Description and performance of the CARMA sectional aerosol microphysical model in CESM2

Abstract

Abstract. We implemented the Community Aerosol and Radiation Model for Atmospheres (CARMA) in both the high and low-top model versions of the Community Earth System Model Version 2 (CESM2). CARMA is a sectional microphysical model, which we use for aerosol in both the troposphere and stratosphere. CARMA is fully coupled to chemistry, clouds, radiation, and transport routines in CESM2. This development enables the comparison of simulations with a sectional (CARMA) and a modal (MAM4) aerosol microphysical model in the same modeling framework. The new implementation of CARMA has been adopted from previous work with some additions that align with the current CESM2 MAM4 implementation. The main updates include an interactive secondary organic aerosol description in CARMA using the volatility basis set (VBS) approach, updated wet removal, and the use of transient emissions of aerosols and trace gases. In addition, we implemented an alternative aerosol nucleation scheme in CARMA, which is also used in MAM4. Detailed comparisons of stratospheric aerosol properties after the Mt Pinatubo eruption reveal the importance of prescribing sulfur injections in a larger region rather than in a single column to better represent the observed evolution of aerosols. Both CARMA and MAM4 in CESM2 are able to represent stratospheric and tropospheric aerosol properties reasonably well compared to observations. Several differences in the performance of the two aerosol models show, in general, an improved representation of aerosols using the sectional aerosol model in CESM2. These include a better representation of the aerosol size distribution after the Mt Pinatubo volcanic eruption in CARMA compared to MAM4. MAM4 produces on average smaller aerosols and less removal than CARMA, which results in a larger total mass. Both CARMA and MAM4 reproduce stratospheric Aerosol Optical Depth (AOD) within the errorbar of the observations between 2001 and 2020, except for recent larger volcanic eruptions that are overestimated by both model configurations. The CARMA background surface area density and aerosol size distribution in the stratosphere and troposphere compare well to observations, with some underestimation of the Aitken-mode size range. MAM4 shows shortcomings in reproducing coarse-mode aerosol distributions in the stratosphere and troposphere. This work outlines additional development needs for CESM2 CARMA to improve the model compared to observations in both the troposphere and stratosphere.

Global Health and Climate Effects of Organic Aerosols from Different Sources

“…59,82 The typical size ranges are 0.06− 0.30 μm for the primary carbon mode, 0.015−0.053 μm for the Aitken mode, and 0.058−0.48 μm for the accumulation mode, respectively. 61 In the CAM-chem simulations in this study, the accumulation mode comprises 93% of OA followed by 6% for the primary carbon mode and 1% for the Aitken mode, which is consistent with Tilmes et al 34 The column version of RRTMG is also used for the offline analysis in Section 3.…”

“…It is noteworthy that the nudged versions of CAM6 were found to reproduce the cloud fraction, cloud phase, and boundary layer structure in the Southern Ocean . Furthermore, CAM-chem with the MAM4 scheme showed good agreement with aircraft measurements of Aitken and accumulation mode number concentrations over the Pacific and Atlantic oceans …”

“…The organic nucleation process is not included in the model at present. The model only accounts for sulfuric acid vapor homogeneous nucleation through a binary parameterization. − The VBS scheme by Hodzic et al is used for all SOA, as implemented by Tilmes et al and further updated for NO x -dependent pathways by Jo et al SOA from S/IVOCs is also calculated on the VBS space, by assuming 60% of POA and 20% of nonmethane VOC (NMVOCs) as SVOC and IVOC emissions, respectively. In addition to OA, both models simulate secondary inorganic aerosols, black carbon, dust, and sea salt aerosols.…”

“…For size distributions of OA, a single geometric mean dry radius of 0.058 μm and a standard deviation of 1.6 are assumed in the bulk aerosol scheme in GEOS-Chem . On the other hand, in CAM-chem, the radius of each mode is calculated online from the number concentration, and a standard deviation of 1.6 is also used for Aitken, accumulation, and primary carbon modes. , The typical size ranges are 0.06–0.30 μm for the primary carbon mode, 0.015–0.053 μm for the Aitken mode, and 0.058–0.48 μm for the accumulation mode, respectively . In the CAM-chem simulations in this study, the accumulation mode comprises 93% of OA followed by 6% for the primary carbon mode and 1% for the Aitken mode, which is consistent with Tilmes et al The column version of RRTMG is also used for the offline analysis in Section .…”

Analysis of the global atmospheric background sulfur budget in a multi-model framework

“…The CARMA aerosol model includes prognostic aerosols for both the troposphere and the stratosphere, as discussed in Yu et al (2015) and additional changes highlighted in Tilmes et al (2023). Apart from pure sulfate aerosol, WACCM6-CARMA is one of two models participating in this study, which include an internally mixed group of aerosol.…”