Natural aerosols and climate: Understanding the unpolluted atmosphere to better understand the impacts of pollution

Transcription

1 : Understanding the unpolluted atmosphere to better understand the impacts of pollution 264 Douglas S. Hamilton Institute for Climate and Atmospheric Science, University of Leeds Aerosols are small solid or liquid particles suspended in the atmosphere and due to their interactions with solar radiation are an important factor in determining present-day climate. Aerosols of a sufficient size directly alter the energy balance of the atmosphere through absorbing and scattering incoming solar radiation, termed aerosol radiation interactions (ARI). The effect of this interaction can be seen through changes to visibility (haze) on polluted days. Additional to the direct effect of aerosols on solar radiation, aerosol cloud interactions (ACI) indirectly alter the energy balance of the atmosphere by changing cloud brightness (albedo) and therefore how much radiation is reflected back to space from clouds. Aerosols above a certain size, with the ability to attract and retain water (hygroscopicity), play a critical role in the atmosphere at supersaturations below a few percent by acting as the seed, termed cloud condensation nucleus (CCN), upon which cloud drops can grow. In the absence of CCN the supersaturations required to freely grow cloud droplets are of the order of several hundred percent, leading to a cloud free world. An increase in CCN concentrations leads to brighter clouds by increasing the number and reducing the average size of the cloud drops for a fixed amount of cloud liquid water (Boucher et al., 2013). The magnitude of the direct aerosol effect on radiation increases linearly with emission strength except at very low aerosol optical depths (the concentration of aerosol in a vertical column of air) (Boucher et al., 1998). ACI are more complex, but to a first order approximation cloud albedo increases logarithmically with aerosol concentration (Pringle et al., 2009). These nonlinearities mean that small changes in aerosol concentrations in regions where the background aerosol concentrations are low will have a larger effect on ARI and cloud albedo than the same increase in a region of higher aerosol concentrations. Globally, increases in aerosol concentrations over the industrial period have helped cool the planet by increasing the amount of solar radiation reflected back to space and, in doing so, have masked the full extent of greenhouse gas warming (Boucher et al., 2013). The magnitude of this cooling, especially the portion relating to ACI, remains one of the greatest sources of uncertainty in our understanding of climate change over the industrial period (Stocker et al., 2013). These uncertainties have hampered efforts to quantify the sensitivity of climate to human activity over the industrial period and increasing our understanding of the different contributers to this uncertainty will improve our perception of future climate change. A large portion of the pre-industrial (PI) to present day (PD) radiative forcing uncertainty from ACI stems from uncertainties in how natural emissions affect aerosol concentrations in the PI (Carslaw et al., 2013; Wilcox et al., 2015). This is due to large uncertainties in the state of the PI atmosphere and the highly non-linear effect of aerosol on cloud albedo described above, especially in warm low-level cloud (stratocumulus) regimes. Global observational records of aerosol number concentrations (a prerequisite of CCN) in the unpolluted preindustrial atmosphere do not exist. While ice cores provide valuable records of past atmospheres, they cannot constrain global CCN number concentrations due to particle number concentrations not being retained (only the mass of the aerosol species) within them, and it is the number concentrations which are important when considering the climate effects of aerosol. While many PD regions are polluted (Andreae, 2007) there remain remote regions where CCN concentrations are still similar to those in an unpolluted state (Hamilton et al., 2014) and can potentially act as an analogue for the pre-industrial atmosphere. Measurements of aerosols in pristine regions, without the influence of anthropogenic (which means man-made and, although often used to describe industrial activity, refers to all human activity) emissions, will help enable the pre-industrial state of the atmosphere to be better understood in those selected environments. Identification of such pristine regions is important to help in constraining the magnitude of anthropogenic climate change from this baseline state as well as providing information on how future changes in natural aerosol concentrations could affect climate (Rap et al., 2013). interactions Natural aerosols can be either directly emitted to the atmosphere (primary