EGU Blogs


Around the world

Around the world

The short lifetime of aerosol particles in the atmosphere means that they tend to be found concentrated in regions close to where they were formed. As they only last for days to weeks in the atmosphere, they don’t travel too far as the Earth’s winds aren’t able to fully mix the air over such a short period.

This short lifetime should also mean that any changes in the emissions of aerosol particles and their gas-phase building blocks will show up quickly when we measure their concentrations in the atmosphere. This contrasts with greenhouse gases, which typically have much-longer lifetimes (decades to centuries), so their concentrations remain much further into the future (even if their emissions were ceased completely).

Below is a collection of images of the global aerosol view from 2001 to 2012 from the Multi-angle Imaging SpectroRadiometer (MISR) on board NASA’s TERRA satellite. Each map is a single year, with the year noted to the bottom right of the image. The maps show the Aerosol Optical Depth (AOD), which is a measure of the amount of aerosol within the atmosphere. As the measurements are made from space, the instrument measures the aerosol in a vertical column below the satellite. Terra orbits the Earth continuously and gradually builds up a picture of the Earth below; MISR itself will typically build up a global picture over a couple of weeks.

Global view of the Aerosol Optical Depth (AOD) from 2001-2012 observed by the Multi-angle Imaging SpectroRadiometer (MISR) on board NASA's TERRA satellite. White spaces indicate there is no data.

Global view of the Aerosol Optical Depth (AOD) from 2001-2012 observed by the Multi-angle Imaging SpectroRadiometer (MISR) on board NASA’s TERRA satellite. White spaces indicate there is no data. High latitudes are excluded as the satellite retrievals don’t work over highly-reflective surfaces (snow/ice). Click on the image for a larger view.

The most persistent feature is the large expanse of aerosol over Africa, which also extends into the Atlantic Ocean. Over North Africa, much of this aerosol is a result of the wind whipping up a storm of Saharan dust, which can then be blown over large distances. Sometimes, the dust even makes its way across the whole Atlantic Ocean and lands in North or South America. Further south, frequent burning occurs to clear savannah areas in Southern Africa, which creates large amounts of smoke.

The Arabian Peninsula, India and South-East Asia (particularly China) are other regions where aerosol is prevalent. Similarly to the Sahara, Saudi Arabia is prone to large dust storms, while India and China have undergone significant economic development recently, which has led to increased burning of fossil fuels. South America also sees a large build up of aerosol over the Amazon Rainforest due to deforestation fires, although the amount of aerosol varies more from year-to-year.

Europe, North America and Australia display relatively low levels of aerosol compared to these other regions. Bear in mind that these are annual averages, so they will miss intense and short-lived pollution events, which do still affect these regions.

The atmosphere above the Earth’s oceans is generally quite clean as far as aerosol is concerned, as the major sources of aerosol are far away. There are deviations from this general observation though, for example, aerosol is transported across the Pacific Ocean from Asia to North America. This can be seen in the top right of each image (as well as in the top left at times). The strong winds in the Southern Ocean also see elevated aerosol amounts, as the wind helps to produce sea spray aerosol.

Below is a map showing the difference between the last four years shown above (2009-2012) and the first four years (2001-2004) in order to highlight changes in aerosol over this period. Overall, there hasn’t been a great change in the global amount of aerosol; Dan Murphy, in a paper in Nature Geoscience (pay walled, sorry), showed that the level of aerosol hasn’t changed much over this period but that there have been regional changes. This is in agreement with several other papers using similar and/or the same measurements (some non-pay walled papers here and here). From a statistical point of view, the trend in global aerosol is negligible and weakly positive i.e. aerosol levels have increased since the turn of the century but the increase isn’t statistically significant.

Difference in Aerosol Optical Depth (AOD) from 2009-2012 to 2001-2004. Data source is the same as above. Blue colours indicate an increase in aerosol, while red colours show a decrease.

Difference in Aerosol Optical Depth (AOD) from 2009-2012 to 2001-2004. Data source is the same as above. Blue colours indicate an increase in aerosol, while red colours show a decrease.

The strongest increases have been over the Arabian Peninsula and India. These increases are likely a consequence of natural causes in the case of Saudi Arabia via increased dust emissions and transport, whereas India is likely a consequence of increased burning of fossil fuels. South America is probably the most notable region as far as aerosol decline is concerned; in recent years, deforestation has decreased compared to the early 2000s combined with some wetter years over the Amazon. Significant burning does still occur though, with 2010 being a notable year where drought conditions meant that a very large amount of smoke built up over the Amazon during the biomass burning season. There have also been declines in aerosol over Europe and the North Eastern USA, which are likely due to decreased emissions of sulphur dioxide from coal power stations.

