The lifetime and spatial distributions of accumulation-mode aerosols in a size range of approximately 0.05-1 μm, and thus their global and regional climate impacts, are primarily constrained by their ...removal via cloud and precipitation (wet removal). However, the microphysical process that predominantly controls the removal efficiency remains unidentified because of observational difficulties. Here, we demonstrate that the activation of aerosols to cloud droplets (nucleation scavenging) predominantly controls the wet removal efficiency of accumulation-mode aerosols, using water-insoluble black carbon as an observable particle tracer during the removal process. From simultaneous ground-based observations of black carbon in air (prior to removal) and in rainwater (after removal) in Tokyo, Japan, we found that the wet removal efficiency depends strongly on particle size, and the size dependence can be explained quantitatively by the observed size-dependent cloud-nucleating ability. Furthermore, our observational method provides an estimate of the effective supersaturation of water vapour in precipitating cloud clusters, a key parameter controlling nucleation scavenging. These novel data firmly indicate the importance of quantitative numerical simulations of the nucleation scavenging process to improve the model's ability to predict the atmospheric aerosol burden and the resultant climate forcings, and enable a new validation of such simulations.
Soot particles strongly absorb solar radiation and contribute to global warming. Also, wetting properties of soot at emission can affect its lifetime. We investigated surface conditions related to ...wetting and hydrophobic properties of fresh soot using data from measurements taken in Tokyo. A cloud condensation nuclei (CCN) counter was used to clarify surface conditions of particles composed mainly of water-insoluble (WI) materials: total and active particles as CCN around critical supersaturation (Sc) of 203-nm-diameter WI particles. Averaged number fractions of inactivated particles as CCN at 1.05% supersaturation (SS), which is Sc of hydrophilic WI particles, were estimated as 1.4%. Number fractions of inactive particles changed less at 1.78%SS during rush hour and increased at 0.89%SS, implying that most of the WI particles included small amounts of water-soluble (WS) materials rather than being completely hydrophobic. Based on transmission electron microscope (TEM) analysis of samples collected during rush hour, 69% of the mostly bare soot particles had Na or K small domains that are regarded as originating in fossil fuels. Based on water dialysis analysis results, some Na and K on soot were WS. Combination results with CCN measurements suggest that these WS materials decrease the Sc of soot. Moreover, the morphological structure of sulfate covering Na and K domains on the soot surface implicates pre-existing sodium and potassium compounds on soot as a trigger of soot aging. However, inactive particles at Sc at poor-hydrophilic particles and soot particles composed solely of WI materials on TEM samples were also found, although they were minor. Such particles, which are unfavorable for obtaining a wettable surface, might retain non-hygroscopicity for a longer period in the atmosphere. Evaluation of long-range soot transport can benefit from consideration of slight and inhomogeneous differences of chemical compounds on soot that occur along with their emission.
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•Wetting and hydrophobic properties of freshly emitted soot in Tokyo were assessed.•CCN was counted around critical supersaturations for non-hygroscopic particles.•Inactive particles changes show that most soot had trace water-soluble materials.•TEM analysis revealed water-soluble Na and K on much rush-hour soot surface.•Existing trace materials on emitted soot can affect aging and lifetime of soot.
Previous studies on the aerosol indirect effect have demonstrated that there are three regimes (aerosol-limited, updraft-limited, and transitional regimes). However, this classification of regimes ...has not been widely used in field observations as it requires measurements of updraft or supersaturation near the cloud base (Smax), which are not easy to perform. In this study, we attempted to classify the regimes using the effective supersaturation (Seff) which is relatively easy to estimate by matching cloud droplet number concentration (Nc) and supersaturation spectra of cloud condensation nuclei. Parcel model simulations were performed to examine the suitability of Seff as a proxy for Smax. Further, 35 low-level cloud cases observed around 458 m over Tokyo were classified into three regimes. In the aerosol-limited regime, Nc increased more rapidly with an increment of aerosols than in the other two regimes, indicating that the regime classification using Seff was sufficiently robust. The aerosol-limited regime in Tokyo appeared frequently when northerly winds prevailed, indicating the impact of local pollution sources around Tokyo on the aerosol cloud interaction. The results imply that anthropogenic aerosol emissions affect the climate of Tokyo.
