To establish the direct climatic and environmental effect of anthropogenic aerosols in East Asia in winter under external, internal, and partial internal mixing (EM, IM and PIM) states, a well-developed regional climate-chemical model RegCCMS is used by carrying out sensitive numerical simulations. Different aerosol mixing states yield different aerosol optical and radiative properties. The regional averaged EM aerosol single scattering albedo is approximately 1.4 times that of IM. The average aerosol effective radiative forcing in the atmosphere ranges from -0.35 to +1.40 W/m(2) with increasing internal mixed aerosols. Due to the absorption of black carbon aerosol, lower air temperatures are increased, which likely weakens the EAWM circulations and makes the atmospheric boundary more stable. Consequently, substantial accumulations of aerosols further appear in most regions of China. This type of interaction will be intensified when more aerosols are internally mixed. Overall, the aerosol mixing states may be important for regional air pollution and climate change assessments. The different aerosol mixing states in East Asia in winter will result in a variation from 0.04 to 0.11 K for the averaged lower air temperature anomaly and from approximately 0.45 to 2.98 mu g/m(3) for the aerosol loading anomaly, respectively, due to the different mixing aerosols.

Black carbon (BC) absorbs solar radiation, leading to a strong but uncertain warming effect on climate. A key challenge in modeling and quantifying BC's radiative effect on climate is predicting enhancements in light absorption that result from internal mixing between BC and other aerosol components. Modeling and laboratory studies show that BC, when mixed with other aerosol components, absorbs more strongly than pure, uncoated BC; however, some ambient observations suggest more variable and weaker absorption enhancement. We show that the lower-than-expected enhancements in ambient measurements result from a combination of two factors. First, the often used spherical, concentric core-shell approximation generally overestimates the absorption by BC. Second, and more importantly, inadequate consideration of heterogeneity in particle-to-particle composition engenders substantial overestimation in absorption by the total particle population, with greater heterogeneity associated with larger model-measurement differences. We show that accounting for these two effects-variability in per-particle composition and deviations from the core-shell approximation-reconciles absorption enhancement predictions with laboratory and field observations and resolves the apparent discrepancy. Furthermore, our consistent model framework provides a path forward for improving predictions of BC's radiative effect on climate.

Recent observational studies suggest that nucleation-scavenging is the principal path to removing black carbon-containing aerosol from the atmosphere, thus affecting black carbon's lifetime and radiative forcing. Modeling the process of nucleation-scavenging is challenging, since black carbon (BC) forms complex internal mixtures with other aerosol species. Here, we examined the impacts of black carbon mixing state on nucleation scavenging using the particle-resolved aerosol model PartMC-MOSAIC. This modeling approach has the unique advantage that complex aerosol mixing states can be represented on a per-particle level. For a scenario library that comprised hundreds of diverse aerosol populations, we quantified nucleation-scavenged BC mass fractions. Consistent with measurements, these vary widely, depending on the amount of BC, the amount of coating and coating material, as well as the environmental supersaturation. We quantified the error in the nucleation-scavenged black carbon mass fraction introduced when assuming an internally mixed distribution, and determined its bounds depending on environmental supersaturation and on the aerosol mixing state index chi. For a given chi value, the error decreased at higher supersaturations. For more externally mixed populations (chi 75%), the error was below 100% for the range of supersaturations (from 0.02% to 1%) investigated here. Accounting for black carbon mixing state and knowledge of the supersaturation of the environment are crucial when determining the amount of black carbon that can be incorporated into clouds.

Seasonal variations in mixing states of aerosols over an urban (Kanpur) and a rural location (Gandhi College) in the Indo-Gangetic Plain (IGP) are determined using the measured and modelled optical properties, and the impact of aerosol mixing state on radiative forcing is examined. Different fractions of black carbon (BC) and water-soluble aerosols in core-shell mixing emerged as the probable mixing state during winter, monsoon and post-monsoon over Kanpur. The degree of mixing, i.e. the percentage mass fraction of aerosols involved in core-shell mixing, is found to exhibit seasonal variations. Owing to the abundance of mineral dust (MD) during the pre-monsoon, MD coated by BC emerges as the most probable mixing state. Top-of-atmosphere (TOA) forcing changes its sign from positive for external mixing to negative for different probable mixing states during the pre-monsoon over both locations, as single scattering albedo is lower for external mixing. However, for other seasons, the TOA forcing is negative for external and different probable core-shell mixing states of aerosols. Surface aerosol forcing for probable mixing state during the post-monsoon is higher (44 W m2) over Kanpur, and is lower (24 W m2) over Gandhi College. A regression between instantaneous model-derived aerosol forcing and AERONET-measured forcing yielded r2 > 0.9, which confirms the robustness of the methodology adopted to retrieve aerosol optical properties and estimate forcing. Heating rates over Kanpur and Gandhi College during the pre-monsoon and monsoon are approximate to 0.75 K d1 and approximate to 0.5 K d1 respectively. Differences exist between measured and model-derived asymmetry parameter, g, owing to the non-sphericity of aerosols. However, aerosol radiative forcing is found to be weakly sensitive to the variation in g due to high (> 0.2) surface albedo. The modelling study provides new insights into the state of aerosol mixing, and indicates that aerosol mixing can vary depending on the type and abundance of aerosol species. Copyright (c) 2012 Royal Meteorological Society