Abstract
Abstract. A modal aerosol module (MAM) has been developed for the Community Atmosphere Model version 5 (CAM5), the atmospheric component of the Community Earth System Model version 1 (CESM1). MAM is capable of simulating the aerosol size distribution and both internal and external mixing between aerosol components, treating numerous complicated aerosol processes and aerosol physical, chemical and optical properties in a physically-based manner. Two MAM versions were developed: a more complete version with seven lognormal modes (MAM7), and a version with three lognormal modes (MAM3) for the purpose of long-term (decades to centuries) simulations. In this paper a description and evaluation of the aerosol module and its two representations are provided. Sensitivity of the aerosol lifecycle to simplifications in the representation of aerosol is discussed. Simulated sulfate and secondary organic aerosol (SOA) mass concentrations are remarkably similar between MAM3 and MAM7. Differences in primary organic matter (POM) and black carbon (BC) concentrations between MAM3 and MAM7 are also small (mostly within 10%). The mineral dust global burden differs by 10% and sea salt burden by 30–40% between MAM3 and MAM7, mainly due to the different size ranges for dust and sea salt modes and different standard deviations of the log-normal size distribution for sea salt modes between MAM3 and MAM7. The model is able to qualitatively capture the observed geographical and temporal variations of aerosol mass and number concentrations, size distributions, and aerosol optical properties. However, there are noticeable biases; e.g., simulated BC concentrations are significantly lower than measurements in the Arctic. There is a low bias in modeled aerosol optical depth on the global scale, especially in the developing countries. These biases in aerosol simulations clearly indicate the need for improvements of aerosol processes (e.g., emission fluxes of anthropogenic aerosols and precursor gases in developing countries, boundary layer nucleation) and properties (e.g., primary aerosol emission size, POM hygroscopicity). In addition, the critical role of cloud properties (e.g., liquid water content, cloud fraction) responsible for the wet scavenging of aerosol is highlighted.
Highlights
Atmospheric aerosol is recognized as one of the most important forcing agents in the climate system (Forster et al, 2007)
The more comprehensive one (MAM7) has 7 log-normal modes and explicitly treats the aging of primary organic matter (POM) and BC from the primary carbon mode into which they are emitted to the accumulation mode where they are mixed with other aerosol species
Other approximations in MAM3 include merging of the MAM7 fine dust and fine sea salt modes into the accumulation mode in MAM3, and merging of the MAM7 coarse dust and coarse sea salt modes into the single coarse mode in MAM3, which is made feasible by the separate geographical sources of sea salt and mineral dust
Summary
Atmospheric aerosol is recognized as one of the most important forcing agents in the climate system (Forster et al, 2007). Aerosol particles span a spectrum of size ranges (10−3 to 101 μm), multiple chemical species (e.g., sulfate, black carbon (BC), organic matter (OM), mineral dust and sea salt), and change through complicated physical and chemical aging in the atmosphere. This diversity and complexity imposes a great challenge to representing aerosol processes and properties in GCMs. There are several methods of aerosol treatments in GCMs. The bulk method only predicts mass mixing ratio of various aerosol species and prescribes fixed aerosol size distributions in order to convert aerosol mass to number mixing ratio.
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