIM that includes NaCl and CaCl2. All chemical compounds were captured in air pumping from the desiccator into a bubbler tank with bidistilled water and then analyzed using ion chromatography. Results. When comparing the obtained results of main DIM components contained in the air inside desiccators with the maximum permissible concentrations, the excess of Cl- was detected both for the highest single concentration of 0.1 mg/m3 and for the average daily concentration of 0.03 mg/m3. When applying DIM at a dose exceeding ten times the recommended norms for liquid the DIM, an excess level of the maximum permissible concentration for chlorine (but not for sodium and calcium) is observed. Conclusion. The method of DIM studying in desiccators is indicative in terms of the choice of concentrations and studying mechanisms of reagent intake for subsequent DIM research conduction using laboratory animals.

Atmospheric Aerosols Elevated Ecosystem Productivity of a Poplar Plantation in Beijing, China

Atmospheric aerosols can influence energy allocation, environmental factors and thus canopy photosynthesis. However, the regulations of aerosol effect on ecosystem productivity are not well understood. Here, we applied aerosols optical properties to quantify the effects of aerosol type and concentration on the environmental factors and associated gross primary productivity (GPP) of a poplar (Populus sp.) plantation during June to August from 2014 to 2016 in Beijing, China. As AOD increased from 0 to 2.5, total photosynthetically active radiation (PAR) decreased by 29%, while the diffuse PAR increased by 39%. Although there was no significant impact of aerosols on air temperature (p > 0.05), aerosols decreased vapor pressure deficit by more than 40%. We found that the plantation GPP changed exponentially with AOD, indicating that aerosols elevated GPP by about 37% under severe aerosol pollution (AOD ≥ 1) compared with background aerosol (AOD < 0.4). Aerosols type also had a significant effect on GPP. We concluded that aerosols could increase the GPP of the poplar plantation and the promotion effect of aerosol on poplar plantation would not be significantly reduction until AOD < 1 under the projected decrease in aerosol loading in the future.

Sensitivity of surface solar radiation to aerosol–radiation and aerosol–cloud interactions over Europe in WRFv3.6.1 climatic runs with fully interactive aerosols

The amount of solar radiation reaching the Earth's surface can be highly determined by atmospheric aerosols, which have been pointed to as the most uncertain climate forcing agents through their direct (scattering and absorption), semi-direct (absorption implying a thermodynamic effect on clouds) and indirect (modification of cloud properties when aerosols act as cloud condensation nuclei) effects. Nonetheless, regional climate models hardly ever dynamically model the atmospheric concentration of aerosols and their interactions with radiation (ARIs) and clouds (ACIs). The objective of this work is to evince the role of modeling ARIs and ACIs in Weather Research and Forecast (WRF) model simulations with fully interactive aerosols (online resolved concentrations) with a focus on summer mean surface downward solar radiation (RSDS) over Europe. Under historical conditions (1991–2010), both ARIs and ACIs reduce RSDS by a few percentage points over central and northern regions. This reduction is larger when only ARIs are resolved, while ACIs counteract the effect of the former by up to half. The response of RSDS to the activation of ARIs and ACIs is mainly led by the aerosol effect on cloud coverage, while the aerosol effect on atmospheric optical depth plays a very minor role, which evinces the importance of semi-direct and indirect aerosol effects. In fact, differences in RSDS among experiments with and without aerosols are smaller under clear-sky conditions. In terms of future projections (2031–2050 vs. 1991–2010), the baseline pattern (from an experiment without aerosols) shows positive signals southward and negative signals northward. While ARIs enhance the former and reduce the latter, ACIs work in the opposite direction and provide a flatter RSDS change pattern, further evincing the opposite impact from semi-direct and indirect effects and the nontrivial influence of the latter.

Biomass burning aerosols in most climate models are too absorbing

Uncertainty in the representation of biomass burning (BB) aerosol composition and optical properties in climate models contributes to a range in modeled aerosol effects on incoming solar radiation. Depending on the model, the top-of-the-atmosphere BB aerosol effect can range from cooling to warming. By relating aerosol absorption relative to extinction and carbonaceous aerosol composition from 12 observational datasets to nine state-of-the-art Earth system models/chemical transport models, we identify varying degrees of overestimation in BB aerosol absorptivity by these models. Modifications to BB aerosol refractive index, size, and mixing state improve the Community Atmosphere Model version 5 (CAM5) agreement with observations, leading to a global change in BB direct radiative effect of −0.07 W m−2, and regional changes of −2 W m−2 (Africa) and −0.5 W m−2 (South America/Temperate). Our findings suggest that current modeled BB contributes less to warming than previously thought, largely due to treatments of aerosol mixing state.

The enhancement of droplet collision by electric charges and atmospheric electric fields

The effects of electric charges and fields on droplet collision–coalescence and the evolution of cloud droplet size distribution are studied numerically. Collision efficiencies for droplet pairs with radii from 2 to 1024 µm and charges from −32 r2 to +32 r2 (in units of elementary charge; droplet radius r in units of µm) in different strengths of downward electric fields (0, 200, and 400 V cm−1) are computed by solving the equations of motion for the droplets. It is seen that the collision efficiency is increased by electric charges and fields, especially for pairs of small droplets. These can be considered as being electrostatic effects. The evolution of the cloud droplet size distribution with the electrostatic effects is simulated using the stochastic collection equation. Results show that the electrostatic effect is not notable for clouds with the initial mean droplet radius of r¯=15 µm or larger. For clouds with the initial r¯=9 µm, the electric charge without a field could evidently accelerate raindrop formation compared to the uncharged condition, and the existence of electric fields further accelerates it. For clouds with the initial r¯=6.5 µm, it is difficult for gravitational collision to occur, and the electric field could significantly enhance the collision process. The results of this study indicate that electrostatic effects can accelerate raindrop formation in natural conditions, particularly for polluted clouds. It is seen that the aerosol effect on the suppression of raindrop formation is significant in polluted clouds, when comparing the three cases with r¯=15, 9, and 6.5 µm. However, the electrostatic effects can accelerate raindrop formation in polluted clouds and mitigate the aerosol effect to some extent.

Investigation of Aerosol Direct Effect over China under El Niño and Its Spatial Distribution Using WRF-Chem

With rapid economic development and urbanization, the air pollution problem over China has drawn great attention. To explore the aerosol direct effect (ADE) over China, two simulations were conducted using WRF-Chem V3.5.1 in the summer of 2015. One was a control run (CTL) including aerosol effect and related physical and chemical processes, and the other one was a sensitivity simulation (SEN), the same as CTL except that aerosol-radiation interactions were turned off. The differences between two tests were analyzed, in particular over regions in South China (SC) and East China (EC). Results showed the following. (1) The large-scale circulation showed a strong El Niño signal, associated with cooling and wet anomalies over EC, while warming and dry anomalies over EC. (2) Due to ADE, there was a significant decrease in precipitation and an increase in AOD over SC and EC, albeit with different mechanisms. (3) In SC, ADE cooled the region reinforcing the El Niño impact and suppressing water vapor fluxes, which led to a more stable atmosphere and weakened water cycle. In EC, ADE caused vertical circulation anomalies opposing the El Niño impact. (4) ADE showed obvious land-sea difference in precipitation and shortwave radiation.