Nitrosyl sulfuric acid and stratospheric aerosols

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Abstract
From information found in the atmospheric and chemical literature, we propose that nitrosyl sulfuric acid (NSA), NOHSO4, may play an important role in stratospheric chemistry. In one study, NSA was observed as a slurry of crystals in about a third of the aerosol particles collected between 15 and 20 km. From the chemical literature, we find that NSA is formed from several reactions between NOy species (averaging +3 oxidation state) and sulfuric acid, if the acid is above 70 wt % at room temperature and above 60 % at stratospheric temperatures. NSA is a non‐volatile ionic solid (NO+HSO4−), melting at 346 K and boiling with decomposition at 633 K. Over time, it could accumulate as a solution in the liquid aerosol, and upon reaching solubility saturation (about 3 % under lower stratospheric conditions) precipitate out as solid crystals, which permits further accumulation. At room temperature, HCl gas reacts with NSA, either the solid or in sulfuric acid solution, to form gaseous nitrosyl chloride, ClNO, which is rapidly photolyzed in the atmosphere to form nitric oxide and atomic chlorine. The overall catalytic cycle for this process can be written in a number of ways, depending upon the NOy species responsible for NSA formation; with NSA production from HONO, the cycle isThe net reaction is thermodynamically favorable for at least three modes of NSA formation. If the reaction probability of forming NSA per collision of NO with a sulfuric acid aerosol is 10−3, 1% of the sulfuric acid would be converted to NSA within 1 day at 20 km altitude. If NSA in the sulfate aerosols is 1% of the sulfuric acid and if the second order rate constant for (HCl + NOHSO4 = H2SO4 + ClNO) in sulfuric acid solution is greater than 10−18 cm3 molec.−1 s−1 under conditions at 20 km altitude, the rate of HCl + NSA is faster than the rate of HO + HCl. In this case, this heterogeneous catalysis is expected to affect the balance of NOy, Cly, and ozone in the lower stratosphere.

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