The upshot of these trends is that since the year 2000, aerosol particles have undergone relatively minor changes on the global scale. The question going forward is whether this will continue and what impact will such changes have on our climate. Thus far, the declines in Europe and North America have been offset by increases in Asia, although the climate-relevant properties of the aerosol may not be the same. Furthermore, much of the decline in aerosol has been driven by reductions in sulphate aerosol but other species, such as secondary organic aerosol and ammonium nitrate, may become comparatively more important. These species are often missing or poorly represented in climate models, which has been suggested as an explanation for the recent pause/hiatus/slowdown in global mean surface temperature.

This is clearly a major issue in climate science currently and the role of aerosol is unclear. I’ll be writing more on this topic in the future, so watch this space!


Header image: Dust and Clouds Dance Over the Sahara, published by NASA Earth Observatory. The photograph was taken by astronaut Alex Gerst on September 8, 2014, from the International Space Station.

MISR analyses and visualizations used in this blog post were produced with the Giovanni online data system, developed and maintained by the NASA GES DISC.

Wuthering heights

Wuthering heights

Aerosol particles typically have short life spans in the atmosphere (days to weeks) but they can travel far and wide in that time. They can be lifted up to new horizons, higher and higher in the atmosphere.

This is important for their impact on our climate, for example, at least 20% of the uncertainty in the climate impact of black carbon aerosol is due to differences in its vertical distribution. This is due to black carbon having a stronger climate impact when located higher in the atmosphere compared with black carbon at the surface.

The majority of the sources of aerosol particles are located on the Earth’s surface but through atmospheric mixing, the particles can be transported to higher altitudes. This variation in aerosol concentrations with altitude is referred to as their vertical profile.

As someone whose research has focussed a lot on aircraft measurements, aerosol vertical profiles are something I think about frequently. Below is an example of a vertical profile from a paper I wrote in 2009, which collated aerosol chemical composition data from 41 research flights around the UK over an eighteen month period.

The profile is for sulphate aerosol, which can arise from both natural sources as well as through fossil fuel burning (particularly coal burning in power plants). We can see that sulphate aerosol peaks in the lowest 3km of the atmosphere and that it varies a lot, with reduced and more consistent concentrations found in the free troposphere above 4km. At the very top of the profile, sulphate appears to increase a little, which is likely due to long-range transport of pollution.

Vertical profile of sulphate aerosol based on aircraft measurements around the UK from Morgan et al., 2009.

Vertical profile of sulphate aerosol based on aircraft measurements around the UK from Morgan et al., 2009, Atmospheric Chemistry & Physics. Black and grey crosses correspond to individual data points, while the red lines in the left hand panel show the 25th, 50th and 75th percentiles for 500m altitude bins. In the right hand panel, the mean profile is shown, along with a representation of the variability in each altitude bin.

It turns out that when comparing observations of aerosol concentrations similar to those above, climate models generally have a tough time replicating the measurements. Personally, I see aerosol vertical profiles as an acid test of model performance as they are a useful means of testing various processes within the models; crucially, the shape of the vertical profile is largely independent of the emission strength of the aerosol sources (which are often highly uncertain), so any discrepancies are easier to ascribe to the processes within the model itself. Essentially, the aerosol vertical profile helps with the detective work that goes into diagnosing issues relating to our understanding of aerosol particles.

Below is an example from Hodnebrog et al. where they compared measurements of black carbon aerosol with a number of climate models. The grey shaded region shows the range from a collection of models and it is quite wide throughout the profile when looking at the 30N-60N and 30S-0 panels (A and B), with some models 10-100 times greater than others in terms of black carbon concentration. This means that there is a large diversity in what the different models think the black carbon concentration is at a given altitude. This is also true at higher altitudes over Japan; the narrower range closer to the surface is likely because the source of black carbon in this region will be from Japan itself, so the lowest portion of the profile will be strongly linked to the emissions from the surface.

Comparison between aircraft observations (black line), the AeroCom phase II model range (shaded area), the AeroCom phase II OsloCTM2 model result (dashed orange line) and various simulations with the OsloCTM2 model (coloured lines). The observations are from the HIPPO3 (a,b) and A-FORCE16 (c) campaigns.