aerosol) or formed by gas-to-particle conversion from suitable pre-cursor gases (secondary aerosol). Typical examples of primary natural aerosol are black carbon (soot) from wildfires, dust particles and sea salt. Secondary natural aerosols are typically formed from emissions of phytoplankton dimethyl sulphide (DMS) in ocean regions, biogenic volatile organic compounds (BVOC) from forest regions and sulphur dioxide (SO 2 ) from volcanic gas emissions. Unlike long-lived greenhouse gases, aerosols are short-lived in the atmosphere with typical atmospheric lifetimes of a few weeks. This combination of varied source types, different emission strengths, and short atmospheric lifetimes means that natural aerosol distributions are temporally and spatially heterogeneous across the globe, with the largest concentrations occurring close to emission sources. Due to the relatively large size of many natural aerosols (e.g. sea salt and dust) they account for the dominant fraction of mass in the atmosphere. However, large increases in smaller sized anthropogenic aerosols have substantially reduced the relative number fraction of natural aerosol in the atmosphere since the PI. As ACI are driven by the total aerosol number and not aerosol mass, the role of natural aerosols will be diminished in comparison to the pre-industrial level; to what extent, however, needs to be investigated further. Natural aerosols and related pre-cursor gases (except those associated with continuously degassing volcanoes) have a seasonal cycle in emission strength due to seasonal variations in local climate (e.g. temperature, light availability or wind speeds). Once in the atmosphere natural aerosols are often a large component of many Earth system

2 feedback cycles and, while these feedbacks are often poorly understood, many have potentially significant impacts on climate (Carslaw et al., 2010). To further understand how natural aerosols affect the climate and the Earth system, as well as the impact that the addition of anthropogenic aerosols has had on these interactions, it is important to first study natural aerosol in environments that are as close to pristine as possible. Modelling the past and present atmosphere The purpose of this paper is to first compare how PD aerosol (year 2000) concentrations have changed since the PI (year 1750) and then to highlight potential regions where aerosol in a pristine environment can be found and measured. The term pristine is defined here as a PD region with aerosol concentrations similar to the PI, although the term is often used misleadingly to imply extremely low aerosol concentrations. Following Hamilton et al. (2014), a similar concentration is one where PD CCN concentrations are within ±20% of PI concentrations. Four grid cells which contain pre-existing measurement stations are then identified as suitable sites where PD aerosol measurements could be taken as analogues for a PI atmosphere for at least part of a year. Although 1750 is not truly pre-human it does represent an appropriate time point for a pre-industrial atmosphere, defined here as being an atmosphere with very low anthropogenic influence yet without going so far back in the historical record that uncertainty in the state of the atmosphere becomes so large as to be inappropriate. It should be noted, however, that this definition of pristine does not in itself guarantee a pristine aerosol environment, as it is conceivable that two time periods could have a similar aerosol concentration but through very different processes. However, for the purpose of this article and to illustrate how aerosol concentrations have changed over the industrial period it is sufficient. The GLObal Model of Aerosol Processes (GLOMAP) (Mann et al., 2010) was used to simulate the 1750 and 2000 aerosol concentrations from the existing experimental set up of Hamilton et al. (2014). In 1750 anthropogenic fossil fuel and anthropogenic volatile organic compound emissions are assumed to be zero, but a small biofuel component exists (mainly due to domestic heating and cooking). Natural emission fluxes of ocean DMS concentrations (Kettle and Andreae, 2000), sea spray and dust to the atmosphere are generated as a function of wind speeds. Both the BVOC and volcanic emissions are set to be the same in both the PI and PD. PI biomass burning emissions are scaled from PD emissions based on how population changes affect historical land cover and crop production. Global modelled daily mean aerosol concentrations are calculated in each grid cell from across the full aerosol size distribution and provide the basis for calculating monthly mean aerosol concentrations. CCN are a subset of the total aerosol population and here are defined as those soluble aerosols with a dry diameter greater than 50nm. Global CCN concentrations reported here are at the 915hPa model level (~850m