Driving from the slopes of Jerusalem toward low-lying Tel Aviv, there you see itit: a dirty haze hovering over the city.

This halo, found above many megacities, is composed of particles emitted by cars and industry, sometimes combined with natural particles like desert dust.

It's easy to spot from afar. But ground stations charged with monitoring urban pollution are located too low to measure this cloud of pollution to its full extent.

“What anyone in a megacity feels is the thick atmospheric layer several hundred meters above the surface," says Prof. Pinhas Alpert, a professor of Atmospheric Sciences at Tel Aviv University, and head of the Porter School of Environmental Studies. It is crucial to measure the particle concentration hundreds of meters overhead, adds Alpert, in order to monitor its effects on residents' health and to aid climate researchers mapping global warming.

Alpert, along with graduate student Olga Shvainshteinand Dr. Pavel Kishcha, thus turned to satellites.

Instead of relying on ground stations, the TAU team collected data from three of NASA's most advanced aerosol-monitoring satellites launched at the beginning of the millennium. Each satellite – Terra, Aqua and MISR – has advantages and shortcomings for measuring aerosols (which are solid particles suspended in gas, such as pollution particles and dust) But combined, the data from the three satellites offer an effective survey of pollution trends in megacities -- urban centers with a population larger than 2 million people – from 2002 to 2010.

The results of the extensive Israeli study – measuring the concentration of aerosols in the 189 largest metropolitan centers around the world – were recently published in the American Journal of Climate Change.

The researchers discovered that the most dramatic rise in aerosol concentration was in cities located in Northeast China, India, the Middle East and Central Africa. Bangalore, India, with a population of almost 8 million people in 2010, experienced an increase of 34 percent in aerosol concentration. Significant increases were also observed in Ibadan, Nigeria, and Hyderabad, India.

Surprisingly, Portland made it to the top of the list of aerosol increases, with an average rise of 53 percent over 8 years, and Seattle was in third place with an increase of 32 percent. Alpert says that these dramatic increases can be explained by natural dust aerosols entering the region from Asia. Further research will be needed to distinguish natural aerosol fluctuations from those caused by manmade pollutants, he explains.

Most decreases in aerosol concentration were observed in the Northeast USA and in Europe. Houston, Texas, showed the greatest decrease  -- 52 percent. Next was the Brazilian city, Curitiba, with a 26 percent dip. In the third and fourth place were Stockholm and Hamburg with a 23 percent and 20 percent decrease in aerosols, respectively. Another Brazilian city, Sao Paulo, also showed a marked decrease – 15 percent.

New York also dipped by 13 percent.

And that urban halo you can see above Tel Aviv? The team that works somewhere beneath this cloud found that it's gotten a bit denser–with a 5.7 percent increase in particles during the years they monitored it. That being said, the different satellites offered somewhat differing general trends for the city.

Alpert notes that the satellites cannot effectively differentiate between aerosol coming from pollution and from naturally-occurring particles, such as desert dust, volcanic particles and even pollen. The assumption is that natural aerosol concentration is steady, and so most of the detected trends in the concentration of aerosol comes from pollutants. But due to the inability to make fine observations on this matter, the present study does not show an absolute ranking of pollution in cities, and focuses on increases and decreases in concentration over the past decade.

 "What's important here is that we have three different impartial judges that can track pollution reduction trends in countries bound to UN treaties," says Alpert. “If some politician comes and claims that he reduces pollutions, we don't need to argue, we just look at the data."