Comparison between aircraft observations (black line), the AeroCom phase II model range (shaded area), the AeroCom phase II OsloCTM2 model result (dashed orange line) and various simulations with the OsloCTM2 model (coloured lines). The observations are from the HIPPO3 (a,b) and A-FORCE16 (c) campaigns. Figure is from Hodnebrog et al., 2014, Nature Climate Communications.

Now, when comparing the thick black line with the grey shaded region, we see that the observations are close to the centre of the model range over Japan (although the observations don’t extend very far in altitude). However, in the other panels, the observations are biased to the lower end of the model range and even falls outside it at high altitudes in panel A. This is a somewhat curious result, as our current understanding of black carbon is that its emissions are generally underestimated; if all else were equal, increasing the emissions of black carbon in models to reflect our current understanding would make such a comparison worse.

This over-prediction of black carbon concentrations, combined with the poor representation of the vertical profile suggests that other facets are playing a role in determining the black carbon vertical profile. The coloured lines in the figure are results from a single model using different assumptions about black carbon. The upshot of this from the paper is that the lifetime of black carbon is likely overestimated in the model, with the paper concluding that:

Increased emissions, together with increased wet removal that reduces the lifetime, yields modelled black carbon vertical profiles that are in strongly improved agreement with recent aircraft observations.

This is consistent with several other studies that have implicated the lifetime of black carbon as being responsible for the overestimation of black carbon at high altitudes. By increasing the amount of black carbon removed by precipitation, these studies can better represent the vertical profile of black carbon. The lifetime of black carbon isn’t likely to be the only culprit though and the discrepancies will likely vary geographically; in the tropics, the Coupled Model Intercomparison Project Phase 5 (CMIP5) models potentially suffer from overenthusiastic convective transport, which leads to more black carbon at higher altitudes when comparing with measurements. As ever, aerosol presents a complex picture.

Black carbon isn’t the only aerosol species subject to these issues, for example, organic aerosol also shows similarly large diversity between models and the agreement with observations isn’t particularly good.

Improving the representation of aerosol vertical profiles is a significant challenge due to a number of factors. Chief among these is the relatively sparse nature of detailed measurements of the aerosol vertical profile; airborne studies are expensive and are typically conducted for short periods e.g. month-long intensive projects. However, the number of airborne research platforms and projects is increasing rapidly, so the potential for improvements in our understanding is likely to increase as more measurements are collected and more measurement-model comparisons are conducted.

Hopefully, the answers are out in the winding, windy atmosphere.


Header image: Facility for Airborne Atmospheric Measurements (FAAM) BAe-146 research aircraft. Image courtesy of FAAM website.

Some of my colleagues are collating a vast number of measurements, with particular focus on airborne observations, for the Global Aerosol Synthesis and Science Project (GASSP), with the aim of creating a database that will be ideal for measurement-model comparisons. If you are interested, I can put you in touch with the relevant people.

If you want to hear and see more about the vertical profile of black carbon (and who doesn’t), then my presentation at AGU Fall Meeting is available online here. The registration is free and I’m told it will be available FOREVER.

EGU 2014: Measuring aerosol climate impacts from Space

In order to understand past climate change and to better project future changes, we need to understand how humans disturbed the radiative balance of our planet. Aerosols are one component of this disruption. The final report from the Intergovernmental Panel on Climate Change (IPCC) physical science basis concluded that aerosols dominate the uncertainty in the total anthropogenic radiative forcing.

Radiative forcing refers to the change in the energy balance of the climate system; carbon dioxide traps energy within our atmosphere, which leads to a warming effect. Aerosols on the other hand stop sunlight from reaching the surface of the Earth, which leads to less energy being retained by the atmosphere and leads to cooling.

I summarised what the IPCC report said about aerosols here, with some further analysis here.

There were several talks at the EGU on Friday that looked to better constrain the important climate relevant properties of aerosol particles in the atmosphere by observing them from space.

Layer cake

Ralph Kahn from NASA’s Goddard Space Center presented measurements of aerosol properties from the Multi-angle Imaging SpectroRadiometer (MISR) instrument, which flies on the TERRA satellite. His abstract is available here.

The MISR instrument views the Earth from nine different angles as it orbits, which allows it to identify 3-D properties of aerosol particles in the atmosphere. One such example that was presented was from Iceland’s Eyjafjallajökull volcano, which famously closed large swathes of European airspace (disrupting EGU 2010 in the process). Such an eruption is an ideal natural laboratory for assessing aerosol properties from satellites with several of the talks during the day making use of it.

