We estimated the source-receptor relationship for surface O₃ in East Asia during the early 2000s using a method that tags O₃ tracers according to their region of chemical production (tagged tracer method) with a global chemical transport model. The estimation demonstrated the importance of intracontinental transport of O₃ inside East Asia as well as of the transport of O₃ from distant source regions. The model well simulated the absolute concentration and seasonal variation of surface O₃ in East Asia and demonstrated significant seasonal differences in the origin of surface O₃. In the cold season (October to March), more than half of surface O₃ in East Asia is attributable to the O₃ transported from distant sources outside of East Asia. In the warm season (April to September), most of the surface O₃ is attributable to O₃ created within East Asia in most areas of East Asia. In spring the contribution of domestically created O₃ accounted for 20% of the surface O₃ in Japan and the Korean Peninsula, 40% in the North China Plain, and around 50% in the southern part of China, and the domestic contribution increased greatly in summer. The contributions of O₃ created in China and the Korean Peninsula to O₃ in Japan were estimated at about 10% and 5%, respectively. We also demonstrated a large contribution (20%) from China to the Korean Peninsula. In the northern and southern parts of China, large contributions of over 10% from East Siberia and the Indochina Peninsula, respectively, were identified. The contribution from intercontinental transport increased with latitude; it was 21% in Northeast China and 13% in Japan and the Korean Peninsula in spring. As for the hourly mean of surface O₃, domestically created O₃ was the main contributor in most areas of East Asia, except for the low O₃ class (90 ppbv). The mean relative contribution of O₃ created in China to O₃ in central Japan was about 10% in every class, but that created in the Korean Peninsula was significant in all except the low O₃ class. We identified the substantial impact of foreign sources on Japan's ambient air quality standard in the high O₃ class (60–90 ppbv) in spring.

The relative contributions of various source regions to the long-term (1980–2005) increasing trend in surface ozone (O₃) over Japan were estimated by a series of tracer-tagging simulations using a global chemical transport model. The model simulated the observed increasing trend in surface O₃, including its seasonal variation and geographical features, in Japan well and demonstrated the relative roles of different source regions in forming this trend. Most of the increasing trend in surface O₃ over Japan ( ∼ 97 %) that was simulated was explained as the sum of trends in contributions of different regions to photochemical O₃ production. The increasing trend in O₃ produced in China accounted for 36 % of the total increasing trend and those in the other northeast Asian regions (the Korean Peninsula, coastal regions in East Asia, and Japan) each accounted for about 12–15 %. Furthermore, the contributions of O₃ created in the entire free troposphere and in western, southern, and southeastern Asian regions also increased, and their increasing trends accounted for 16 and 7 % of the total trend, respectively. The impact of interannual variations in climate, in methane concentration, and in emission of O₃ precursors from different source regions on the relative contributions of O₃ created in each region estimated above was also investigated. The variation of climate and the increase in methane concentration together caused the increase of photochemical O₃ production in several regions, and represented about 19 % of the total increasing trend in surface O₃ over Japan. The increase in emission of O₃ precursors in China caused an increase of photochemical O₃ production not only in China itself but also in the other northeast Asian regions and accounted for about 46 % of the total increase in surface O₃ over Japan. Similarly, the relative impact of O₃ precursor emission changes in the Korean Peninsula and Japan were estimated as about 16 and 4 % of the total increasing trend, respectively. The O₃ precursor emission change in regions other than northeast Asia caused increases in surface O₃ over Japan mainly through increasing photochemical O₃ production in western, southern, and southeast Asia and the free troposphere and accounted for about 16 % of the total.

A model inter-comparison of secondary pollutant simulations over urban areas in Japan, the first phase of Japan’s study for reference air quality modeling (J-STREAM Phase I), was conducted using 32 model settings. Simulated hourly concentrations of nitric oxide (NO) and nitrogen dioxide (NO2), which are primary pollutant precursors of particulate matter with a diameter of 2.5 µm or less (PM2.5), showed good agreement with the observed concentrations, but most of the simulated hourly sulfur oxide (SO2) concentrations were much higher than the observations. Simulated concentrations of PM2.5 and its components were compared to daily observed concentrations by using the filter pack method at selected ambient air pollution monitoring stations (AAPMSs) for each season. In general, most models showed good agreement with the observed total PM2.5 mass concentration levels in each season and provided goal or criteria levels of model ensemble statistics in warmer seasons. The good performances of these models were associated with the simulated reproducibility of some dominant components, sulfates (SO42−) and ammonium (NH4+). The other simulated PM2.5 components, i.e., nitrates (NO3−), elemental carbon (EC), and organic carbon (OC), often show clear deviations from the observations. The considerable underestimations (approximately 30 µg/m3 for total PM2.5) of all participant models found on heavily polluted days with approximately 40–50 µg/m3 for total PM2.5 indicated some problems in the simulated local meteorology such as the atmospheric stability. This model inter-comparison suggests that these deviations may be owing to a need for further improvements both in the emission inventories and additional formation pathways in chemical transport models, and meteorological conditions also require improvement to simulate elevated atmospheric pollutants. Additional accumulated observations are likely needed to further evaluate the simulated concentrations and improve the model performance.

