I saw a well argued statistic several years ago which suggested that the pollution from all the vehicles in the world amounts to just 0.6% of all the rubbish spewed into the atmosphere.[Some extremely dubious guesstimates, from some extremely dodgy sources, claim a full 30% which has the appearance of extreme exaggeration. It is my belief that these figures simply did not take into account the effect of volcanos as these are erratic by nature.]
Look at this picture and that lower claim seems considerably more believable!
LINK: http://www.dailymail.co.uk/sciencetech/article-1195215/Stunning-pictures-hole-clouds-astronauts-witness-volcano-eruption-International-Space-Station.html