above sea level), which corresponds to the cloud base for low level stratiform clouds. The effect that changes in meteorology have had on aerosol concentrations between the 2 years has been eliminated in the simulations by using the same year 2008 reanalysis data, updated every 6h from the European Centre for Medium-range Weather Forecast. Global changes in aerosol concentrations from 1750 to 2000 Figure 1 shows the fraction of the Earth that is defined as being pristine as a function of aerosol particle size, i.e. counting all particles above this size. In all months of the year the pristine fraction of the Earth (PD aerosol concentrations similar to PI [±20%], see earlier) depends on the particle size. Anthropogenic aerosols tend to occur at smaller particle sizes, resulting in the fraction of the Earth defined as pristine being smallest within the Aitken mode (aerosol particle sizes of nm), with a minimum pristine coverage when the particle size lies between 50 and 100nm, depending on month. Natural aerosols increasingly dominate at larger particle sizes, resulting in a steady increase in the pristine fraction of the Earth with increasing particle size throughout the accumulation mode (aerosol particle sizes of nm). At the boundary with coarse mode aerosol (aerosol particle size of >1000nm) pristine coverage of the Earth is over 90% in all months of the year. As ice nuclei are typically large (>500nm) aerosol particles, such as dust (Atkinson et al., 2013), the very high pristine fraction of the Earth which is shown at these larger aerosol sizes could be important when considering changes to ice nuclei concentrations over the industrial period. There is little seasonality in pristine coverage within the naturally dominated accumulation size range compared to a distinct seasonality in the anthropogenically effected Aitken size range. As particle size increases, the in-cloud supersaturation required to activate the aerosol into cloud drops decreases. Using the definition that soluble aerosol above a dry diameter of 50nm are classed as CCN, Figure 2 shows maps of the monthly mean percentage change in CCN concentrations from 1750 to 2000 in each grid cell. The maps show that although the anthropogenic fraction of PD CCN is variable, the atmosphere has undergone a significant change from one being primarily influenced by natural emissions to one which is now dominated by pollutants in many regions (see also Box 1). The PI-to-PD change in CCN concentration due to anthropogenic activity is most noticeable in the Northern Hemisphere (NH), where higher levels of pollution also occur. In particular, over the low- to midlatitude NH regions the increase in CCN concentrations are usually over 100% and can exceed 1000% in highly polluted continental regions. In some NH regions (e.g. Western Europe) pollution levels are now in decline due to air quality legislation, so the natural fraction of aerosol will increase in the future even if natural emissions themselves remain constant. While the magnitude of the effect on climate that changing the atmosphere back towards one dominated by natural aerosols is unknown, any reduction to the negative radiative forcing provided by aerosols increases the overall net positive radiative forcing from greenhouse gases by removing the counterbalance they provided. Figure 1. How changing the aerosol size of interest alters the change in fraction of the Earth defined as pristine (1750 and 2000 aerosol concentrations are similar (±20%)). 265

3 example, in the Indian Ocean region the Intertropical Convergence Zone (ITCZ) provides an effective barrier to the range that continental pollution can travel. As the position of the ITCZ alters over the course of the year, from the summer when it positioned over India to the winter when it is over the Indian Ocean, the extent to which pollution can travel also changes and therefore alters the extent of the coverage in pristine aerosol environments. Micro-pristine environments with low aerosol concentrations can be created within a single cloud with high precipitation levels as the aerosol entering the cloud is effectively immediately washed out. However, as these clouds would also exist in the PI they may not be overly instructive in shedding light on the pristine state of larger-scale cloud systems, which is what is important when considering the radiative forcing of climate due to anthropogenic activity. Present day observational sites close to a PI aerosol state Figure 2. The monthly mean percent change in CCN number concentration at 915hPa (~850m above sea level). Box 1 Annual average global modelled CCN concentrations have increased by a factor of 2.2 over the industrial period ( ) at the low-level warm cloud base (915hPa) due to additional man-made emissions. This increase is larger in the Northern Hemisphere due to the different hemispheric distributions of pollution sources. Nevertheless, there remain locations close to an unpolluted state in remote regions of the world, (generally concentrated in the Southern Hemisphere). 