When I first came to BA, in 2007, it was stinkier. Most of the cabs were still Peugot 504 diesels, and the older colectivos put out huge clouds of black smoke too. But by 2010, the vast majority of taxis were CNG- mainly because the government has subsidized it, and the gas companies built CNG only gas stations around the city. That helped a lot. So the taxistas switched, to save money.
And, if you look at colectivos today, most have a little sticker by the fuel filler door that says Euro 5. Still, admittedly, a bit behind Europe itself, which is now mandating Euro 6 diesel, but a lot less polluting than the stuff they used to use.
My experience has been the biggest pollutant in Buenos Aires air-wise is pollen from all the street trees. I take Claritin.