9:00 AM - 9:15 AM
[AAS11-07] Tropospheric ozone increase in Northern Vietnam and evaluation of rice yield loss
Keywords:Tropospheric ozone, Rice yield loss, Vietnam
Tropospheric ozone (hereafter referred to as TO3), although small in quantity, contributes to global warming and climate change as the third greenhouse gas. Because It has large oxidizing capacity and it can directly contact organisms on the earth's surface, it significantly affects the growth of crops and forests, as well as human health.
In the period 2019-2020, a group from Ibaraki University and Vietnam-Japan University conducted observations of TO3 at Hanoi, northern Vietnam, and found that its concentrations were often very high in this region. Vietnam is the world's third largest rice exporter, and rice production in northern Vietnam accounts for 29.4% (in 2022) of Vietnam's total production, but no studies have yet been conducted on the effects of ozone on rice production in this region. The objective of this study was to determine the factors responsible for the seasonal increase in tropospheric ozone and to quantitatively assess its impact on rice production in the northern Vietnam based on observation and model TO3 data.
Direct observation of TO3 began in December 2018 and were conducted with UV absorption method (Dylec Model 1100) on the 5th floor of the My Dinh Campus of the Vietnam-Japan University in the western part of downtown Hanoi, together with black carbon aerosol and PM2.5 observations. The observation was often interrupted due to COVID-19 and other reasons. Surface ozone concentrations and ozone precursor distributions calculated by NASA's Goddard Earth Observing System Composition Forecasts (GEOS-CF) model were also used to examine regional TO3 distribution and ozone transport over Indochina region. The model data at a grid including Hanoi were compared with the direct observation data and generally agreed well except for the systematically high daytime maximum and systematically low nighttime minimum in the direct observation data, which may be due to locally high NOx concentrations in the Hanoi urban area.
Direct observations indicated that there is a seasonal increase in TO3 in spring and fall. The model-calculated distributions of TO3 and ozone precursor gas concentrations and wind speeds were examined during these seasons. The results showed that TO3 and ozone precursor gas concentrations were high in northern Vietnam, especially in the Hong River (Red River) Delta where Hanoi is located, indicating that ozone is basically increasing regionally due to photochemical generation due to reginal air pollution. In addition, transport of TO3 from China to the north of the Hong River delta by the northeast winter monsoon in the first half of the spring season, and that of TO3 produced by biomass burning from Laos mountainous area and Thailand to the south of the Hong River delta by the southwest summer monsoon in the second half of the spring season.
Overall, the ozone exposure indices were higher in the monsoon crop than in the winter-spring crop. In the winter-spring crop season, the exposure indices tended to be higher in the northwest than in the southeast, probably due to the transport of polluted air from the north and the inflow of relatively clean air over the East Sea to the south of Hong River delta during the winter and early spring when the northeast monsoon prevails. During the monsoon crop season, exposure indices tended to be higher in the Hong River Delta, especially closer to its center where Hanoi is located, due to the regional ozone increase during the fall season. The mean and standard deviation of the yield reduction rates for the 28 provinces in the northern Vietnam were 16.2 ± 3.0% for the 2019 monsoon crop and 11.8 ± 2.1% for the 2019 winter-spring crop, indicating that the impact of ozone cannot be ignored. The product of this average yield reduction in each province and the amount of rice produced in that province is the actual rice yield reduction. The impact was found to be particularly large for monsoon crops with large ozone exposure in the Hong River Delta and south of the Hong River Delta, where rice production is high.
Morikawa et al. (1980) showed that the ripening period, the period during which rice plants bear fruit, is more affected by ozone than the growing periods of the sprouting and internode elongation periods. Contrary to the case where differences of ozone sensitivity with the growing periods was not taken into account, the yield reduction rate of the winter-spring crop, in which ozone increased during the ripening period, was slightly higher than that of the monsoon crop, in which ozone decreased during the ripening period.
In the period 2019-2020, a group from Ibaraki University and Vietnam-Japan University conducted observations of TO3 at Hanoi, northern Vietnam, and found that its concentrations were often very high in this region. Vietnam is the world's third largest rice exporter, and rice production in northern Vietnam accounts for 29.4% (in 2022) of Vietnam's total production, but no studies have yet been conducted on the effects of ozone on rice production in this region. The objective of this study was to determine the factors responsible for the seasonal increase in tropospheric ozone and to quantitatively assess its impact on rice production in the northern Vietnam based on observation and model TO3 data.
Direct observation of TO3 began in December 2018 and were conducted with UV absorption method (Dylec Model 1100) on the 5th floor of the My Dinh Campus of the Vietnam-Japan University in the western part of downtown Hanoi, together with black carbon aerosol and PM2.5 observations. The observation was often interrupted due to COVID-19 and other reasons. Surface ozone concentrations and ozone precursor distributions calculated by NASA's Goddard Earth Observing System Composition Forecasts (GEOS-CF) model were also used to examine regional TO3 distribution and ozone transport over Indochina region. The model data at a grid including Hanoi were compared with the direct observation data and generally agreed well except for the systematically high daytime maximum and systematically low nighttime minimum in the direct observation data, which may be due to locally high NOx concentrations in the Hanoi urban area.
Direct observations indicated that there is a seasonal increase in TO3 in spring and fall. The model-calculated distributions of TO3 and ozone precursor gas concentrations and wind speeds were examined during these seasons. The results showed that TO3 and ozone precursor gas concentrations were high in northern Vietnam, especially in the Hong River (Red River) Delta where Hanoi is located, indicating that ozone is basically increasing regionally due to photochemical generation due to reginal air pollution. In addition, transport of TO3 from China to the north of the Hong River delta by the northeast winter monsoon in the first half of the spring season, and that of TO3 produced by biomass burning from Laos mountainous area and Thailand to the south of the Hong River delta by the southwest summer monsoon in the second half of the spring season.
Overall, the ozone exposure indices were higher in the monsoon crop than in the winter-spring crop. In the winter-spring crop season, the exposure indices tended to be higher in the northwest than in the southeast, probably due to the transport of polluted air from the north and the inflow of relatively clean air over the East Sea to the south of Hong River delta during the winter and early spring when the northeast monsoon prevails. During the monsoon crop season, exposure indices tended to be higher in the Hong River Delta, especially closer to its center where Hanoi is located, due to the regional ozone increase during the fall season. The mean and standard deviation of the yield reduction rates for the 28 provinces in the northern Vietnam were 16.2 ± 3.0% for the 2019 monsoon crop and 11.8 ± 2.1% for the 2019 winter-spring crop, indicating that the impact of ozone cannot be ignored. The product of this average yield reduction in each province and the amount of rice produced in that province is the actual rice yield reduction. The impact was found to be particularly large for monsoon crops with large ozone exposure in the Hong River Delta and south of the Hong River Delta, where rice production is high.
Morikawa et al. (1980) showed that the ripening period, the period during which rice plants bear fruit, is more affected by ozone than the growing periods of the sprouting and internode elongation periods. Contrary to the case where differences of ozone sensitivity with the growing periods was not taken into account, the yield reduction rate of the winter-spring crop, in which ozone increased during the ripening period, was slightly higher than that of the monsoon crop, in which ozone decreased during the ripening period.