In 2014, Qianlai Zhang from Indiana University and others studied the aerosol direct effect (radiative effect) on the surface energy fluxes of global terrestrial ecosystems during 2003-2010.

Background: the effects of aerosols can be divided into

(1) Direct radiative effects

(1.1) Aerosol reduces the total downward solar radiation, but increases the diffuse radiation. The former is bad for vegetation, while the latter is good.

(1.2) Observational studies on the direct radiative effects at site, regional, and global levels are abundant

(1.3) Modeling studies on the direct radiative effects are also abundant at site and regional levels, but few at global levels

(2) Indirect effects

(2.1) Such effects happen through aerosol-cloud interactions, which are not considered in this study.

Methods

(1) Two models are coupled

(1.1) Atmospheric radiative transfer model (uses MODIS-based global aerosol properties, estimates direct and diffuse radiations)

(1.1.1) The aerosol loading data shows highest levels in northern India, China, and the Sahel.

(1.2) Terrestrial ecosystem model (the integrated Terrestrial ecosystem model, iTem; run at 1 deg resolution)

(2) Two experiments are conducted

(2.1) S0: Aerosol effect (uses transient solar radiation data estimated by the atmospheric radiative transfer model)

(2.2) S1: No aerosol effect (uses the solar radiation data estimated by the atmospheric radiative model without considering the aerosol loadings)

Findings

(1) Global mean

(1.1) Aerosol loadings decrease the mean latent heat flux by 2.4 Wm-2, from 46.00 Wm-2 to 43.60 Wm-2

(1.2) Aerosol loadings decrease the sensible heat flux by 16 Wm-2, from 95.26 Wm-2 to 79.57 Wm-2

(1.3) Global mean surface soil moisture increases by 0.5%. This is because (i) surface water evapotranspiration is inhibited by the cooler land surface, (ii) aerosol-induced sensible and latent heat decreases are positively correlated with leaf area index (LAI) decreases, further reducing transpiration.

(1.4) Global mean water evaporative fraction increases by 4%.

(2) Spatial distributions

(2.1) Aerosol direct effects on the sensible heat flux are universally negative or zero across the globe. The highest reductions coincide with where the highest aerosol loadings occur, but are larger in spatial extents - i.e. Central Africa, the Indian subcontinent, and China. Also, the Amazon basin is a hotspot.

(2.2) Aerosol direct effects on the latent heat flux are also universally negative or zero across the globe. But compared to sensible heat, the hotspot in Central Africa is much muted, whereas the hotspot in the Amazon basin is strengthened. This is because in high-LAI systems (e.g. forests), the effect of reduced transpiration will be especially apparent.

(2.3) Aerosol direct effects on the surface soil moisture are universally positive or zero across the globe. The hotspots of increases are mainly the region below Sahel in Central Africa, India, and China. The Amazon region shows slight increases, but hardly as much as the previously mentioned two hotspots.

(3) Seasonal-latitudinal distributions

(3.1) For sensible heat, the strongest effect is in summer in each hemisphere, and is the strongest between 20S and 20N. The latitudinal pattern should be related to the aerosol emission patterns.

(3.2) For latent heat, the strongest effect coincide with latitudes of high LAI, but not always with the season where vegetation productivity is the highest. [Perhaps this is because leaf area changes, percentage-wise, are the most apparent in the northern mid-latitudes during the transition seasons when leaves are growing out or senescing.]

#aerosols_direct_effect
#land_atmosphere_coupled_simulations
#land_energy_fluxes
#soil_moisture
#evaporative_fraction

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2014GL061640