Pyruvic acid, an efficient catalyst in SO3 hydrolysis and effective clustering agent in sulfuric-acid-based new particle formation

The role of pyruvic acid (PA), one of the most abundant alpha-keto carboxylic acids in the atmosphere, was investigated both in the SO3 hydrolysis reaction to form sulfuric acid (SA) and in SA-based aerosol particle formation using quantum chemical calculations and a cluster dynamics model. We found that the PA-catalyzed SO3 hydrolysis is a thermodynamically driven transformation process, proceeding with a negative Gibbs free-energy barrier, ca. 1 kcal mol 1 at 298 K, similar to 6.50 kcal mol(-1) lower than that in the water-catalyzed SO3 hydrolysis. Results indicated that the PA-catalyzed reaction can potentially compete with the water-catalyzed SO3 reaction in SA production, especially in dry and polluted areas, where it is found to be similar to 2 orders of magnitude more efficient that the water-catalyzed reaction. Given the effective stabilization of the PA-catalyzed SO3 hydrolysis product as SA center dot 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 relevant tropospheric temperatures and concentrations of SA (106 molec : cm(3)), PA (1010 molec : cm 3) and NH3 (10(11) and 5 similar to 10(11) 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)center dot PA center dot(NH3)(2) cluster was found to contribute by similar to 100% to the net flux to aerosol particle formation. At higher temperatures (258 and 278 K), however, the net flux to the particle formation is dominated by pure SA-NH3 clusters, while PA would rather evaporate from the clusters at high temperatures and not contribute to the particle formation. The enhancing effect of PA was examined by evaluating the ratio of the ternary SA-PA-NH3 cluster formation rate to binary SA-NH3 cluster formation rate. Our results show that while the enhancement factor of PA to the particle formation rate is almost insensitive to investigated temperatures and concentrations, it can be as high as 4.7 similar to 102 at 238K and [NH3] = 1.3 similar to 10(11) molec : cm(3). This indicates that PA may actively participate in aerosol formation, only in cold regions of the troposphere and highly NH3-polluted environments. The inclusion of this mechanism in aerosol models may reduce uncertainties that prevail in modeling the aerosol impact on climate.

Automated summary

Using quantum chemical calculations and a cluster dynamics model, the role of pyruvic acid (PA) in the formation of sulfuric acid (SA) and aerosol particles was investigated. It was found that PA can potentially compete with water-catalyzed reaction in SA production and enhance particle formation in dry and polluted areas. The inclusion of this mechanism in aerosol models may reduce uncertainties in modeling the aerosol impact on climate.