The image below shows how MISR was able to measure the height of the plume of ash thrown into the atmosphere by the eruption. The colouring on the left of the image shows the height of the plume; notice how the plume starts out higher (from 4-6km) and then sinks lower (below 3km).


Satellite image of the plume of volcanic ash from the Eyjafjallajökull eruption from 7 May 2010. The image on the left is the natural colour image, while the version on the left has the plume height measured by MISR overlaid. Source: NASA/GSFC/LaRC/JPL MISR Team, retrieved from here.

In terms of volcanic ash, such information is useful for aircraft safety. From a climate perspective, the height of aerosol layers in the atmosphere can strongly influence the strength of the radiative forcing and the vertical distribution of aerosol is something that is represented poorly in climate models.

Cloudy with a chance of aerosols

Jens Redemann from the Bay Area Environmental Research Institute and the NASA AMES research center in the USA presented work that took advantage of a collection of satellites that fly in formation high above the Earth. His abstract is available here.

The satellites are known as the ‘A-Train’ and by combining the data from multiple instruments operating in quick succession, he was able to extract more detailed and valuable information than could be achieved with any one single instrument. Their calculations for clear-sky conditions agreed well with model-based results used in the 2007 IPCC report, with more work planned to compare with more recent model estimates.

However, there were more significant differences when looking at so called all-sky conditions i.e. when clouds are added to the mix. Several of the other talks in the session considered methods to improve retrievals of aerosol properties in the vicinity of clouds, with some encouraging results. The famous image of the Earth taken from Apollo 17 is known as the ‘Blue Marble’ but as an atmospheric scientist, I’m often more struck by how white the globe is when observing it from space. This is an important consideration for assessing the role of aerosols when it comes to climate change.

Anthropogenic, vegetable or mineral

Both Kahn and Redemann presented work aimed at categorising aerosol particles into different types e.g. urban pollution, biomass burning smoke, desert dust. Different types of pollution tend to have differing properties, so distinguishing between them can improve estimates of aerosol climate effects.

Another important distinction is separation into natural and human-caused components, so that the anthropogenic influence can be characterised. This is complicated though by human sourced pollution interacting with natural emissions; such aerosol particles are dazed and confused about their origins.

Such information would be particularly valuable as there is a large degree of diversity in climate model estimates of the various aerosol types, even though their estimate of the overall aerosol burden in the atmosphere is quite similar. Such discrepancies are troubling for future projections of climate change as they will introduce significant errors.

I love it when a plan comes together

Ralph Kahn ended his talk with a call for renewed focus to combine satellite and measurements made more directly (known as in-situ) in order to determine aerosol radiative effects independently of climate models. 

Bringing all of this information together is a big opportunity to better constrain the aerosol radiative forcing, which will greatly improve our ability to project future climate changes at both the global and regional scale.

Aerosols and the pause

There is a new commentary piece in Nature Geoscience by Gavin Schmidt and colleagues on ‘Reconciling warming trends’. The paper investigates several potential causes for the discrepancy between climate model projections and the recent ‘slowdown’ in global surface temperatures, which is nicely illustrated on Ed Hawkins’ Climate Lab Book blog.

One of the aspects the paper looks at is the influence of aerosol particles in the troposphere (the lower part of the atmosphere), which tend to exert a cooling influence on our climate. Of the aspects they looked at, aerosol particles are the most uncertain.

As I summarised in this previous post, the final report from the Intergovernmental Panel on Climate Change (IPCC) on the physical science basis concluded that aerosols was continue to dominate the uncertainty in the human influence on climate. They said that a complete understanding of past and future climate change requires a thorough assessment of aerosol-cloud-radiation interactions.

With this in mind, I want to delve into the details of how the chemical make-up of aerosol particles varies across the globe and how this is important for our climate and how this relates to the warming trends paper.

What are aerosols made of?

Aerosols are made up of a large variety of different chemical species, which vary by the time of day, seasonally, regionally and a host of other factors. There is no single type of aerosol in our atmosphere.

The figure below attempts to pull together a huge collection of studies that have been measuring the major aerosol chemical species in different regions of the globe. We can see on the map that many of the sites are in North America and Western Europe, where large coordinated surface networks have been making such measurements for decades. Measurements elsewhere are much more limited, particularly in the Southern Hemisphere.

