To firm up these figures, a team from UC San Diego, US, has measured the size-resolved mixing state, optical properties and ageing timescales for soot particles in Riverside, California, and Mexico City. They carried out the measurements in situ using aerosol time-of-flight mass spectrometry.

The researchers found that soot particles became completely coated with non-absorbing species, such as sulphate, ammonium, organics, nitrate and water within three hours of sunrise, in a process known as ageing. The particles also became spherical. The presence of a non-absorbent coating actually tends to increase the absorption efficiency of the particles, increasing their warming effect.

"Modeling studies typically assume that it takes much longer for this process to occur – a day or so," researcher Ryan Moffet, who is now at Lawrence Berkeley National Laboratory, US, told environmentalresearchweb. "Our findings highlight how quickly aerosol properties change in the real atmosphere. These changes will strongly affect their radiative influence."

According to Moffet, the condensation of "water loving" material on a soot particle will increase its chances of being removed by rain, decreasing the amount of time it spends in the atmosphere and reducing its radiative impact.

"Incorporating these measurements into model simulations is essential to unraveling the effect the particles will have on climate," he added. "We seek to answer the question: does the coating really have an effect, or is this effect counterbalanced by their shorter atmospheric lifetime?"

Moffet and colleague Kimberly Prather found that fresh non-spherical soot particles dominated the measured absorption during peak traffic periods (6.00–9.00 a.m.). Ageing of the particles enhanced their absorption by a maximum of 1.6 times that of fresh soot. Soot particles in Riverside had thicker coatings and a larger absorption enhancement than in Mexico City, where meteorological conditions tend to flush out the basin each day.

"We chose to perform these single particle measurements because they allowed us to single out the soot particles from a very complex population of different particles," he said. "Not only did we see the particles change chemically, but optically as well. The optical measurements allowed us to 'see' the particle shape change from non-spherical to spherical. This is tough to do for microscopic objects."

The researchers reported their work in PNAS.