Particle Size and Mixing State of Freshly Emitted Black Carbon from Different Combustion Sources in China

Modeling studies have highlighted that accurate simulations of radiative effect of black carbon (BC) require knowledge about the particle size and mixing state of freshly emitted BC from combustion sources. However, the information is absent in China due to lack of available measurements. In this study, we present the particle size and mixing state of fresh BC emitted from diesel vehicles (DV), brick kilns (BK), residential crop residue burnings (CR), and residential firewood burnings (FW) in September-October 2014 at North China Plain by field measurement. The mass median diameters of BC cores (whole particles including cores and coatings) above the limit of measurement (i.e., > 70 nm) from these sources are ∼155 (∼194), ∼230 (∼306), ∼250 (∼438) and ∼273 (∼426) nm, respectively, and corresponding size ratios (i.e., mixing state) are ∼1.25, ∼1.33, ∼1.75, and ∼1.56, respectively. Compared with the values commonly used in model based on the laboratory experiments and the field measurements in developed countries, larger particle sizes and higher mixing sate of freshly emitted BC in China may enhance their light absorption and cloud condensation nuclei activities during atmospheric transport. The available data could be used to improve future model development on radiative effect of BC in China.

SSH-Aerosol v1.1: A Modular Box Model to Simulate the Evolution of Primary and Secondary Aerosols

Particles are emitted by different sources and are also formed in the atmosphere. Despite the large impact of atmospheric particles on health and climate, large uncertainties remain concerning their representation in models. To reduce these uncertainties as much as possible, a representation of the main processes involved in aerosol dynamics and chemistry is necessary. For that purpose, SSH-aerosol was developed to represent the evolution of the mass and number concentrations of primary and secondary particles, across different scales, using state-of-the-art modules, taking into account processes that are usually not considered in air-quality or climate modelling. For example, the particle mixing state and the growth of ultra-fine particles are taken into account in the aerosol dynamics, the affinity of semi-volatile organic compounds with water and viscosity are taken into account in the partitioning between the gas and particle phases of organics and the formation of extremely low-volatility organic compounds from biogenic precursors is represented. SSH-aerosol is modular and can be used with different levels of complexity. It may be used as standalone to analyse chamber measurements. It is also designed to be easily coupled to 3D models, adapting the level of complexity to the spatial scale studied.

Simplification of Carbon Bond Mechanism IV (CBM-IV) under Different Initial Conditions by Using Concentration Sensitivity Analysis

Carbon Bond Mechanism IV (CBM-IV) is a widely used reaction mechanism in which VOCs are grouped according to the molecular structure. In the present study, we applied a sensitivity analysis on the CBM-IV mechanism to clarify the importance of each reaction under two different initial conditions (urban and low-NO_x scenarios). The reactions that exert minor influence on the reaction system are then screened out from the mechanism, so that a reduced version of the CBM-IV mechanism under specific initial conditions can be obtained. We found that in a typical urban condition, 11 reactions can be removed from the original CBM-IV mechanism, and the deviation is less than 5% between the results using the original CBM-IV mechanism and the reduced mechanism. Moreover, in a low-NO_x initial condition, two more reactions, both of which are nitrogen-associated reactions, can be screened out from the reaction mechanism, while the accuracy of the simulation is still maintained. It is estimated that the reduction of the CBM-IV mechanism can save 11–14% of the computing time in the calculation of the chemistry in a box model simulation.

A Review of the Representation of Aerosol Mixing State in Atmospheric Models

Aerosol mixing state significantly affects concentrations of cloud condensation nuclei (CCN), wet removal rates, thermodynamic properties, heterogeneous chemistry, and aerosol optical properties, with implications for human health and climate. Over the last two decades, significant research effort has gone into finding computationally-efficient methods for representing the most important aspects of aerosol mixing state in air pollution, weather prediction, and climate models. In this review, we summarize the interactions between mixing-state and aerosol hygroscopicity, optical properties, equilibrium thermodynamics and heterogeneous chemistry. We focus on the effects of simplified assumptions of aerosol mixing state on CCN concentrations, wet deposition, and aerosol absorption. We also summarize previous approaches for representing aerosol mixing state in atmospheric models, and we make recommendations regarding the representation of aerosol mixing state in future modelling studies.