It is a scientific term (usually). If you take something (in this case, the air that is inhaled by the animal with the good nose) and divide that thing by any number (in this case 1 trillion) you can categorize the contents of that thing. Here is a small example:. you have 100 parts 20 parts is oxygen 79 parts is nitrogen 1 part is smell of food you can then say, the animal can smell 1 part per hundred. So in your example, if you divide the air by 1,000,000,000,000 parts - the animal can detect something that is only a few parts inside that 1 trillion. sorry - that's the best I can do :)

"Go down to" can refer to the low end of a range of measurements. For example, "in second gear, this car's recommended speed range goes up to 70 kph and down to 15 kph." Here, it means "this animal's nose is so sensitive that its sensitivity range goes down to concentrations of a few parts per trillion." For comparison, dogs can detect some odors in concentrations of a few parts per trillion, while humans require parts per billion. The sensitivity of a dog's nose goes down to parts per trillion; ours only goes down to parts per billion. The dog's nose is more than a thousand times more sensitive than ours is.

i heard it from bbc learning language .. i suppose it means around ppt or part per trillion ..is a factor in chemistry