Climate change

EGU 2014: Measuring aerosol climate impacts from Space

Will Morgan May 4, 2014

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.

EGU 2014 Day 1: A day in the life of an aerosol particle

Will Morgan April 29, 2014

My first day at EGU 2014 in Vienna was principally spent listening to various speakers describe the life and death of tiny particles in the atmosphere, known as aerosols. These aerosol particles come from a variety of sources – one of the major sources is through burning of fossil fuels, which produces a cocktail of pollutants that form these particles. They can also arise from natural sources, such as bursting bubbles on the ocean surface, strong odours from trees and high winds whipping up sandstorms in the desert.

Pinning these sources down is tough but we also have to understand how they evolve in the atmosphere from their birth, their growth during their adolescent years and ultimately their adult years, where they can influence our climate and health. At some point, they are removed from the atmosphere where they become an ex-aerosol. Understanding these different changes is necessary if we are going to be able to understand their impact both in the past, present and future.

Baby steps

One of the major routes for an aerosol to be born is via ‘nucleation’ where the particles form tiny clusters, which are around 100,000 times smaller than the width of a human hair. These clusters form due to the combination of different gas phase molecules, which given the right cocktail and conditions, can condense to form these initial tiny particles. I’ve previously written about these early steps here.

There was work presented here at the EGU by Jasmin Tröstl from the Paul Scherrer Institute in Switzerland showing that chemical species known as oxidised organics take part in this initial process. The abstract for the work is available here.

For a long time, sulphuric acid was thought to be the vital ingredient for this nucleation process but recent work at a laboratory at CERN (known as the cleanest box in the world) has illustrated the importance of several other species. You can read more about two studies in Nature that were published in the past few years on these here and here. Oxidised organic species are abundant in the atmosphere, so it isn’t a huge surprise that they are important but it has only been through the development of the new laboratory at CERN and sophisticated new instrumentation that the importance of this key ingredient has been demonstrated.

The difficult years

The same study also illustrated that these oxidised organic species were vital for the growth of these nucleated particles. This is the key stage for such particles as they essentially either grow or suffer an early death. When they start out, they are too small to become cloud particles, which is their main route to impacting our climate. So without growing they will never know the wet embrace of a cloud droplet.

Not only did the oxidised organics strongly increase the growth of these particles but their addition was enough to reconcile the laboratory measurements with observations of the real world. This is an enlightening step as it has previously proven difficult to mix up the right cocktail to represent what really goes on in the atmosphere, which suggests a deficit in our knowledge of this important process.

All grown up

Once they reach adulthood, these particles become important from a human health and climate perspective. They can build-up in the atmosphere over a matter of hours or days and influence our lives.

Rongrong Shen from Karlsruhe Institute of Technology, Germany, presented measurements of spring time pollution in Beijing during 2012, focusing on the chemical makeup of the pollution. Her abstract is available here. Beijing is well known as a hotspot for pollution, with over 20 million people living in the city and over 5 million vehicles on the road frequently creating a heavy chemical soup. The average concentration for PM2.5 (aerosol particles with a diameter less than 2.5µm) was 89µg/m³, which is far in excess of what is considered healthy. Even the ‘clear’ days in terms of visibility saw average concentrations of around 45µg/m³. The World Health Organisation guidelines recommend the daily average values should remain below 25µg/m³, while annual values should be 10µg/m³ or lower.

Haze over Beijing and surrounding region from 22 March 2007. Image credit: NASA Earth Observatory

More severe pollution episodes were typically driven by species such as sulphate and nitrate, which are known as ‘secondary’ species. This means that they start out as a gas and then condense onto pre-existing aerosol, such as nucleated particles or direct emissions from car exhausts and other forms of combustion. The results also indicated that such episodes were not solely driven by emissions within the city; the wider region played a role, including industrial sources and other Chinese cities. This is a common feature of pollution episodes in Western Europe also, which I wrote about recently here and here.

This is an ex-aerosol

Urs Baltensperger from the Paul Scherrer Institute, Switzerland gave the Vilhelm Bjerknes Medal Lecture and included a discussion of the fate of aerosols in the atmosphere. His abstract is available here. Aerosols are typically removed from the atmosphere via crashing into something, such as the ground, or by forming cloud droplets. These cloud droplets either evaporate, leaving an aerosol particle behind or they can grow to form rain, which removes the aerosol from the atmosphere. The rainfall can also washout other aerosols by catching them on the way down.

He referred to several previous studies, including measurements very early in the aerosol life cycle in an urban environment (Paris) and more mature aerosol at a high altitude site in the Swiss Alps at the Jungfraujoch.

The urban study illustrated that aerosol particles are quite diverse in this environment, which affects how readily they would form cloud droplets. Black carbon is known to be a poor candidate for making a cloud droplet, which the study showed. However, the results also illustrated that adding some other chemical components to the mix can vastly increase the likelihood of the particle joining the cloud droplet gang. This is important as the removal of black carbon from the atmosphere is poorly understood and can have significant implications when trying to predict its climate impact.

At the high altitude site at ‘the top of Europe’, the aerosol properties are more uniform. This makes it somewhat easier to predict how many particles will form a cloud droplet. This is an important result for models of aerosol impacts, as such a situation is more reflective of the scales that atmospheric models work in, particularly for climate change studies. This result is not true everywhere though, so as aerosol scientists we need to work towards understanding the differences across the globe, so that we can understand the ultimate fate of aerosol particles.

Image of the Swiss Alps during a research flight on the FAAM BAe-146 research aircraft. Photo credit: Will Morgan

That concludes this diary in the life of an aerosol particle; they have a hard and complex life, which often lasts just a few days or maybe weeks.

I’ll be back with more later this week.

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Edit 03/05/14: Urs Baltensperger was originally spelt incorrectly.

Aerosols and the pause

Will Morgan February 28, 2014

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.

Fires in South East Asia

Will Morgan February 25, 2014

Smoke from a number of agricultural fires is currently blanketing Thailand and Cambodia. This is shown below in the satellite image from the MODIS instrument on the TERRA satellite. The red dots are classed as ‘thermal anomalies’ by the satellite instrument and are usually indicative of fires burning in these locations.

The majority of the fires are occurring in grass and cropland areas, which are the bale brown portions of the land surface in the image. This is indicative of agricultural burning, where farmers clear land and use the fires to recycle nutrients ahead of the growing season. In South-East Asia, the fire season usually runs from January to April/May.

Image of fires in South East Asia and the associated smoke haze from 24th February 2014 from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on the TERRA satellite. Image courtesy of the NASA Earth Observatory. Click on the image for a larger view.

While the burning is beneficial to farmers, it isn’t too good for air quality in the region. The smoke generated by the fires has built up over Cambodia and Thailand (contrast the clearness of the image in the top-left over Myanmar with the haze to the east). The MODIS instrument on the satellite can also measure the amount of pollution in the atmosphere. This is known as the aerosol optical depth, which is well above 0.5 across the region. For context, a fairly polluted day in North-Western Europe might seen an aerosol optical depth of around 0.2.

This build up of pollution is harmful to health and can also cool or warm the atmosphere depending on the properties of the smoke. The side affects of such changes are uncertain but they could for instance alter atmospheric circulation patterns and rainfall.

Fires are a frequent occurrence across the globe and their impact can have long lasting consequences on our health and climate.