Effect of composition variations in cloud droplet populations on aqueous‐phase chemistry

Abstract

Abstract. We prove that the use of a cloud or fog droplet population's volume weighted average pH results in the underestimation of the actual rate of sulfate production for most atmospheric conditions. To quantify the magnitude of this error, we have developed two aqueous-phase chemistry models: a droplet size-resolved model and a bulk chemistry model. The discrepancy between the results of these two models indicates the magnitude of the error introduced by using bulk aqueous-phase properties. This error depends mainly on the availability of gas-phase species (SO:, 03, H:O:, and NH3), the aerosol size/composition distribution, and the residence time of the air parcel in cloud containing air. The ratio of predicted sulfate production between the two models for the cases studied here varies from as low as unity to as high as 30. The largest ratios occur during the first few minutes of cloud formation. After this peak the difference in sulfate production rates between the two models decreases rapidly. For the scenarios simulated, the largest error introduced by the bulk modeling approach at the end of a cloud event was underprediction of the sulfate production by a factor of 2. The magnitude of the sulfate underprediction by the bulk model decreases with increasing initial levels of gas-phase NH3 and H202 and is rather insensitive to the gasphase O3 and SO2 concentrations.

The magnitude and causes of uncertainty in global model simulations of cloud condensation nuclei

“…The rate of the reaction SO 2 + O 3 → SO 4 is controlled by the pH of cloud water (Gurciullo and Pandis, 1997;Kreidenweis et al, 2003) and has been identified as an important uncertainty in the global sulfur cycle (Faloona, 2009). We assume this reaction occurs in low-level clouds (Fig.…”

Mixing state of biomass burning particles by single particle aerosol mass spectrometer in the urban area of PRD, China

“…These results suggest different formation pathways for sulfate and nitrate. Particle-phase sulfate resulted predominantly from sulfur dioxide oxidation in the cloud free atmosphere (Tyndall and Ravishankara, 1991) and, more commonly, SO 2 conversion in clouds and fogs (Gurciullo and Pandis, 1997;Zhang et al, 1999). The preferential enrichment of nitrate in inorganic ions-rich particles was evidence of a predominant gas-phase source for nitric acid and precursors, followed by partitioning and heterogeneous/aqueous chemistry.…”

Current Status of Air Quality Models for Particulate Matter

“…The microphysics of cloud and fog droplets affects the chemical composition of the droplets as a function of their size. 73,74 Consequently, it may be necessary to add some treatment of the droplet size distribution in the 3-D PM models that currently assume that all droplets have the same chemical composition.…”