Impacts of black carbon mixing state on black carbon nucleation scavenging: Insights from a particle‐resolved model

Abstract

[1] This paper presents an advancement of the recently developed particle-resolved aerosol model PartMC-MOSAIC (Particle Monte Carlo-Model for Simulating Aerosol Interactions and Chemistry) to investigate the impacts of mixing state on cloud droplet formation and to provide a tool for the quantification of errors in cloud properties introduced by simplifying mixing state assumptions. We coupled PartMC-MOSAIC with a cloud parcel model. We initialized the cloud parcel simulation with hourly PartMC-MOSAIC model output from a 48-hour urban plume simulation. The cloud parcel model then explicitly simulated activation and condensational growth of the particles as the parcel underwent cooling at a specified rate and the particles of the aerosol population competed for water vapor. We used this capability to quantify the relative importance of size information versus composition information for the prediction of the cloud droplet number fraction, mass fraction of black carbon that is nucleation-scavenged, cloud droplet effective radius, and relative dispersion of the droplet size distribution by introducing averaging of particle-resolved information within prescribed bins. For the cloud droplet number fraction, both composition averaging and size-bin averaging individually led to an error of less than 25% for all cloud parcel simulations, while averaging in both size bins and composition resulted in errors of up to 34% for the base case cooling rate of 0.5 K/min. In contrast, for the nucleation-scavenged black carbon mass fraction, the results for size-bin averaging tracked the reference case well, while composition averaging, with or without size-bin averaging, led to overestimation of this quantity by up to 600%.Citation: Ching, J., N. Riemer, and M. West (2012), Impacts of black carbon mixing state on black carbon nucleation scavenging: Insights from a particle-resolved model,

Metrics to quantify the importance of mixing state for CCN activity

“…In contrast, closure studies show that, in areas close to emission sources, a certain degree of external mixing needs to be assumed to obtain good closure. This is confirmed by modeling studies showing that, without introducing fresh emissions, aging processes transform the mixing state of aerosol populations so that the CCN properties can be deduced from the bulk aerosol composition, and mixing state details become negligible Ching et al, 2012;Fierce et al, 2013). However, the timescale for this transformation depends on the local conditions, such as total number concentration of existing CCN and the amount of condensable aerosol material (Fierce et al, 2015).…”

“…To construct the scenario library, the BC emission rate was set to 100 % (E100), 25 % (E25), and 2.5 % (E2.5) of the base case, and the number concentration of background aerosol particles was set to 100 % (B100) and 10 % (B10) of the base case. Combining the emission cases and the background aerosol concentration cases results in six scenarios, and the gas-phase emissions for these six scenarios were the same as in Ching et al (2012). For the base case (E100-B100) we performed two additional simulations by reducing the emission rate of all gaseous components by 50 % (G50) and 25 % (G25) of the case presented in Ching et al (2012).…”

“…For example, Ching et al (2012) quantified the impact of aerosol mixing state on cloud droplet formation, and Fierce et al (2013) investigated the sensitivity of CCN activity to mixing state characteristics at emission. The model was also used to explain the observed diurnal variations in aerosol hygroscopicity on the North China Plain (Liu et al, 2011) and to characterize the evolution of aerosol mixing state in a ship plume (Tian et al, 2014).…”

“…The eight scenarios are derived from the base case, which is the scenario presented in Ching et al (2012). To construct the scenario library, the BC emission rate was set to 100 % (E100), 25 % (E25), and 2.5 % (E2.5) of the base case, and the number concentration of background aerosol particles was set to 100 % (B100) and 10 % (B10) of the base case.…”

“…Previous work has approached this question from a Lagrangian point of view, considering the aging history of the particle population Ching et al, 2012Ching et al, , 2016a as a plume of aerosol particles is evolving. While this approach yields valuable process-level understanding, it is difficult to apply to field observations because following a particle population in the real atmosphere is inherently challenging.…”