Trace ammonia in laboratory air reacts easily with sulfuric acid aerosol samples to form crystalline ammonium sulfate. Using argon atmospheres, however, we have succeeded in protecting sampling surfaces from ammonia contamination. We find that atmospheric aerosols treated in this way contain only sulfuric acid. After an hour exposed to laboratory air, these same samples convert to ammonium sulfate. We are continuing to collect aerosol particles, using argon control, to determine if the absence of crystalline sulfate is common. But so far there is no evidence that aerosols are being neutralized by ammonia in the stratosphere.