266 In general, the Arctic is a region strongly affected by long-range transport of anthropogenic pollution. However, due to increased fire suppression in the PD model, results shown in Figure 2 suggest an overall reduction in summertime PD CCN concentrations in parts of the high latitude boreal regions of Canada and Siberia. This illustrates the point that the overall anthropogenic influence on aerosol concentrations does not always equate to an overall increase in aerosol concentrations across the industrial period. The Southern Hemisphere (SH) is generally less affected by pollution than the NH, especially over remote marine regions which cover large portions of this hemisphere. Figure 2 shows the long-range transport of pollutants reaching Antarctica from May to June (peaking in September) and although this region of the world is very remote from civilization it is not always exempt from an anthropogenic influence. The Brazilian rainforest is another remote region often assumed to be representative of the PI, and is often called a green ocean. However, model results show that the large increases in man-made biomass burning emissions since 1750 in regions to the west (1 S, 74 W) and south (10 S, 60 W) of the rainforest have yielded the highest global increase in 1750-to-2000 CCN concentrations in April and October respectively. Even so, the way in which CCN number concentrations respond to the fire emissions from South American biomass burning regions has remained similar over the industrial period (Hamilton et al., 2014) and therefore even though the magnitude of aerosol concentrations has changed, South American biomass burning regions are still informative of how aerosol behaves in a PI-like environment. Regions where CCN concentrations are still predominantly natural in origin, and therefore closest to pristine, are coloured light blue in Figure 2 (PD CCN ±20% PI levels). Due to the periodic nature of pollution events pristine regions exhibit a seasonal cycle in their extent, often with a large hemispheric contrasts seen. As well as emission source, the environmental and meteorological conditions that aerosol particles travel through can also affect aerosol composition and lifetime and therefore affect how close a region is to pristine. For The monthly mean fractional change in CCN concentrations in Figure 2 is useful for illustrating the general PI-to-PD change in aerosol concentrations across the globe and shows remote regions can often contain a small but pervasive anthropogenic aerosol component. The background state in these regions is therefore no longer indicative of a pristine PI background, but rather an altered PD background which is influenced by anthropogenic activity. Ideally, pristine aerosol measurements should be made in regions as similar to the PI as possible. To evaluate suitable measurement locations a more detailed daily analysis of the PI and PD CCN concentrations in specific grid cells is more suitable. This is because when using monthly average CCN values, small pollution events can make the whole month appear mildly polluted. An advantage of comparing daily changes in CCN concentrations is to capture this episodic nature of pollution events, especially in remote regions. Unfortunately the majority of regions with a pristine aerosol environment are located in marine locations where making measurements is logistically more difficult than on land. However, the modelling results identified that certain existing measurement stations are located within potentially suitable pristine regions. Figure 3 shows examples of four measurement locations where daily PI and PD CCN concentrations are similar. There are two SH locations that include Amsterdam Island (Figure 3(a)) and American Samoa (Figure 3(b)), and two NH locations that include the high-latitude measurement stations of Alert (Figure 3(c)) and Barrow (Figure 3(d)). Under the assumption that the boundary layer is well mixed, the

