Photochemical oxidants (Ox; mainly O3) are a concern in East Asia. Because of the prevailing westerly wind in the midlatitudes, O3 concentration generally shows a high in spring over Kyushu Island, ...western Japan, and Ox warnings have been issued in spring. However, the record from 2000 to 2021 of Ox warning days in Kyushu Island contains one warning case in autumn 2020. Interestingly, a typhoon had passed the day before this Ox warning. To relate these events, a modelling simulation was conducted and it showed the transboundary O3 transport from the Asian continent to the western coast of Japan due to the strong wind field determined by the location of Typhoon Haishen (2020). The sensitivity simulations for changing Chinese anthropogenic sources suggested that both nitrogen oxides (NOx) and volatile organic compound (VOC) emission regulations in China could decrease high O3 over the downwind region of Japan. Furthermore, VOC emission regulation in China led to an overall O3 decrease in East Asia, whereas NOx emission regulation in China had complex effects of decreasing (increasing) O3 during the daytime (nighttime) over China. The association between air quality and meteorology related to typhoons should be considered along with global warming in the future.
The severe aerosol pollution in East Asia has been a focus of much research. In Japan, the environmental quality standard (EQS) for PM2.5 was established in 2009 (daily average, 35 μg m−3; annual ...average, 15 μg m−3), and its achievement rate was below 50% during the early 2010s. Then, the PM2.5 concentration gradually decreased, the achievement rate improved, and the EQS for PM2.5 was finally achieved (100%) in fiscal year (FY) 2021. Because transboundary aerosol pollution is an important factor in Japanese air quality, here we analysed the long-term dataset of the satellite-measured fine-mode aerosol optical depth (AODf) over the East Asian ocean to reveal the changes in the transboundary aerosol over East Asia. Overall, a decrease in AODf was seen over the entire East Asian ocean during the period analysed. A gradual declining trend in AODf was measured (−4% to −5%/year over the adjacent ocean around Japan) and corresponded well to the trend in PM2.5 concentration observed in Japan (−5.3%/year) during FY2010–FY2021. Due to the domestic contribution in Japan, the negative trend was slightly greater for Japanese PM2.5 concentration than for AODf over the adjacent ocean around Japan, and we concluded that the main reason for the dramatic air quality improvement in PM2.5 in Japan was driven by the improvement of transboundary aerosol pollution over East Asia. In addition, the 12 year analysis period (FY2010 to FY2021) was divided into three parts: stagnation (FY2010 to FY2014), in which PM2.5 and AODf remained the same as they were in FY2010; improvement (FY2015 to FY2018), in which PM2.5 and AODf declined dramatically; and achievement (FY2019 to FY2021), in which PM2.5 and AODf declined further.
Abstract
Changes in the aerosol composition of sulfate (SO
4
2−
) and nitrate (NO
3
−
) from 2012 to 2019 have been captured as a paradigm shift in the region downwind of China. Specifically, SO
4
2−
...dramatically decreased and NO
3
−
dramatically increased over downwind locations such as western Japan due to the faster reduction of SO
2
emissions than NO
x
emissions and the almost constant trend of NH
3
emissions from China. Emissions from China sharply decreased during COVID-19 lockdowns in February–March 2020, after which China’s economic situation seemed to recover going into 2021. Given this substantial change in Chinese emissions, it is necessary to clarify the impact of long-range PM
2.5
transport into the leeward of East Asia. In this study, ground-based aerosol compositions observed at three sites in western Japan were analysed. The concentrations of PM
2.5
, SO
4
2−
and NO
3
−
decreased in 2020 (during COVID-19) compared with 2018–2019 (before COVID-19). In 2021 (after COVID-19), PM
2.5
and NO
3
−
increased and SO
4
2−
was unchanged. This suggests the returning long-range PM
2.5
transport in 2021. From numerical simulations, the status of Chinese emissions during COVID-19 did not explain this returning impact in 2021. This study shows that the status of Chinese emissions in 2021 recovered to that before COVID-19.