Meteorology and emission sources are the two main factors determining concentrations of air pollutants, including fine particulate matter. A regional air quality modeling system was used to analyze the sources of fine-particulate air pollution in Hanoi, Vietnam, in December 2010. The impacts of precipitation and winds on PM_2.5 concentrations was investigated. Precipitation was negatively correlated with PM_2.5 concentrations. However, winds showed both positive and negative correlations with PM_2.5 concentrations, depending on wind direction (WD) and the level of upwind concentrations. Sensitivity simulations were conducted to investigate the contribution of local and non-local emissions sources on total PM_2.5 by perturbing the emission inputs of the model. Overall, local and non-local sources contributed equally to the total PM_2.5 in Hanoi. Local emission sources comprised 57% of the total PM_2.5 concentrations for the high PM_2.5 pollution levels, while only comprising 42% of the total PM_2.5 for low levels of PM_2.5 concentrations. In Hanoi’s urban areas, local sources contributed more to the total PM_2.5 than non-local sources. In contrast, non-local sources were the main contributors to the PM_2.5 in Hanoi’s rural areas. Additional sensitivity simulations were conducted to identify the main local emission sources of PM_2.5 concentrations in December 2010. The industrial and residential sectors collectively comprised 79% of the total PM_2.5 concentrations while the transport and power sectors comprised only 2% and 3%, respectively. This is the first case study which used a regional air quality modeling system to provide new and informative insights into PM_2.5 air pollution in Hanoi by estimating the contributions of local and non-local emissions sources, as well as the contribution of local emission sectors to PM_2.5 concentrations in Hanoi.

Simulations using a climate model are used to investigate the possible impact of increasing emissions of carbonaceous aerosols on near‐surface temperature in the mid‐20th century. The annual global mean near‐surface temperature change from the mid‐20th century onward is reasonably described by a model that is forced by changes in most of the known climate forcing agents including an increase in carbonaceous aerosols, though it can also be well reproduced without increases in carbonaceous aerosols. However, if we consider spatio‐temporal structure of the changes in the near‐surface temperature, an increase in carbonaceous aerosols is definitely required for the model to represent changes in the near‐surface temperature in the mid‐century, in particular, cooling trends in the tropical and subtropical continents. The significance of an increase in carbonaceous aerosols as an indispensable contributor to mid‐20th century temperature changes is confirmed with the use of an optimal fingerprinting methodology.

Pattern scaling is an efficient way to generate projections of regional climate change for various emission scenarios. This approach assumes that the spatial pattern of changes per degree of global warming (scaling pattern) is the same among emission scenarios. The hypothesis was tested for the scaling pattern of precipitation by focusing on the scenario dependence of aerosol scaling patterns. The scenario dependence of aerosol scaling patterns induced the scenario dependence of the surface shortwave radiation scaling pattern. The scenario dependence of the surface shortwave radiation scaling pattern over the ocean tended to induce the scenario dependence of evaporation scaling patterns. The scenario dependence of evaporation scaling patterns led to the scenario dependence of precipitation scaling patterns locally and downwind. Contrariwise, when the scenario dependence of aerosol scaling patterns occurred over land, the scenario dependence of surface shortwave radiation scaling patterns induced the scenario dependence of the scaling patterns of evaporation, surface longwave radiation, and sensible heat. Consequently, the scenario dependence of evaporation scaling patterns was smaller over land, and the scenario dependence of precipitation scaling patterns tended to be insignificant. Moreover, the scenario dependence of the southern annular mode and polar amplification caused some of the scenario dependence of precipitation scaling patterns. In this study, only one global climate mode was analyzed. In addition, sensitivity experiments that remove aerosol emissions from some regions or some kinds of aerosols are ideal to separate the impacts of aerosols. Thus, an analysis of the dependencies of precipitation scaling pattern among global climate models and the sensitivity experiments are required in future work.