•Low-level clouds observed over Tokyo were classified into three ACI regimes.•Classification was based on the effective supersaturation relatively easy to estimate.•Twomey effect was prominent under the aerosol-limited regime.•Aerosol-limited regime appeared frequently when northerly winds prevailed.
Abstract
Quantitative simulation of an aerosol’s lifecycle by regional-scale and global-scale atmospheric models is mandatory for unbiased analysis and prediction of aerosol radiative forcing and ...climate change. Globally, aerosol deposition is dominated by the rainout process, which is mostly triggered by activation of aerosols to liquid droplets in supersaturated domains of precipitating clouds. However, the actual environmental supersaturation value that aerosols experience in precipitating clouds is difficult for models to predict, and it has never been constrained by observations; as a result, there is large uncertainty in atmospheric aerosol simulations. Here, by a particle-tracer analysis of 37 rainfall events in East Asia, near the largest source region of anthropogenic aerosols in the northern hemisphere, we observed that the environmental supersaturation actually experienced by the removed aerosols in precipitating clouds averaged 0.08 ± 0.03% and ranged from 0.03 to 0.2%. Simulations by a mixing-state-resolved global aerosol model showed that the simulated long-range transport efficiency and global atmospheric burden of black carbon aerosols can be changed by a factor of two or three as a result of a change in the environmental supersaturation in precipitating clouds within just 0.08 ± 0.03%. This result is attributable to the fact that the sensitivity of an aerosol’s rainout efficiency to environmental supersaturation is higher for the less-aged black carbon concentrated near source regions. Our results suggest that observational constraints of environmental supersaturation in precipitating clouds, particularly near source regions, are of fundamental importance for accurate simulation of the atmospheric burden of black carbon and other aerosols.
Black carbon (BC) particles cause adverse health effects and contribute to the heating of the atmosphere by absorbing visible solar radiation. Efforts have been made to reduce BC emissions, ...especially in urban areas; however, long-term measurements of BC mass concentration (MBC) are very limited in Japan. We report MBC measurements conducted in Tokyo from 2003 to 2017, showing that MBC decreased by a factor of 3 from 2003 to 2010 and was stable from 2010 to 2017. Fine particulate concentrations (PM2.5) decreased by a much smaller factor during 2003–2010. The diurnal variations of BC size distributions suggest that the BC in Tokyo originates mainly from local sources, even after 2010. Our three-dimensional model calculations show that BC from the Asian continent contributes a small portion (about 20%) of the annual average MBC in the Kanto region of Japan, which includes Tokyo. This indicates that continued reduction of BC emissions inside Japan should be effective in further decreasing MBC.
Reliable measurement of the size-resolved number concentration (size distribution) of solid particles dispersed in water or melted ice is of critical importance in many geoscientific observational ...studies. Because physical and chemical properties of particles can be measured more unambiguously and accurately in rarefied media (air) than in condensed media (water), particle measurement after aerosolization using a nebulizer is a significant method for the observation of solid particles dispersed in water. We propose a mathematical theory for estimating the original size distribution of solid particles in water from the measured size distribution of aerosolized particles. We assume that the size distribution of water droplets produced by a nebulizer is given. The size distribution of solid particles in water can be estimated by solving a system of nonlinear equations. The complexity in solving the equations increases rapidly with the computational resolution of particle size and the assumed maximum number of particles within each droplet. For such a system of equations, we found rigorous error bounds of a true solution using INTLAB, an interval arithmetic package. Our theoretical framework will be useful in many fields in geoscience as a fundamental scheme to quantify solid particles in water. In particular, an application of the proposed theoretical method is shown to be useful for the quantitative observations of the size distribution of black carbon particles in rainwater.
•Accurate measurement of size distribution of solid particles in water.•We assume a use of nebulizer to aerosolize solid particles in water.•We model the stochastic agglomerations of particles upon nebulization.
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