(2017), Smaller desert dust cooling effect estimated from analysis of desert dust size and abundance, Nature Geoscience,10,[274][275][276][277][278] Desert dust aerosols affect Earth's global energy balance through direct interactions with radiation, and through indirect interactions with clouds and ecosystems. But the magnitudes of these effects are so uncertain that it remains unclear whether atmospheric dust has a net warming or cooling effect on global climate. Consequently, it is still uncertain whether large changes in atmospheric dust loading over the past century have slowed or accelerated anthropogenic climate change, or what the effects of potential future changes in dust loading will be. Here we present an analysis of the size and abundance of dust aerosols to constrain the direct radiative effect of dust. Using observational data on dust abundance, in situ measurements of dust optical properties and size distribution, and climate and atmospheric chemical transport model simulations of dust lifetime, we find that the dust found in the atmosphere is substantially coarser than represented in current global climate models. Since coarse dust warms climate, the global dust direct radiative effect is likely to be less cooling than the ~-0.4 W/m 2 estimated by models in a current global aerosol model ensemble. Instead, we constrain the dust direct radiative effect to a range between -0.48 and +0.20 W/m 2 , which includes the possibility that dust causes a net warming of the planet.The direct radiative effect (DRE) of desert dust aerosols on global climate depends sensitively on both the size distribution and atmospheric abundance of dust 1-3 . However, current global model estimates of the atmospheric loading of dust with geometric diameter D ≤ 10 µm (PM10) vary widely from ~6 to 30 Tg [4][5][6][7] . Similarly, the size distribution of atmospheric dust varies substantially across models, with the fraction of dust in the clay size range (D ≤ 2 µm) varying by over a factor of three 8 . This uncertainty in dust size and abundance is partially driven by a critical limitation of global models: the need to prescribe poorly known attributes of dust particles. In particular, the assumed dust optical properties and size distribution at emission greatly affect the resultant size-resolved dust loading 1,6 . Each model parameterizes these properties differently, and in a manner not always consistent with experimental results [8][9][10] . This divergence in assumed dust properties contributes to a wide range of estimates of the sizeresolved global dust loading 6,8 . Because fine dust cools global climate whereas coarse dust (D ≥ 5 μm) likely warms it 3 , this uncertainty in size-resolved dust loading contributes to a wide spread in model estimates of the dust DRE 1,3,9,[11][12][13][14] . Since the use of global models alone is thus unlikely to substantially narrow the uncertainty on dust climate effects 15 , we develop an alternative approach to determine the size-resolved global dust loading, which we subsequently use ...

Abstract. A new version of the Global Model of AerosolProcesses (GLOMAP) is described, which uses a twomoment pseudo-modal aerosol dynamics approach rather than the original two-moment bin scheme. GLOMAP-mode simulates the multi-component global aerosol, resolving sulfate, sea-salt, dust, black carbon (BC) and particulate organic matter (POM), the latter including primary and biogenic secondary POM. Aerosol processes are simulated in a size-resolved manner including primary emissions, secondary particle formation by binary homogeneous nucleation of sulfuric acid and water, particle growth by coagulation, condensation and cloud-processing and removal by dry deposition, in-cloud and below-cloud scavenging. A series of benchmark observational datasets are assembled against which the skill of the model is assessed in terms of normalised mean bias (b) and correlation coefficient (R). Overall, the model performs well against the datasets in simulating concentrations of aerosol precursor gases, chemically speciated particle mass, condensation nuclei (CN) and cloud condensation nuclei (CCN). Surface sulfate, sea-salt and dust mass concentrations are all captured well, while BC and POM are biased low (but correlate well). Surface CN concentrations compare reasonably well in free troposphere and marine sites, but are underestimated at continental and coastal sites related to underestimation of either primary particle emissions or new particle formation. The model compares well against a compilation of CCN observations coverCorrespondence to: G. W. Mann (gmann@env.leeds.ac.uk) ing a range of environments and against vertical profiles of size-resolved particle concentrations over Europe. The simulated global burden, lifetime and wet removal of each of the simulated aerosol components is also examined and each lies close to multi-model medians from the AEROCOM model intercomparison exercise.

