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Abstract. The role of pyruvic acid (PA), one of the most abundant α-keto carboxylic acids in the atmosphere, was investigated both in the SO3hydrolysis reaction to form sulfuric acid (SA) and in SA-based aerosol particle formation using quantum chemical calculations and a cluster dynamicsmodel. We found that the PA-catalyzed SO3 hydrolysis is a thermodynamically driven transformation process, proceeding with a negativeGibbs free-energy barrier, ca. −1 kcal mol−1 at 298 K, ∼ 6.50 kcal mol−1 lower than that in thewater-catalyzed SO3 hydrolysis. Results indicated that the PA-catalyzed reaction can potentially compete with the water-catalyzedSO3 reaction in SA production, especially in dry and polluted areas, where it is found to be ∼ 2 orders of magnitude moreefficient that the water-catalyzed reaction. Given the effective stabilization of the PA-catalyzed SO3 hydrolysis product asSA⚫PA cluster, we proceeded to examine the PA clustering efficiency in a sulfuric-acid–pyruvic-acid–ammonia (SA-PA-NH3) system. Our thermodynamic data used in the Atmospheric Cluster Dynamics Code indicated that under relevanttropospheric temperatures and concentrations of SA (106 molec.cm-3), PA (1010 molec.cm-3) and NH3 (1011 and5 × 1011 molec.cm-3), PA-enhanced particle formation involves clusters containing at most one PA molecule. Namely, under these monomer concentrations and 238 K, the(SA)2⚫PA⚫(NH3)2 cluster was found to contribute by ∼ 100 % to the net flux to aerosol particle formation. At higher temperatures (258 and 278 K), however, the net flux to the particleformation is dominated by pure SA-NH3 clusters, while PA would rather evaporate from the clusters at high temperatures and notcontribute to the particle formation. The enhancing effect of PA was examined by evaluating the ratio of the ternarySA-PA-NH3 cluster formation rate to binary SA-NH3 cluster formation rate. Our results show that while theenhancement factor of PA to the particle formation rate is almost insensitive to investigated temperatures and concentrations, it can be as high as4.7 × 102 at 238 K and [NH3] = 1.3 × 1011 molec.cm-3. This indicates that PA mayactively participate in aerosol formation, only in cold regions of the troposphere and highly NH3-polluted environments. The inclusion ofthis mechanism in aerosol models may reduce uncertainties that prevail in modeling the aerosol impact on climate.

atmospheric chemistry and physics

Pyruvic acid, an efficient catalyst in SO&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; hydrolysis and effective clustering agent in sulfuric-acid-based new particle formation