It's an age-old question. Over the past decade, researchers have found that a significantly large portion of nutrient getting to Lake Tahoe is falling from the sky. Their ability to distinguish between sources has been limited by the small amount of funding available for that kind of research. Still, evidence suggests that a substantial fraction of airborne nutrients does come from out of the basin. Their objective was to determine how much atmospheric deposition of nitrogenous nutrients occurs during the dry season.
Ammonia is mostly from fertilizers and partly from auto exhausts.
Gertler explained that ammonia in the Tahoe Basin was most likely to be from local sources. Ammonium nitrate, on the other hand, takes a while to form in the atmosphere and is apt to be from a distant source. Nitric acid and ozone are produced by sunlight acting on auto exhaust pollutants and are also formed downwind of the pollution source. However, when Tahoe Basin air is stagnant, auto exhaust hangs around, baking in the sun and producing secondary pollutants, such as nitric acid. DRI researchers found evidence that some nutrients do indeed come from the west.