Abstract. Atmospheric black carbon (BC) is a leading climate warming agent, yet uncertainties on the global direct radiative forcing (DRF) remain large. Here we expand a global model simulation (GEOS-Chem) of BC to include the absorption enhancement associated with BC coating and separately treat both the aging and physical properties of fossil-fuel and biomass-burning BC. In addition we develop a global simulation of brown carbon (BrC) from both secondary (aromatic) and primary (biomass burning and biofuel) sources. The global mean lifetime of BC in this simulation (4.4 days) is substantially lower compared to the AeroCom I model means (7.3 days), and as a result, this model captures both the mass concentrations measured in nearsource airborne field campaigns (ARCTAS, EUCAARI) and surface sites within 30 %, and in remote regions (HIPPO) within a factor of 2. We show that the new BC optical properties together with the inclusion of BrC reduces the model bias in absorption aerosol optical depth (AAOD) at multiple wavelengths by more than 50 % at AERONET sites worldwide. However our improved model still underestimates AAOD by a factor of 1.4 to 2.8 regionally, with the largest underestimates in regions influenced by fire. Using the RRTMG model integrated with GEOS-Chem we estimate that the all-sky top-of-atmosphere DRF of BC is +0.13 Wm −2 (0.08 Wm −2 from anthropogenic sources and 0.05 Wm −2 from biomass burning). If we scale our model to match AERONET AAOD observations we estimate the DRF of BC is +0.21 Wm −2 , with an additional +0.11 Wm −2 of warming from BrC. Uncertainties in size, optical properties, observations, and emissions suggest an overall uncertainty in BC DRF of −80 %/+140 %. Our estimates are at the lower end of the 0.2-1.0 Wm −2 range from previous studies, and substantially less than the +0.6 Wm −2 DRF estimated in the IPCC 5th Assessment Report. We suggest that the DRF of BC has previously been overestimated due to the overestimation of the BC lifetime (including the effect on the vertical profile) and the incorrect attribution of BrC absorption to BC.

Abstract. The direct radiative effect (DRE) of aerosols, which is the instantaneous radiative impact of all atmospheric particles on the Earth's energy balance, is sometimes confused with the direct radiative forcing (DRF), which is the change in DRE from pre-industrial to present-day (not including climate feedbacks). In this study we couple a global chemical transport model (GEOS-Chem) with a radiative transfer model (RRTMG) to contrast these concepts. We estimate a global mean all-sky aerosol DRF of −0.36 Wm −2 and a DRE of −1.83 Wm −2 for 2010. Therefore, natural sources of aerosol (here including fire) affect the global energy balance over four times more than do present-day anthropogenic aerosols. If global anthropogenic emissions of aerosols and their precursors continue to decline as projected in recent scenarios due to effective pollution emission controls, the DRF will shrink (−0.22 Wm −2 for 2100). Secondary metrics, like DRE, that quantify temporal changes in both natural and anthropogenic aerosol burdens are therefore needed to quantify the total effect of aerosols on climate.

[1] We use a suite of satellite observations (Moderate Resolution Imaging Spectroradiometer (MODIS), Multiangle Imaging Spectroradiometer (MISR), Cloud-Aerosol Lidar With Orthogonal Polarization (CALIOP)) to investigate the processes of long-range transport of dust represented in the global GEOS-Chem model in [2006][2007][2008]. A multiyear mean of African dust transport is developed and used to test the representation of the variability in the model. We find that both MODIS and MISR correlate well with the majority of Aerosol Robotic Network observations in the region (r > 0.8). However, MODIS aerosol optical depth (AOD) appears to be biased low (>0.05) relative to MISR in Saharan regions during summer. We find that GEOS-Chem captures much of the variability in AOD when compared with MISR and MODIS (r > 0.6) and represents the vertical structure in aerosol extinction over outflow regions well when compared to CALIOP. Including a realistic representation of the submicron-size distribution of dust reduces simulated AOD by $25% over North Africa and improves agreement with observations. The lifetime of the simulated dust is typically a few days (25%-50%) shorter than inferred from MODIS observations, suggesting overvigorous wet removal, confirmed by comparison with rain rate observations from the Tropical Rainfall Measuring Mission satellite. The simulation captures the seasonality of deposition in Florida and the observed magnitude and variability of dust concentrations at Barbados from 2006 to 2008 (r = 0.74), indicating a good simulation of the impacts of North African dust on air quality in North America. We estimate that 218 AE 48 Tg of dust is annually deposited into the Atlantic and calculate a lower estimate for the dust deposited in the Caribbean and Amazon to be 26 AE 5 Tg yr À1 and 17 AE 5 Tg yr À1, respectively. This suggests that the dust deposition in the Amazon derived from satellites may be an upper limit.