4 (a) (b) (c) (d) Figure 3. Daily mean CCN number concentrations over a 1 year period in the pre-industrial (1750) and present-day (2000) at four atmospheric monitoring stations: (a) Amsterdam Island, (b) American Samoa, (c) Barrow and (d) Alert. CCN concentrations at 850m above sea level and the CCN concentrations in lower model levels (where stations typically exist) are comparable. Due to model resolution limitations the grid cells that contain the island locations are treated as ocean. As the air masses reaching these islands also travel over the surrounding ocean regions it is assumed that this discrepancy will have little to no effect on the analysis. In general, both of the island locations are more similar in their CCN profiles throughout the whole of the year than the Arctic stations, which are typically only pristine during the summer months. Amsterdam Island is situated close to the Southern Ocean and is located in a grid cell which is particularly close to a PI aerosol state. This is a region of the world that would particularly benefit from previously untaken measurements of CCN concentrations, ideally with a focus on how natural ocean emissions (such as DMS) interact with aerosols and the climate without influence from pollution. American Samoa is located in the remote South West Pacific, and modelled results also show this as a good choice for understanding pristine aerosol. Measurements of aerosol at American Samoa both during an extreme ENSO (El Niño Southern Oscillation) event and during a more stable period would also be highly valuable in furthering our understanding of natural aerosol Earth system interactions. For example, ENSO has been shown to increase Indonesian biomass burning emissions through decreases to local precipitation levels (Chrastansky and Rotstayn, 2012). The resulting increase in aerosol concentrations could then provide a feedback upon the local climate by further reducing precipitation levels by increasing cloud lifetimes. A better understanding of these feedbacks between natural aerosols and climate is essential for future climate sensitivity studies. In the NH PD CCN concentrations are lower in both Alert in Northern Canada and Barrow in Alaska over the summer months due to the modelled reductions in wildfire emissions, with this reduction being more pronounced in Barrow than Alert. During the winter months long-range transport of pollution builds up in the Arctic regions, consistent with the seasonal cycle of Arctic haze which is controlled by scavenging processes (Browse et al., 2012). Nevertheless both of these Northern Hemisphere sites are potential candidates for measuring aerosol in a pristine environment for parts of the year as well as how natural wildfires affect aerosol concentrations. Conclusion There is a large diversity in natural aerosols, driven by a combination of different source types, atmospheric chemistry interactions, and lifetimes. Anthropogenic activity has greatly increased aerosol number concentrations since 1750 and, in doing so, has reduced the number fraction attributable to natural aerosol, especially in the NH. The result of this increase may suppress natural aerosol-climate interactions (Spracklen and Rap, 2013) in the PD compared to the PI. Regions where anthropogenic emissions are rapidly changing (e.g. large decreases in Europe and increases in China) will again alter the natural-to-anthropogenic aerosol fraction over the coming decades. Studies into the strength of natural aerosol Earth system-climate feedbacks, as well as the effect that anthropogenic emissions has had on these feedbacks, will benefit from improved representations of these couplings and processes in future versions of Earth system models. In particular, improved representation of both natural wildfire and anthropogenic fire emissions in PI and PD model simulations will help understand the different impacts these two different sources of fire have on the climate. Model results have been shown here to be useful in highlighting those regions that are potentially suitable sites for making observations of natural aerosol in pristinelike environments. Pristine sites are often marine in nature and located in the SH, which has suitable island locations for taking measurements from. Some high latitude NH pristine regions, such as Alaska and Yukon, also exist, but these pristine environments are more transient than in the SH. Precursor gases forming secondary aerosols (i.e. aerosol formed from volcanic SO 2, oceanic DMS and terrestrial BVOC emissions) are contributing most of the uncertainty in PI-to-PD cloud albedo radiative forcing calculations from natural emissions (Regayre et al., 2014). It is therefore beneficial to first focus on making aerosol measurements of these species in pristine regions. Finding observationally suitable methods of separating polluted and pristine atmospheric conditions requires careful consideration in future campaigns. Currently operational definitions of polluted vs. unpolluted often include using tracers of pollution, such as black carbon, to filter out polluted days, but as these have both a natural and anthropogenic source it may not be the ideal solution. The assumption that using low aerosol concentrations as a filter for separating out pristine regions may not always be suitable either, as many regions in the PI contained high levels of aerosol from fire emissions. Future studies will benefit from defining observational criteria for studying pristine regions by 267

