Comment on “Comparison of ozone measurement methods in biomass burning smoke: an evaluation under field and laboratory conditions” (Long et al. 2021)

Long et al (2021) conducted a detailed study of possible interferents in measurements of surface O3 by UV spectroscopy, which measures the UV transmission in ambient and O3 scrubbed air. While we appreciate the careful work done in this analysis, there were several omissions and, in one case, the type of scrubber used was mis-identified as manganese dioxide (MnO2), when in fact it was manganese chloride (MnCl2). This misidentification led to the erroneous conclusion that all UV-based O3 instruments employing solid-phase catalytic scrubbers exhibit significant positive artifacts, whereas previous research found this not to be the case when employing MnO2 scrubber types. While the Long study, and our results, confirm the substantial bias in instruments employing an MnCl2 scrubber, a replication of the earlier work with an MnO2 scrubber type and no humidity correction is needed.

The drivers and health risks of unexpected surface ozone enhancements over the Sichuan Basin, China, in 2020

Following a continuous increase in the surface ozone (O3) level from 2013 to 2019, the overall summertime O3 concentrations across China showed a significant reduction in 2020. In contrast to this overall reduction in surface O3 across China, unexpected surface O3 enhancements of 10.2 ± 0.8 ppbv (23.4 %) were observed in May–June 2020 (relative to 2019) over the Sichuan Basin (SCB), China. In this study, we use high-resolution nested-grid GEOS-Chem simulation, the eXtreme Gradient Boosting (XGBoost) machine learning method, and the exposure–response relationship to determine the drivers and evaluate the health risks due to the unexpected surface O3 enhancements. We first use the XGBoost machine learning method to correct the GEOS-Chem model–measurement O3 discrepancy over the SCB. The relative contributions of meteorology and anthropogenic emission changes to the unexpected surface O3 enhancements are then quantified with a combination of GEOS-Chem and XGBoost models. In order to assess the health risks caused by the unexpected O3 enhancements over the SCB, total premature mortalities are estimated. The results show that changes in anthropogenic emissions caused a 0.9 ± 0.1 ppbv O3 reduction, whereas changes in meteorology caused an 11.1 ± 0.7 ppbv O3 increase in May–June 2020 relative to 2019. The meteorology-induced surface O3 increase is mainly attributed to an increase in temperature and decreases in precipitation, specific humidity, and cloud fractions over the SCB and surrounding regions in May–June 2020 relative to 2019. These changes in meteorology combined with the complex basin effect enhance biogenic emissions of volatile organic compounds (VOCs) and nitrogen oxides (NOx), speed up O3 chemical production, and inhibit the ventilation of O3 and its precursors; therefore, they account for the surface O3 enhancements over the SCB. The total premature mortality due to the unexpected surface O3 enhancements over the SCB has increased by 89.8 % in May–June 2020 relative to 2019.

High-resolution modeling of the distribution of surface air pollutants and their intercontinental transport by a global tropospheric atmospheric chemistry source–receptor model (GNAQPMS-SM)

Many efforts have been devoted to quantifying the impact of intercontinental transport on global air quality by using global chemical transport models with horizontal resolutions of hundreds of kilometers in recent decades. In this study, a global online air quality source–receptor model (GNAQPMS-SM) is designed to effectively compute the contributions of various regions to ambient pollutant concentrations. The newly developed model is able to quantify source–receptor (S-R) relationships in one simulation without introducing errors by nonlinear chemistry. We calculate the surface and planetary boundary layer (PBL) S-R relationships in 19 regions over the whole globe for ozone (O3), black carbon (BC), and non-sea-salt sulfate (nss-sulfate) by conducting a high-resolution (0.5&deg &times 0.5&deg) simulation for the year 2018. The model exhibits a realistic capacity in reproducing the spatial distributions and seasonal variations of tropospheric O3, carbon monoxide, and aerosols at global and regional scales – Europe (EUR), North America (NAM), and East Asia (EA). The correlation coefficient (R) and normalized mean bias (NMB) for seasonal O3 at global background and urban–rural sites ranged from 0.49 to 0.87 and −2 % to 14.97 %, respectively. For aerosols, the R and NMB in EUR, NAM, and EA mostly exceed 0.6 and are within ±15 %. These statistical parameters based on this global simulation can match those of regional models in key regions. The simulated tropospheric nitrogen dioxide and aerosol optical depths are generally in agreement with satellite observations. The model overestimates ozone concentrations in the upper troposphere and stratosphere in the tropics, midlatitude, and polar regions of the Southern Hemisphere due to the use of a simplified stratospheric ozone scheme and/or biases in estimated stratosphere–troposphere exchange dynamics. We find that surface O3 can travel a long distance and contributes a non-negligible fraction to downwind regions. Non-local source transport explains approximately 35 %–60 % of surface O3 in EA, South Asia (SAS), EUR, and NAM. The O3 exported from EUR can also be transported across the Arctic Ocean to the North Pacific and contributes nearly 5 %–7.5 % to the North Pacific. BC is directly linked to local emissions, and each BC source region mainly contributes to itself and surrounding regions. For nss-sulfate, contributions of long-range transport account for 15 %–30 % within the PBL in EA, SAS, EUR, and NAM. Our estimated international transport of BC and nss-sulfate is lower than that from the Hemispheric Transport of Air Pollution (HTAP) assessment report in 2010, but most surface O3 results are within the range. This difference may be related to the different simulation years, emission inventories, vertical and horizontal resolutions, and S-R revealing methods. Additional emission sensitivity simulation shows a negative O3 response in receptor region EA in January from EA. The difference between two methods in estimated S-R relationships of nss-sulfate and O3 are mainly due to ignoring the nonlinearity of pollutants during chemical processes. The S-R relationship of aerosols within EA subcontinent is also assessed. The model that we developed creates a link between the scientific community and policymakers. Finally, the results are discussed in the context of future model development and analysis opportunities.

