When the humidity in the atmosphere is relatively high, the particles naturally present, also known as aerosol droplets, grow to play a significant role in the chemistry and climate of Earth. These particles are produced from either clean or polluted air, after emissions of gases that nucleate and condense while in the atmosphere. Many times this process is affected by the presence of organic (2-oxocarboxylic) acids that have been characterized to be present over polluted cities.
Click here for original story, Organic 2-oxocarboxylic acids provide link to formation of radicals and reactive oxygen species in atmospheric particles
Source: Phys.org