Temporal and seasonal variations in the frequency of high-concentration precipitation samples at EANET sites in Japan: 20 years of daily wet deposition data

Over the past decades, the atmospheric environment in Northeast Asia has changed dynamically. The Acid Deposition Monitoring Network in East Asia (EANET) has accumulated wet deposition monitoring data through daily sampling at many sites. Daily precipitation samples may reflect air pollutant concentrations more dynamically than samples collected at longer intervals, such as weekly or biweekly. Among daily precipitation data accumulated over the 20 years from 2000 to 2019 at the 10 EANET sites in Japan, we selected high-concentration data within the top 2% range for non-sea salt components, namely H⁺, NH₄⁺, nss-Ca²⁺, NO₃⁻, and nss-SO₄²⁻, respectively. Approximately 67% of the high-concentration data were from 2005 to 2014. Thereafter, the number of high-concentration data points was limited in recent years. Most of the high-concentration data were obtained in spring (March–May; approximately 50%) and winter (December–February; approximately 31%). These results are partially consistent with the air pollution conditions in mainland China, and the high concentration phenomenon is thought to be influenced by transboundary air pollution. Trajectory analysis performed on the samples with the highest concentration data in the winter-spring season confirmed that, in most cases, air masses from the continent were transported to each site. The seasonal differences in the coexisting high-concentration components in the samples suggested that counterions of SO₄²⁻ and NO₃⁻ were changed from H⁺ to Ca²⁺ from winter to spring due to the transport of spring dust storm from the continent. Thus, the daily high-concentration data dynamically reflected the air mass flow in the region.

Graphical Abstract