The lockdown measures due to COVID-19 affected the industry, transportation and other human activities within China in early 2020, and subsequently the emissions of air pollutants. The decrease of ...atmospheric NO2 due to the COVID-19 lockdown and other factors were quantitively analyzed based on the surface concentrations by in-situ observations, the tropospheric vertical column densities (VCDs) by different satellite retrievals including OMI and TROPOMI, and the model simulations by GEOS-Chem. The results indicated that due to the COVID-19 lockdown, the surface NO2 concentrations decreased by 42% ± 8% and 26% ± 9% over China in February and March 2020, respectively. The tropospheric NO2 VCDs based on both OMI and high quality (quality assurance value (QA) ≥ 0.75) TROPOMI showed similar results as the surface NO2 concentrations. The daily variations of atmospheric NO2 during the first quarter (Q1) of 2020 were not only affected by the COVID-19 lockdown, but also by the Spring Festival (SF) holiday (January 24–30, 2020) as well as the meteorology changes due to seasonal transition. The SF holiday effect resulted in a NO2 reduction from 8 days before SF to 21 days after it (i.e. January 17 - February 15), with a maximum of 37%. From the 6 days after SF (January 31) to the end of March, the COVID-19 lockdown played an important role in the NO2 reduction, with a maximum of 51%. The meteorology changes due to seasonal transition resulted in a nearly linear decreasing trend of 25% and 40% reduction over the 90 days for the NO2 concentrations and VCDs, respectively. Comparisons between different datasets indicated that medium quality (QA ≥ 0.5) TROPOMI retrievals might suffer large biases in some periods, and thus attention must be paid when they are used for analyses, data assimilations and emission inversions.
Display omitted
•NO2 in China decreased by 42% and 26% in February and March 2020 due to COVID-19.•The NO2 VCDs by satellites were consistent with the surface NO2 concentrations.•Spring Festival holiday led to a NO2 decrease in 30 days with a maximum of 37%.•COVID-19 led to a NO2 decrease from 6 days after SF with a maximum of 51%.•NO2 concentrations and VCDs decreased by 25% and 40% in 90 days due to meteorology.
A rapid decrease in PM
concentrations in China has been observed in response to the enactment of strong emission control policies. From 2012 to 2017, total emissions of SO
and NO
from China decreased ...by approximately 63% and 24%, respectively. Simultaneously, decreases in the PM
concentration in Japan have been observed since 2014, and the proportion of stations that satisfy the PM
environmental standard (daily, 35 µg/m
; annual average, 15 µg/m
) increased from 37.8% in fiscal year (FY) 2014 (April 2014 to March 2015) to 89.9% in FY 2017. However, the quantitative relationship between the PM
improvement in China and the PM
concentration in downwind regions is not well understood. Here, we (1) quantitatively evaluate the impacts of Chinese environmental improvements on downwind areas using source/receptor analysis with a chemical transport model, and (2) show that these rapid emissions reductions improved PM
concentrations both in China and its downwind regions, but the difference between SO
and NO
reduction rates led to greater production of nitrates (e.g., NH
NO
) due to a chemical imbalance in the ammonia-nitric acid-sulfuric acid-water system. Observations from a clean remote island in western Japan and numerical modeling confirmed this paradigm shift.
Numerical models have been developed to elucidate air pollution caused by sulfate aerosols (SO4 2–). However, typical models generally underestimate SO4 2–, and oxidation processes have not been ...validated. This study improves the modeling of SO4 2– formation processes using the mass-independent oxygen isotopic composition 17O-excess; Δ17O(SO4 2–), which reflects pathways from sulfur dioxide (SO2) to SO4 2–, at the background site in Japan throughout 2015. The standard setting in the Community Multiscale Air Quality (CMAQ) model captured SO4 2– concentration, whereas Δ17O(SO4 2–) was underestimated, suggesting that oxidation processes were not correctly represented. The dust inline calculation improved Δ17O(SO4 2–) because dust-derived increases in cloud-water pH promoted acidity-driven SO4 2– production, but Δ17O(SO4 2–) was still overestimated during winter as a result. Increasing solubilities of the transition-metal ions, such as iron, which are a highly uncertain modeling parameter, decreased the overestimated Δ17O(SO4 2–) in winter. Thus, dust and high metal solubility are essential factors for SO4 2– formation in the region downstream of China. It was estimated that the remaining mismatch of Δ17O(SO4 2–) between the observation and model can be explained by the proposed SO4 2– formation mechanisms in Chinese pollution. These accurately modeled SO4 2– formation mechanisms validated by Δ17O(SO4 2–) will contribute to emission regulation strategies required for better air quality and precise climate change predictions over East Asia.