Understanding the cooling effect of recent volcanoes is of particular interest in the context of the post-2000 slowing of the rate of global warming. Satellite observations of aerosol optical depth above 15 km have demonstrated that small-magnitude volcanic eruptions substantially perturb incoming solar radiation. Here we use lidar, Aerosol Robotic Network, and balloon-borne observations to provide evidence that currently available satellite databases neglect substantial amounts of volcanic aerosol between the tropopause and 15 km at middle to high latitudes and therefore underestimate total radiative forcing resulting from the recent eruptions. Incorporating these estimates into a simple climate model, we determine the global volcanic aerosol forcing since 2000 to be À0.19 ± 0.09 Wm À2 . This translates into an estimated global cooling of 0.05 to 0.12°C. We conclude that recent volcanic events are responsible for more post-2000 cooling than is implied by satellite databases that neglect volcanic aerosol effects below 15 km.

Abstract. The role of mineral dust in climate and ecosystems has been largely quantified using global climate and chemistry model simulations of dust emission, transport, and deposition. However, differences between these model simulations are substantial, with estimates of global dust aerosol optical depth (AOD) that vary by over a factor of 5. Here we develop an observationally based estimate of the global dust AOD, using multiple satellite platforms, in situ AOD observations and four state-of-the-science global models over 2004–2008. We estimate that the global dust AOD at 550 nm is 0.030 ± 0.005 (1σ), higher than the AeroCom model median (0.023) and substantially narrowing the uncertainty. The methodology used provides regional, seasonal dust AOD and the associated statistical uncertainty for key dust regions around the globe with which model dust schemes can be evaluated. Exploring the regional and seasonal differences in dust AOD between our observationally based estimate and the four models in this study, we find that emissions in Africa are often overrepresented at the expense of Asian and Middle Eastern emissions and that dust removal appears to be too rapid in most models.

Humans make extensive use of fire to clear forests and vegetation and to prepare and maintain land for agriculture 8,9 . Emissions of particulate matter (PM) from fires can dominate atmospheric concentrations particularly during the dry season 6,10 . Inhalation of PM from fires has adverse impacts on human health, including increased hospital admissions and premature mortality 3,4,11,12 .Rapid deforestation is occurring across the tropics 5 . Between 1976 and 2010, more than 750 000 km 2 of the Brazilian Amazon was deforested, equivalent to ~15% of the original forested area 1 . Recently, Brazil has achieved well documented reductions in deforestation rates 1,5,6 .Over Figure S1). Reduction in deforestation rates have numerous social and environmental benefits 1 . We were interested in whether the reduction in deforestation rates has also improved air quality across Brazil.Satellite-derived datasets of fire occurrence show the total number of active fire counts across Amazonia is positively related to both deforestation rates and occurrence of drought 1,7,13,14 .During 2001 to 2010, years with high deforestation rates had a factor 2 greater incidence of fire compared to years with low deforestation rates 1 . Significant declines in fire frequency across Brazil have occurred over this period, with the largest reductions in regions of high cumulative deforestation 7 .We used three different datasets of satellite-derived fire emissions 2,15,16 available over 20022,15,16 available over to 2011 to further explore the relationship between deforestation and PM emissions from fire. Substantial fire emissions occur across Brazil (Fig. 2), accounting for 12-16% of global particulate emissions from fire (Supplementary Table S1). In South America, particulate emissions from fire are greatest across southeast Amazonia where there is rapid deforestation (Fig. 2). Tropical forests of central Amazonia have little fire emission because high moisture, dense forest canopies and little deforestation mean fires are a rare occurrence 17,18 . Regions with frequent agricultural fires also have lower total fire emission compared to regions of active deforestation, because agricultural fires result in a factor 3-5 lower emission per unit area burned due to lower fuel loads 19 .One satellite fire dataset classifies emissions according to fire type 2 , allowing the specific contribution of deforestation fires to be estimated (see Methods). Deforestation fires only account for 20% of global total particulate fire emissions but 64% of Brazil's total, meaning deforestation fires dominate regional air quality impacts. Classification of fire types is an uncertainty -deforestation fires may spread out of the deforested area into surrounding forest, where they are classified as a different fire type not associated with deforestation. Throughout our analysis, we therefore analyse both total particulate fire emissions and emissions specifically classified as from deforestation fires.Over 2001 to 2011, Amazonia experienced drought conditions du...