5 incorporating modelling results alongside other observational metrics. Acknowledgements This research has received funding from the Natural Environment Research Council AEROS project (project number NE/ G006172/1) and GASSP project (project number NE/J024252/1), and the National Centre for Atmospheric Science. D.S.H would like to thank the Natural Environment Research Council and Met Office in funding his PhD, Kirsty Pringle and Lindsay Lee in providing data and Ken Carslaw, Dominick Spracklen, Steven Turnock and 2 anonymous reviewers for their thoughtful comments. References Andreae MO Aerosols before pollution. Science 315: Atkinson JD, Murray BJ, Woodhouse MT et al The importance of feldspar for ice nucleation by mineral dust in mixedphase clouds. Nature 498: Boucher O, Schwartz SE, Ackerman TP et al Intercomparison of models representing direct shortwave radiative forcing by sulfate aerosols. J. Geophys. Res. 103: Boucher O, Randall D, Artaxo P et al Clouds and aerosols, in Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, chapter 7. Stocker TF, Qin D, Plattner G-K et al. (eds). Cambridge University Press: Cambridge, UK and New York, NY, pp Browse J, Carslaw KS, Arnold SR et al The scavenging processes controlling the seasonal cycle in Arctic sulphate and black carbon aerosol. Atmos. Chem. Phys. 12: Carslaw KS, Boucher O, Spracklen DV et al A review of natural aerosol interactions and feedbacks within the Earth system. Atmos. Chem. Phys. 10: Carslaw KS, Lee LA, Reddington CL et al Large contribution of natural aerosols to uncertainty in indirect forcing. Nature 503: Chrastansky A, Rotstayn LD The effe ct of ENSO-induced rainfall and circulation changes on the direct and indirect radiative forcing from Indonesian biomass-burning aerosols. Atmos. Chem. Phys. 12: Hamilton DS, Lee LA, Pringle KJ et al Occurrence of pristine aerosol environments on a polluted planet. Proc. Natl. Acad. Sci. U.S.A. 111: Kettle A, Andreae M Flux of dimethylsulf ide from the oceans: a comparison of updated data sets and flux models. J. Geophys. Res. 105: Mann GW, Carslaw KS, Spracklen DV et al Description and evaluation of GLOMAP-mode: a modal global aerosol microphysics model for the UKCA composition-climate model. Geosci. Model Dev. 3: Pringle KJ, Carslaw KS, Spracklen DV et al Th e relationship between aerosol and cloud drop number concentrations in a global aerosol microphysics model. Atmos. Chem. Phys. 9: Rap A, Scott CE, Spracklen DV et al Natural aeroso l direct and indirect radiative effects. Geophys. Res. Lett. 40: Regayre LA, Pringle KJ, Booth BBB et al Uncertainty in the magnitude of aerosol-cloud radiative forcing over recent decades. Geophys. Res. Lett. 41: Spracklen DV, Rap A Natural aerosol-climate feedbacks suppre ssed by anthropogenic aerosol. Geophys. Res. Lett. 40: 1 4. Stocker T, Qin D, Plattner G et al IPCC 2013: summary for po licy makers, in Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Stocker T, Qin D, Plattner G et al. (eds). Cambridge University Press: Cambridge, UK and New York, NY, pp Wilcox LJ, Highwood EJ, Booth BBB et al Quantifying sources of inter-mo del diversity in the cloud albedo effect. Geophys. Res. Lett. 42: Correspondence to: Douglas S. Hamilton d.hamilton@leeds.ac.uk 2015 Royal Meteorological Society doi: /wea.2540 Implications of event attribution for loss and damage policy 268 Hannah R. Parker, 1 Rosalind J. Cornforth, 1 Emily Boyd, 2 Rachel James, 3 Friederike E. L. Otto 3 and Myles R. Allen 3,4 1 Department of Meteorology, University of Reading 2 Geography and Environmental Science, University of Reading 3 Environmental Change Institute, University of Oxford 4 Department of Physics, University of Oxford Introduction Extreme weather events are widespread across the globe. As an example, Figure 1 highlights the significant precipitation and temperature extremes that occurred just during the month of January The extremes identified in Figure 1 are those that were significant from a meteorological perspective. Many other events also occur which have important impacts on both human lives and livelihoods, and the biophysical environment, precipitating questions on their causes. The climate system is changing and global average temperatures are increasing, and it has been shown that it is extremely likely that the majority of this warming over the last decades has been due to anthropogenic forcings (Bindoff et al., 2013). Yet how anthropogenic influences on the climate system are affecting extreme events is less well understood. How extreme events are changing, and could do in the future, is uncertain. How were the events in Figure 1 affected by human influence on the climate? What does this mean for the climate policies that aim to deal with their impacts? Addressing the impacts of extreme weather events Although the impacts of climate change and extreme events can be large in developed countries, people s livelihoods are much