Reply on RC1

Abstract

The role of pyruvic acid (PA), one of the most abundant α-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, ~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 ~two orders of magnitude more efficient that the water-catalyzed reaction. Given the effective stabilization of the PA-catalyzed SO3 hydrolysis product as SA•PA cluster, we proceeded to examine the PA clustering efficiency in 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 cm3), PA (1010 cm3) and NH3 (1011 and 5 × 1011 cm3), of the PA-containing clusters, only clusters with one PA molecule, namely (SA)2•PA•(NH3)2, can participate to the particle formation, contributing by ~100 % to the net flux to aerosol particle formation at 238 K, exclusively. At higher temperatures (258 K 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 of 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 × 102 at 238 K and [NH3] = 1.3 × 1011 molecule 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 definitely reduce uncertainties that prevail in modeling the aerosol impact on climate.