Latest seasonal trend of aerosol, particulate matter and ozone in Delhi

In this work, latest seasonal variation of aerosol, particulate matter and ozone in Delhi has been studied. Observations show that during winter, concentration of surface O3 is low and that of PM2.5 and PM10 is high. Aerosol size and aerosol content increases during winter. Decrease in surface ozone is explainable by gas phase and heterogeneous chemistry. An interesting feature is, along with surface ozone, total ozone also shows a low value during winter. This is a characteristic of ozone in Indo-Gangetic plain. Indo-Gangetic plain is covered by mild to heavy fog during most of the days in winter. It is possible that increase in size and content of aerosol and PM particles coupled with low temperature, low solar flux and high humidity is the cause of fog formation during winter in Indo-Gangetic plain.

Examining the competing effects of contemporary land management vs. land cover changes on global air quality

Our work explores the impact of two important dimensions of land system changes, land use and land cover change (LULCC) as well as direct agricultural reactive nitrogen (Nr) emissions from soils, on ozone (O3) and fine particulate matter (PM2.5) in terms of air quality over contemporary (1992 to 2014) timescales. We account for LULCC and agricultural Nr emissions changes with consistent remote sensing products and new global emission inventories respectively estimating their impacts on global surface O3 and PM2.5 concentrations as well as Nr deposition using the GEOS-Chem global chemical transport model. Over this time period, our model results show that agricultural Nr emission changes cause a reduction of annual mean PM2.5 levels over Europe and northern Asia (up to −2.1 µg m−3) while increasing PM2.5 levels in India, China and the eastern US (up to +3.5 µg m−3). Land cover changes induce small reductions in PM2.5 (up to −0.7 µg m−3) over Amazonia, China and India due to reduced biogenic volatile organic compound (BVOC) emissions and enhanced deposition of aerosol precursor gases (e.g., NO2, SO2). Agricultural Nr emission changes only lead to minor changes (up to ±0.6 ppbv) in annual mean surface O3 levels, mainly over China, India and Myanmar. Meanwhile, our model result suggests a stronger impact of LULCC on surface O3 over the time period across South America; the combination of changes in dry deposition and isoprene emissions results in −0.8 to +1.2 ppbv surface ozone changes. The enhancement of dry deposition reduces the surface ozone level (up to −1 ppbv) over southern China, the eastern US and central Africa. The enhancement of soil NO emission due to crop expansion also contributes to surface ozone changes (up to +0.6 ppbv) over sub-Saharan Africa. In certain regions, the combined effects of LULCC and agricultural Nr emission changes on O3 and PM2.5 air quality can be comparable (>20 %) to anthropogenic emission changes over the same time period. Finally, we calculate that the increase in global agricultural Nr emissions leads to a net increase in global land area (+3.67×106km2) that potentially faces exceedance of the critical Nr load (>5 kg N ha−1 yr−1). Our result demonstrates the impacts of contemporary LULCC and agricultural Nr emission changes on PM2.5 and O3 in terms of air quality, as well as the importance of land system changes for air quality over multidecadal timescales.

Reflections Based on Pollution Changes Brought by COVID-19 Lockdown in Shanghai

COVID-19 and its variants have been changing the world. The spread of variants brings severe effects to the global economy and to human’s lives and health, as well as to society. Lockdown is proven to be effective in stopping the spread. It also provides a chance to study natural environmental changes with humanity’s limited interference. This paper aims to evaluate the impact of lockdown on five major airborne pollutants, i.e., NO2, SO2, O3, PM2.5 and PM10, in the three different functional regions of Chongming, Xuhui and Jinshan of Shanghai. Changes in the same pollutants from the three regions over the same/different periods were all studied and compared. Overall, the COVID-19 lockdown has changed pollutant concentrations in the long and short terms. Concentrations of four pollutants decreased, except for that of earth surface O3, which increased. SO2 had significant correlations with all other pollutants. PM2.5 and PM10 are more externally input than locally produced. NO2, SO2 and PM levels sharply reduced in Jinshan and Xuhui due to the limited usage of fossil fuel. Lockdown improved the air quality. People now have a chance to rethink the value of life and the harmony between economic progress and environmental protection. This is helpful to establish sustainable societies.

