Impact of Isolated Atmospheric Aging processes on the Cloud Condensation Nuclei-activation of Soot Particles

Abstract

<p><strong>Abstract.</strong> The largest contributors to the uncertainty in assessing the anthropogenic contribution in radiative forcing are the direct and indirect effects of aerosol particles on the Earth's radiative budget. Soot particles are of special interest since their properties can change significantly due to aging processes once they are emitted to the atmosphere. Probably the largest obstacle for the investigation of these processes in the laboratory is the long atmospheric lifetime of one week, demanding tailored experiments that cover this time span. This work presents results on the ability of two types of soot to act as cloud condensation nuclei (CCN) after exposure to atmospherically relevant levels of ozone and humidity. Aging times of up to 12 h were achieved by successful application of the continuous-flow stirred tank reactor (CSTR) concept while allowing for size-selection of particles prior to the aging step. 100 nm particles rich in organic carbon (OC) that were initially CCN-inactive showed significant CCN-activity at supersaturations (SS) down to 0.3 % after 10 h of exposure to 200 ppb of ozone. While this process was not affected by different levels of relative humidity in the range 5–75 %, a high sensitivity towards the ambient/reaction temperature was observed. Soot particles with a lower OC-content demanded an approximately four-fold longer aging duration to show CCN-activity for the same SS. Prior to the slow change in the CCN-activity, a rapid increase in the particle diameter was detected which occurred within several minutes. This study highlights the applicability of the CSTR-approach for the simulation of atmospheric aging processes, as aging durations beyond 12 h can be achieved in comparably small aerosol chamber volumes (< 3 m<sup>3</sup>). Implementation of our measurement results on the CCN-activity of soot particles retrieved from measurements at atmospherically relevant conditions into a global aerosol-climate model showed a statistically significant increase in the regional and global CCN burden and cloud droplet number concentration (CDNC).</p>

Ozone Concentration versus Temperature: Atmospheric Aging of Soot Particles

“…The experimental setup and the standard operation procedure were described in detail in the works of Friebel and Mensah and Friebel et al; therefore, it is described only briefly here. In Figure , a schematic of the setup is shown.…”

“…To characterize the fresh aerosol, all instruments could be switched to a bypass line to measure the aerosol properties upstream of the chamber. A full list of instruments deployed can be found in the work of Friebel et al In the following we focus on the CCN activity measured with a cloud condensation nuclei counter (CCNC; Droplet Measurement Technologies) and on the particle diameter retrieved from measurments with a scanning mobility particle sizer (SMPS, Model 3936, TSI Inc.).…”

“…The ratio between young, medium-aged, and old particles is defined by the particle age distribution (PAD) . The PAD can be obtained from the residence time distribution (RTD) taking particle losses into account (see the work of Friebel et al for more details).…”

“…Experimental results obtained from the work presented herein are presented separately for each experiment conducted at a specific O 3 concentration (25 ppb: circle → 200 ppb: pentagon; 100 nm; the full data set is presented in Table ). Times sign: CBK particles (100 nm, 200 ppb O 3 , 0–12 h; Friebel et al, 2019). Star: ethylene soot particles (222 nm, 20.000 ppb O 3 , 100 s; Lambe et al, 2015).…”

“…The implementation of aging processes requires a framework that describes the conversion of soot particles into CCN. Friebel et al show that oxidation of soot particles with O 3 can significantly increase the number of CCN in the atmosphere and lead to a higher cloud droplet number concentration (CDNC). However, these results are based on a first-order reaction kinetic and do not consider any influence of the temperature.…”