5:15 PM - 7:15 PM
[AAS11-P07] Uptake of black carbon aerosols in air masses from China and Siberia into low-level clouds over the western North Pacific in summer
Keywords:Black Carbon Aerosols, Cloud uptake, Aircraft observations, Biomass burning
Black carbon aerosol (BC) is a light-absorbing particle emitted by incomplete combustion and, in addition to its direct effects, has quasi-direct effects that affect cloud properties and lifetime when it is taken up into clouds. Uptake of BC into clouds is also important because main BC removal process is wet deposition. However, the quantitative relationship between BC cloud uptake rate and cloud parameters such as cloud water content is still poorly understood. The objective of this study was to evaluate BC cloud uptake rates by separating its concentration changes due to mixing dilution from simultaneous direct observations of clouds and BC by aircraft, and found the relationship between the BC cloud uptake rates and cloud parameters.
This study used data obtained from aircraft observations conducted off the eastern coast of Hokkaido, Japan, from July to August 2022, aiming to clarify the characteristics of the summer lower clouds over the western North Pacific, which are important for the radiation budget, and the dynamics of marine and anthropogenic aerosols that affect the clouds. A King Air aircraft of Diamond Air Service, Inc. was used for the observation flights from Memanbetsu Airport. The aircraft flew above, in and below the clouds scanning them to obtain data on aerosol interactions with lower clouds over the ocean and cloud properties, including cloud water content (CLWC), cloud particle density, Vertical profiles of BC concentration, aerosol composition, concentrations of carbon monoxide (CO), nitrous oxide (N2O), and water vapor (H2O), and other data were measured. We focused on the data taken in July 28-30, 2022, when high concentrations of CO and BC were observed in the region where lower-level clouds existed at altitudes of 1~3 km.
The origin and transport paths of the high-CO/BC air masses above and below the clouds were estimated from the relationship between CO, N2O, and H2O concentrations, measured many times scanning clouds during each flight in this period, and from backward trajectory analysis. The air masses with relatively high concentrations of BC and CO observed in this study were assumed to be air masses with a positive correlation between CO and N2O, which are thought to originate in China, and air masses with relatively high water vapor concentrations, and air masses with a large increase in CO but no change in N2O concentration are thought to originate from biomass burning in Eastern Siberia. These were transported at altitudes above 2 km, above the low-level clouds, which were formed by contacting air masses with low concentrations of CO and BC and high-water vapor concentrations, which existed at low altitudes over the northern Pacific Ocean. When the CO, N2O, and H2O concentrations within a cloud had all equal mixing rate between the concentrations of each gas above and below the cloud, the air within the cloud was considered to be a mixture of the air masses above and below it at the time of the observation. The mixing ratio of high CO/BC air masses above the cloud to the air within the cloud in those profiles was about 45% in most cases and 10% in some cases.
For each of these four profiles, the BC uptake rate into the cloud was estimated from the relationship between CO and BC concentrations within, above and below the cloud. The median values of the rate ranged from about 10% to 60%, and the mid 50% ranges were from about10% to about 70%. The BC uptake rate into clouds were significantly different in the three profiles observed on July 30, when the same Siberian-derived air mass was mixed with a similar mixing ratio to the North Pacific Ocean air mass below the clouds. Correlation between the BC uptake rates and the CLWC values of the corresponding low-level clouds were not significant due to the large variation in both variables, an inverse correlation-like relationship was found. These results indicate that the rate of BC uptake into clouds varies greatly depending on the properties of the BC particles themselves and cloud conditions, even within the same cloud, and that it also varies depending on the history of air entering and leaving the cloud, making it difficult to estimate with the method in this study. We will investigate the relationship between the properties of BC particles and their uptake rate into clouds by examining the differences in BC particle size and its internal mixing state in air with different cloud uptake rates within clouds and those above the clouds.
This study used data obtained from aircraft observations conducted off the eastern coast of Hokkaido, Japan, from July to August 2022, aiming to clarify the characteristics of the summer lower clouds over the western North Pacific, which are important for the radiation budget, and the dynamics of marine and anthropogenic aerosols that affect the clouds. A King Air aircraft of Diamond Air Service, Inc. was used for the observation flights from Memanbetsu Airport. The aircraft flew above, in and below the clouds scanning them to obtain data on aerosol interactions with lower clouds over the ocean and cloud properties, including cloud water content (CLWC), cloud particle density, Vertical profiles of BC concentration, aerosol composition, concentrations of carbon monoxide (CO), nitrous oxide (N2O), and water vapor (H2O), and other data were measured. We focused on the data taken in July 28-30, 2022, when high concentrations of CO and BC were observed in the region where lower-level clouds existed at altitudes of 1~3 km.
The origin and transport paths of the high-CO/BC air masses above and below the clouds were estimated from the relationship between CO, N2O, and H2O concentrations, measured many times scanning clouds during each flight in this period, and from backward trajectory analysis. The air masses with relatively high concentrations of BC and CO observed in this study were assumed to be air masses with a positive correlation between CO and N2O, which are thought to originate in China, and air masses with relatively high water vapor concentrations, and air masses with a large increase in CO but no change in N2O concentration are thought to originate from biomass burning in Eastern Siberia. These were transported at altitudes above 2 km, above the low-level clouds, which were formed by contacting air masses with low concentrations of CO and BC and high-water vapor concentrations, which existed at low altitudes over the northern Pacific Ocean. When the CO, N2O, and H2O concentrations within a cloud had all equal mixing rate between the concentrations of each gas above and below the cloud, the air within the cloud was considered to be a mixture of the air masses above and below it at the time of the observation. The mixing ratio of high CO/BC air masses above the cloud to the air within the cloud in those profiles was about 45% in most cases and 10% in some cases.
For each of these four profiles, the BC uptake rate into the cloud was estimated from the relationship between CO and BC concentrations within, above and below the cloud. The median values of the rate ranged from about 10% to 60%, and the mid 50% ranges were from about10% to about 70%. The BC uptake rate into clouds were significantly different in the three profiles observed on July 30, when the same Siberian-derived air mass was mixed with a similar mixing ratio to the North Pacific Ocean air mass below the clouds. Correlation between the BC uptake rates and the CLWC values of the corresponding low-level clouds were not significant due to the large variation in both variables, an inverse correlation-like relationship was found. These results indicate that the rate of BC uptake into clouds varies greatly depending on the properties of the BC particles themselves and cloud conditions, even within the same cloud, and that it also varies depending on the history of air entering and leaving the cloud, making it difficult to estimate with the method in this study. We will investigate the relationship between the properties of BC particles and their uptake rate into clouds by examining the differences in BC particle size and its internal mixing state in air with different cloud uptake rates within clouds and those above the clouds.