The drivers and health risks of the unexpected surface ozone enhancements over the Sichuan basin, China in 2020

After a continuous increase in surface ozone (O3) level from 2013 to 2019, the overall summertime O3 concentration across China showed a significant reduction in 2020. In contrast to this overall reduction in surface O3 across China, unexpected surface O3 enhancements of 10.2 ± 0.8 ppbv (23.4 %) were observed in May–June 2020 vs. 2019 over the Sichuan basin (SCB), China. In this study, we use high resolution nested-grid GEOS-Chem simulation, the eXtreme Gradient Boosting (XGBoost) machine learning method and the exposure−response relationship to determine the drivers and evaluate the health risks of the unexpected surface O3 enhancements. We first use the XGBoost machine learning method to correct the GEOS-Chem model-to-measurement O3 discrepancy over the SCB. The relative contributions of meteorology and anthropogenic emissions changes to the unexpected surface O3 enhancements are then quantified with the combination of GEOS-Chem and XGBoost models. In order to assess the health risks caused by the unexpected O3 enhancements over the SCB, total premature death mortalities are estimated. The results show that changes in anthropogenic emissions caused 0.9 ± 0.1 ppbv of O3 reduction and changes in meteorology caused 11.1 ± 0.7 ppbv of O3 increase in May–June 2020 vs. 2019. The meteorology-induced surface O3 increase is mainly attributed to significant increases in temperature and downward potential vorticity, and decreases in precipitation, specific humidity and cloud fractions over the SCB and surrounding regions in May–June 2020 vs. 2019. These changes in meteorology combined with the complex basin effect enhance downward transport of O3 from upper troposphere, enhance biogenic emissions of volatile organic compounds (VOCs) and nitrogen oxides (NOx), speed up O3 chemical production, and inhabit the ventilation of O3 and its precursors, and therefore account for the surface O3 enhancements over the SCB. The total premature mortality due to the unexpected surface O3 enhancements over the SCB has increased by 89.8 % in May–June 2020 vs. 2019.

Land-surface forcing and anthropogenic heat modulate ozone by meteorology: A perspective from the Yangtze River Delta region

With the rapid advance in urbanization, land-surface forcing related to the urban expansion and anthropogenic heat (AH) release from human activities significantly affect the urban climate and in turn the air quality. Focusing on the Yangtze River Delta (YRD) region, a highly urbanized place with sever ozone (O3) pollution and complex geography, we estimate the impacts of land-surface forcing and AH on meteorology (meteorological factors and local circulations) and O3 using the WRF-chem model, which can enhance our understanding about the formation of O3 pollution in those rapidly developing regions with unique geographical features as most of our results can be supported by previous studies conducted in other regions in the world. Regional O3 pollution episodes occur frequently (26 times per year) in the YRD in recent years. These O3 pollution episodes are usually under calm conditions characterized by high temperature (over 20 °C), low relative humidity (less than 80 %), light wind (less than 3 m s−1) and shallow cloud cover (less than 5). In this case, high O3 mainly appears during the daytime influenced by the local circulations (the sea and the lake breezes). The change in land-surface forcing can cause an increase in 2-m temperature (T2) by maximum 3 °C, an increase in planetary boundary layer height (PBLH) by maximum 500 m and a decrease in 10-m wind speed (WS10) by maximum 1.5 m s−1, and surface O3 can increase by maximum 20 μg m−3 eventually. Furthermore, the expansion of coastal cities enhances the sea-breeze below 500 m. During the advance of the sea-breeze front inland, the upward air flow induced by the front makes well vertical mixing of O3. However, once the sea-breeze is fully formed, further progression inland is stalled, thus the O3 removal by the low sea-breeze will be weakened and surface O3 can be 10 μg m−3 higher in the case with cities than no-cities. The expansion of lakeside cities can extend the lifetime of the lake-breeze from the noon to the afternoon. Since the net effect of the lake-breeze is to accelerate the vertical mixing in the boundary layer, the surface O3 can increase as much as 30 μg m−3 in lakeside cities. Compared with the effects from land-surface forcing, the impacts of AH are relatively small. And the changes mainly appear in and around cities where AH emission is large. There are increases in T2, PBLH, WS10 and surface O3 when AH are taken into account, with the increment about 0.2 °C, 75 m, 0.3 m s−1 and 4 μg m−3, respectively. Additionally, AH can affect the urban-breeze circulations, meteorological factors and O3 concentration, but its effect on local circulations, such as the sea and the lake breezes, seems to be limited.