Megacities: Megapollution

Smog over city

26 November 2007 by James Allen

James Allan travelled to one of the world's most polluted cities to analyse atmospheric emissions.

Atmospheric scientists are increasingly turning their attention to megacities. These vast, sprawling metropolises get this moniker when their populations top 10 million. Many of them are in developing countries where pollution is rife. Often lax or non-existent emissions control lead to large volumes of pollution spewing straight into the atmosphere which can be a major headache for city authorities.

But the scale of the problem goes beyond traditional city limits; scientists have tracked pollution from individual megacities across entire regions and even around the globe. Given that it is estimated that 60 percent of the world's population will be living in cities by 2030, identifying and describing megacity emissions will give us the best idea of what to expect in the future.

While emissions from human activities are by no means unique to the largest cities in the world, the sheer quantities do mean they provide ideal locations to study and understand them better.

Of all megacities, Mexico City has historically ranked as one of the most polluted.

Of all megacities, the Mexico City metropolitan area has historically ranked as one of the most polluted, thanks in part to its location. It lies in a basin over two kilometres above sea level, which acts as an efficient pollution trap.

Vehicle and industrial emissions, combined with intense tropical sunlight, produce huge amounts of photochemical smog which can irritate eyes, damage lungs, trigger heart problems and harm plants.

In March 2006, the atmospheric conditions around the city attracted the attentions of an international group of scientists under the guise of the MILAGRO project (Megacities Initiative: Local & Global Research Observations). As part of this effort, I flew to Mexico to join researchers from the Universidad Nacional Autónoma de México and the University of California, San Diego, amongst others.

Blessed with burning biomass

The work could not have contrasted more with my previous assignment at Mace Head on the west coast of Ireland, officially the cleanest air in Western Europe. In comparison, taking measurements around Mexico City was like shooting fish in a very smoggy barrel. We were doubly blessed at one point when a large grass fire allowed us to study biomass burning, another pressing atmospheric phenomenon.

Measuring pollution around Mexico City was like shooting fish in a very smoggy barrel.

Our group deployed instruments at Paso de Cortes, a mountain pass to the south-east of the city and one of the escape routes for emissions. I measured the number, size and composition of particulates, also known as aerosols. These are tiny particles suspended in the atmosphere and which range in size from a few nanometres to tens of microns.

To perform these measurements, I used a variety of different instruments (the Aerosol Mass Spectrometer being my personal favourite), as well as the more traditional method of collecting particles on filters to send to laboratories for analysis.

Particulate matter in the atmosphere is divided into two types: primary and secondary. Primary particles are emitted directly into the atmosphere from a range of sources including wildfires, sand, dust and sea-salt spray, as well as soot from domestic burning, industry and transport.

Secondary particulates are formed when gases such as sulphur dioxide, oxides of nitrogen and volatile organic carbon species (VOCs) react in the presence of sunlight to produce sulphate, nitrate and involatile organics that condense on to existing particles, substantially increasing both their size and mass.

Aerosols are a hot topic right now because as well as harming health, they are known to be a big factor in the atmosphere's radiation balance. The mechanisms are pretty complicated. Aerosols can both absorb and scatter the sun's energy, which cause competing warming and cooling effects respectively.

Megacities populations and average suspended particulate mass concentrations in µg/m3

population: 35 million
particulates: 43

Mexico City
population: 18·7 million
particulates: 69

New York
population: 18·3 million
particulates: 23

São Paulo
population: 17·9 million
particulates: 46

population: 17·4 million
particulates: 79

population: 14·1 million
particulates: 187

population: 13·8 million
particulates: 153

Buenos Aires
population: 13 million
particulates: no data

population 12·8 million
particulates: 87

population: 12·3 million
particulates: 103

Pollution plumes

The degrees by which they do so have a very complex relationship with the size and exact composition of the particles. In urban emission plumes, we know that sooty primary particles are efficient absorbers of solar radiation, but as they accumulate more secondary mass and get larger they are able to scatter more radiation. This is a major area of study within MILAGRO.

We measured the aerosols simultaneously at several different sites and using aircraft. This allowed us to map changes in composition and optical properties as the emissions age in the atmosphere. Climate predictions need to take accurate account of these phenomena, but there are gaps in the science.

For example, the secondary material produced from VOCs can represent over half of the fine particulate matter in polluted air, but for reasons not completely understood, even the best models currently under-predict it by a large factor. This is a major area of research in atmospheric aerosol science.

Aerosols also affect climate by the way they interact with clouds, which develop when water condenses on particles. Clouds are very efficient at reflecting energy back to space, especially when a large number of droplets form. A particle that will act as a host for a cloud droplet is known as a cloud condensation nucleus (CCN). Whether it develops to a droplet depends on both its size and chemical composition. City emissions perturb cloud properties by increasing CCN numbers.

Seeds of clouds

The effect this has on climate is generally accepted to be one of overall cooling (although not enough to offset the warming of carbon dioxide), but the exact amount and extent is currently highly uncertain. To probe this, we used a CCN counter at Paso de Cortes, a device that simulates conditions similar to those found in clouds and counts how many droplets form.

It is important to study the evolution of the pollution because small, sooty particles generally make poor CCN but as they accumulate more secondary mass over time, they become more viable. The fact that the urban plume contained greatly elevated numbers of CCN was not in doubt, but in order to accurately simulate the effects of pollution in cloud models, we must be able to quantitatively predict the CCN behaviour based on the aerosol composition.

This is not a straightforward exercise, but it is something we are actively pursuing by developing detailed thermodynamic models. Validating these models against real-world data is a vital part of this development, which is why measurements such as these are so important.

The bottom line is that understanding and quantifying aerosols is a key challenge in climate science. In fact, the two biggest radiative forcing error bars in the latest UN Intergovernmental Panel on Climate Change report, published earlier this year, relate directly to aerosols.

Big uncertainties and gaps in our knowledge continue to surround these processes but researchers are addressing these with a concerted combination of laboratory experiments, theoretical work and targeted field measurements (both intensive and long-term), of which projects like MILAGRO represent an important part.

James Allan is the aerosol measurement postdoctoral research associate in the Composition Directorate of the National Centre for Atmospheric Science and is based at the University of Manchester. He is also a co-investigator in two awards (ACES, ADIENT) within the Aerosol Properties, Processes & Influences on the Earth's Climate (APPRAISE) directed programme. Since starting his PhD in 2000, he has averaged three major field projects a year, most of which have been international collaborations.