Capture of decline in spring phytoplankton biomass derived from COVID-19 lockdown effect in the Yellow Sea offshore waters

The Yellow Sea, characterized as a high-productivity ecosystem, is considered to be significantly attributable to high nutrient supply via atmospheric deposition. We observed a significant decline in phytoplankton biomass (~30%) over the Yellow Sea during February-May 2020 (period of COVID-19 lockdown effect) compared to the same period in 2015-2019 (period of no effect of COVID-19 lockdown). Several possible factors, such as variations in irradiance, vertical mixing, and river discharges, were not major contributors. Through the analysis of transportation and the constituents of atmospheric pollutants from Northern China (main source regions) to the Yellow Sea, we suggest that the decline in phytoplankton biomass over the Yellow Sea is mainly attributed to decreased atmospheric nutrient deposition due to the COVID-19 lockdown effect, because of decreased anthropogenic emissions in Northern China. Thus, attention should be focused on the Yellow Sea ecosystem response to increasing anthropogenic activities by lifting the COVID-19 lockdown restrictions.

Assessment of the clear-sky bias issue using continuous PM10 data from two AERONET sites in Korea

A bias in clear-sky conditions that will be involved in estimating particulate matter (PM) concentration from aerosol optical depth (AOD) was examined using PM₁₀ from two Aerosol Robotic Network sites in Korea. The study periods were between 2004 and 2007 at Anmyon and between 2003 and 2011 at Gosan, when both PM₁₀ and AOD were available. Mean PM₁₀ when AOD was available (PM_AOD) was higher than that from all PM₁₀ data (PM_all) by 5.1 and 9.9μg/m³ at Anmyon and Gosan, which accounted for 11% and 26% of PM_all, respectively. Because of a difference between mean PM₁₀ under daytime clear-sky conditions (PM_clear) and PM_AOD, the variations in ΔPM₁₀, the difference of PM_all from PM_clear rather than from PM_AOD, were investigated. Although monthly variations in ΔPM₁₀ at the two sites were different, they were positively correlated to those in ΔT, similarly defined as ΔPM₁₀ except for temperature, at both sites. ΔPM₁₀ at Anmyon decreased to a negative value in January due to an influence of the Siberian continental high-pressure system while ΔPM₁₀ at Gosan was high in winter due to an effect of photochemical production at higher temperatures than at Anmyon.

Chemical characteristics of long-range-transported fine particulate matter at Gosan, Jeju Island, in the spring and fall of 2008, 2009, 2011, and 2012

Carbonaceous species (organic carbon [OC] and elemental carbon [EC]) and inorganic ions of particulate matter less than 2.5 μm (PM2.5) were measured to investigate the chemical characteristics of long-range-transported (LTP) PM2.5 at Gosan, Jeju Island, in Korea in the spring and fall of 2008-2012 (excluding 2010). On average, the non-sea-salt (nss) sulfate (4.2 µg/m3) was the most dominant species in the spring, followed by OC (2.6 µg/m3), nitrate (2.1 µg/m3), ammonium (1.7 µg/m3), and EC (0.6 µg/m3). In the fall, the nss-sulfate (4.7 µg/m3) was also the most dominant species, followed by OC (4.0 µg/m3), ammonium (1.7 µg/m3), nitrate (1.1 µg/m3), and EC (0.7 µg/m3). Both sulfate and OC were higher in the fall than in the spring, possibly due to more common northwest air masses (i.e., coming from China and Korea polluted areas) and more frequent biomass burnings in the fall. There was no clear difference in the EC between the spring and fall. The correlation between OC and EC was not strong; thus, the OC measured at Gosan was likely transported across a long distance and was not necessarily produced in a manner similar to the EC. Distinct types of LTP events (i.e., sulfate-dominant LTP versus OC-dominant LTP) were observed. In the sulfate-dominant LTP events, air masses directly arrived at Gosan without passing over the Korean Peninsula from the industrial area of China within 48 hr. During these events, the aerosol optical depth (AOD) increased to 1.63. Ionic balance data suggest that the long-range transported aerosols are acidic. In the OC-dominant LTP event, a higher residence time of air masses in Korea was observed (the air masses departing from the mainland of China arrived at the sampling site after passing Korea within 60-80 hr).

IMPLICATIONS

In Northeast Asia, various natural and anthropogenic sources contribute to the complex chemical components and affect local/regional air quality and climate change. Chemical characteristics of long-range-transported (LTP) PM2.5 were investigated during spring and fall of 2008, 2009, 2011, and 2012. Based on air mass types, sulfate-dominant LTP and OC-dominant LTP were observed. A long-term variation and chemical characteristics of PM2.5 along with air mass and satellite data are required to better understand long-range-transported aerosols.

Dry deposition of PM10 over the Yellow Sea during Asian dust events from 2001 to 2007

Dry deposition velocities and fluxes of PM10 during Asian dust events over the Yellow Sea from 2001 to 2007 were investigated using observation data in Qingdao, China and Jeju, Korea. The dry deposition velocities of PM10 during dust events over the Yellow Sea ranged from 0.19 to 8.17 cm/sec, with an average of 3.38 cm/sec. Dry deposition fluxes of PM10 during dust events over the Yellow Sea were in the range of 68.5-2647.1 mg/(m2 x day), with an average of 545.4 mg/(m2 x day), which is 2-10 times higher than those reported by other studies for both dust and non-dust periods. It was estimated that 2.6 x 10(11) -48.7 x 10(11) g dust particles deposit to the Yellow Sea during dust events through dry deposition every year. Compared with the results in previous studies, it was found that the dry deposition of PM10 over the Yellow Sea during dust events in the years with high frequency of dust could account for a large or overwhelming fraction of the annual total dry deposition. Backward air mass trajectory analysis showed that dust events influenced Jeju mainly originated from the desert regions located in Mongolia and Inner Mongolia, China. There were 119 backward trajectories influenced both Qingdao and Jeju during 15 dust events from 2001 to 2007, accounting for 61.3% of the total trajectories of 194, indicating that Qingdao and Jeju were usually on the same pathway of dust transport downwind from source areas.

Factors affecting the level and pattern of polycyclic aromatic hydrocarbons (PAHs) at Gosan, Korea during a dust period

Polycyclic aromatic hydrocarbons (PAHs) in both gas and total suspended particles were measured at Gosan, Jeju Island in Korea, a remote background site, for 15 days (March 29-April 12, 2002). During the sampling period, a severe three-day Asian dust (AD) event originating from Mongolia and northern China was observed throughout the Korean Peninsula and Jeju Island. In addition, pollution (PO) and normal (NO) periods were also identified based on the levels of anthropogenic pollutants. Despite a large difference of PM(10) concentrations between the AD and PO periods, the levels of particulate PAHs in both periods were comparable (2.7±1.0 and 2.4±0.5 ng m(-3), respectively) since they were determined by the concentration of anthropogenic PM(2.5) transported from industrial areas of China. In the AD period, the level of gaseous PAHs, which were mostly from local sources, was the lowest due to strong winds; the gas/particle partitioning was close to equilibrium as the effect of long-range transport was manifested. The results of backward air trajectories, correlation analysis, and diagnostic ratios show that long-range transport of particulate PAHs produced by coal/biomass burning in China could strongly affect the levels and patterns of PAHs at Gosan, Korea.