This is one of the major difficulties in understanding aerosols; they are difficult to measure, the techniques used are typically quite labour intensive and they require access to well developed infrastructure and supporting measurements.


Summary of aerosol chemical species split across different regions based on surface measurements. The map shows the locations of the sites. For each area, the panels represent the median, the 25th to 75th percentiles (box), and the 10th to 90th percentiles (whiskers) for each aerosol component. The colour code for the measured components is shown in the key where SO4 refers to sulphate, OC is organic aerosol (strictly organic carbon), NO3 is nitrate, NH4 is ammonium, EC is black carbon (strictly elemental carbon), mineral refers to mineral dust e.g. from deserts and sea-salt refers to aerosols produced by ocean waves. The figure is 7.13 from the IPCC report, where the numerous references for the data are included. Click on the image for a larger view. Source: IPCC.

Bearing this in mind, we are able to draw some broad conclusions on their chemical make-up. We can see that in all of the non-marine regions, the aerosol is broadly made up of sulphate, organic carbon, nitrate, ammonium and black carbon, with the influence of mineral dust varying depending on location e.g. concentrations are much greater in locations such as Africa and Asia where large deserts exist.

Of these species, sulphate, nitrate, ammonium and black carbon are predominantly man-made in origin, while organic carbon can be a consequence of both natural and man-made emissions. We see that concentrations of organic carbon and nitrate are broadly equivalent to those of sulphate, with organic carbon being much greater in South America, Oceania, Africa and parts of Asia.

Where is the aerosol?

One of the key properties of aerosol particles in the troposphere is that they have a short lifetime, as they are removed from the atmosphere by rain or simply by crashing into things (terrible drivers those aerosol particles). This typically means that the largest concentrations occur near their sources; they are regionally very important in a climate context but their impact evens out at larger scales, although they do still exert a global influence.

Below is an illustration of this based on satellite measurements combined with a forecast model system, which shows ‘hotspots’ of aerosol particles over China, India and Southern Africa in particular. The metric used here is the aerosol optical depth, which is a measure of the amount of aerosol present in a vertical slice through the atmosphere.

Horizontal and vertical distributions of aerosol amount based on a combination of the European Centre for Medium Range Weather Forecasts (ECMWF) Integrated Forecast System model with satellite measurements from the Moderate Resolution Imaging Spectrometer (MODIS) averaged over the period 2003–2010 and the Cloud–Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument for the year 2010. The figure is 7.14 in the IPCC report and references for studies are available there. Click on the image for a larger view. Source: IPCC.

Comparing the two figures, we see that the enhanced aerosol optical depth in Asia is consistent with the large aerosol concentrations measured by the surface based networks. Of the man-made species, organic carbon is a big driver here, with nitrate, sulphate and ammonium also playing a major role. These aerosol particles are formed due to the substantial emissions from the region.

What about the pause?

Of these species, sulphate and black carbon are the major species that climate models have historically focussed on. Organic carbon is generally included, although there are large discrepancies in relation to measurements, particularly related to the secondary organic aerosol component. Nitrate has received comparatively little attention and those that have included it have shown that it is an important aerosol species both today and over the 21st Century. Drew Shindell and colleagues when comparing aerosol optical depth from satellite measurements and climate models concluded that:

A portion of the negative bias in many models is due to missing nitrate and secondary organic aerosol.

I’ve been involved in research showing how nitrate is important in North-Western Europe and how it enhances the aerosol radiative forcing in the region (many others have done so also by the way). Nitrate has long been known to be important in North America, particularly in California, where large urban and agricultural emissions coincide. Several studies have investigated its importance in East Asia, showing that it is particularly prevalent during the winter.

The ‘Reconciling warming trends’ commentary notes that nitrate was only included in two of the models that contributed to the Coupled Model Intercomparison Project (CMIP5) and that including this across all models brings them closer to the observed surface temperature trend. They also note that underestimating the secondary organic aerosol contribution would have a similar effect but don’t put a value on this.

While this certainly isn’t the last word on the importance of these species for climate change and research on the ‘pause’ will undoubtedly continue, further recognition of their contribution to atmospheric aerosol is a step in the right direction. There are many unanswered questions in this realm, particularly relating to the large differences across climate models in terms of what contribution each species makes to regional and global aerosol.

Bringing the aerosol measurement and modelling communities together more to address such issues is likely the way forward. This is a big focus in many aerosol related research projects these days…we’re trying, honest.