Impact of isolated atmospheric aging processes on the cloud condensation nuclei activation of soot particles

Abstract

Abstract. 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 into the atmosphere. Probably the largest obstacle for the investigation of these processes in the laboratory is the long atmospheric lifetime of 1 week, requiring tailored experiments that cover this time span. This work presents results on the ability of two types of soot, obtained using a miniCAST soot generator, to act as cloud condensation nuclei (CCN) after exposure to atmospherically relevant levels of ozone (O3) 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. Particles of 100 nm diameter and 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 O3. While this process was not affected by different levels of relative humidity in the range of 5 %–75 %, a high sensitivity towards the ambient/reaction temperature was observed. Soot particles with a lower OC content required an approximately 4-fold longer aging duration to show CCN activity at 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 m3). Implementation of our measurement results in a global aerosol-climate model, ECHAM6.3-HAM2.3, showed a statistically significant increase in the regional and global CCN burden and cloud droplet number concentration.

Ozone Concentration versus Temperature: Atmospheric Aging of Soot Particles

“…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 24 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). 56 The PAD can be obtained from the residence time distribution (RTD) 57 taking particle losses into account (see the work of Friebel et al 24 for more details).…”

“…The implementation of aging processes requires a framework that describes the conversion of soot particles into CCN. Friebel et al 24 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.…”

Influence of the dry aerosol particle size distribution and morphology on the cloud condensation nuclei activation. An experimental and theoretical investigation

“…Such large uncertainties result from the combination of several difficult-to-predict behaviors of the soot particles in the atmosphere when compared to mineral and biogenic aerosols. For instance, their small size and low density enable a long lifetime in the atmosphere, which can reach several weeks (Govardhan et al, 2017). Their complex morphology and large specific surface allow many possible surface interactions that can deeply affect their reactivity (Monge et al, 2010;Browne et al, 2015).…”

Assessing the potential for simplification in global climate model cloud microphysics

“…-The model version of Neubauer et al (2019) contains a mistake in the calculation of the hygroscopicity parameter in the aerosol activation parameterization, leading to an underestimation of the individual aerosol mode solubility. The calculation was updated in Friebel et al (2019) and subsequently used in Lohmann et al (2020) and this correction is also applied here.…”