Abstract
Air pollution over the Seto Inland Sea (SIS) is among the most severe of any region in Japan and is considered to be affected by both long-range and local pollution. To unravel the long-term ...trends of aerosol pollution over this region, in this study, measurements from the moderate resolution imaging spectroradiometer instrument onboard the Terra satellite were analyzed over two decades, from 2001 to 2020. Fine-mode aerosol optical depth (AOD
f
) was calculated to estimate the amount of aerosol produced by anthropogenic emissions. The results showed that the AOD
f
over the SIS increased from 2001 to 2004, had a flat trend from 2005 to 2009, and decreased from 2010 to 2020. To clarify the impact of long-range transport from the Asian continent to the SIS, the AOD
f
over the Yellow Sea was also investigated and was found to increase and level off during the 2000s, after which it decreased, especially after 2014. This decrease can be attributed to emission regulations in China. The above analysis suggests that the aerosol pollution status in the SIS during the late 2010s was similar to that during the early 2000s. Over the SIS, the lowest AOD
f
value was found in 2020, with the values in January–March and June–July approximately 30% and 30%–60% lower than the average values during the same periods in 2018–2019, respectively. The reduction found in January–March could be related to the decline in the long-range transport with restrictions on human activity due to the COVID-19 pandemic. Meanwhile, the reduction during June–July could be related to the decline of local emission sources. Considering the large SO
2
decline in 2020, regulations on SO
2
emitted from ships that started from 1 January 2020 are one possible factor for the improvement of aerosol pollution over the SIS in 2020.
To mitigate tropospheric ozone (O3) pollution with proper and effective emission regulations, diagnostics for the O3-sensitive regime are critical. In this study, we analyzed the satellite-measured ...formaldehyde (HCHO) and nitrogen dioxide (NO2) column densities and derived the HCHO to NO2 ratio (FNR) from 2005 to 2019. Over China, there was a clear increase in the NO2 column during the first 5-year period and a subsequent decrease after 2010. Over the Republic of Korea and Japan, there was a continuous decline in the NO2 column over 15 years. Over the entire East Asia, a substantial increase in the HCHO column was identified during 2015–2019. Therefore, FNR increased over almost all of East Asia, especially during 2015–2019. This increasing trend in FNR indicated the gradual shift from a volatile organic compound (VOC)-sensitive to a nitrogen oxide (NOx)-sensitive regime. The long-term changes in HCHO and NO2 columns generally corresponded to anthropogenic non-methane VOC (NMVOC) and NOx emissions trends; however, anthropogenic sources did not explain the increasing HCHO column during 2015–2019. Because of the reduction in anthropogenic sources, the relative importance of biogenic NMVOC sources has been increasing and could have a larger impact on changing the O3-sensitive regime over East Asia.
Stratospheric intrusion and trans-Pacific transport have been recognized as a potential source of tropospheric ozone over the US. The state-of-the-science Community Multiscale Air Quality (CMAQ) ...modeling system has recently been extended for hemispheric-scale modeling applications (referred to as H-CMAQ). In this study, H-CMAQ is applied to study the stratospheric intrusion and trans-Pacific transport during April 2010. The results will be presented in two companion papers. In this Part 1 paper, model evaluation for tropospheric ozone (O
) is presented. Observations at the surface, by ozonesondes and airplane, and by satellite across the Northern Hemisphere are used to evaluate the model performance for O
. H-CMAQ is able to capture surface and boundary layer (defined as surface to 750hPa) O
with a normalized mean bias (NMB) of -10%; however, a systematic underestimation with an NMB up to -30% is found in the free troposphere (defined as 750-250hPa). In addition, a new air mass characterization method is developed to distinguish influences of stratosphere-troposphere transport (STT) from the effects of photochemistry on O
levels. This method is developed based on the ratio of O
and an inert tracer indicating stratospheric O
to examine the importance of photochemistry, and sequential intrusion from upper layer. During April 2010, on a monthly average basis, the relationship between surface O
mixing ratios and estimated stratospheric air masses in the troposphere show a slight negative slope, indicating that high surface O
values are primarily affected by other factors (i.e., emissions), whereas this relationship shows a slight positive slope at elevated sites, indicating that STT has a possible impact at elevated sites. STT shows large day-to-day variations, and STT impacts can either originate from the same air mass over the entire US with an eastward movement found during early April, or stem from different air masses at different locations indicated during late April. Based on this newly established air mass characterization technique, this study can contribute to understanding the role of STT and also the implied importance of emissions leading to high surface O
. Further research focused on emissions is discussed in a subsequent paper (Part 2).
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