Evolution of Ozone Pollution in China: What Track Will It Follow?

Increasing surface ozone (O3) concentrations has emerged as a key air pollution problem in many urban regions worldwide in the last decade. A longstanding major issue in tackling ozone pollution is the identification of the O3 formation regime and its sensitivity to precursor emissions. In this work, we propose a new transformed empirical kinetic modeling approach (EKMA) to diagnose the O3 formation regime using regulatory O3 and NO2 observation datasets, which are easily accessible. We demonstrate that mapping of monitored O3 and NO2 data on the modeled regional O3-NO2 relationship diagram can illustrate the ozone formation regime and historical evolution of O3 precursors of the region. By applying this new approach, we show that for most urban regions of China, the O3 formation is currently associated with a volatile organic compound (VOC)-limited regime, which is located within the zone of daytime-produced O3 (DPO3) to an 8h-NO2 concentration ratio below 8.3 ([DPO3]/[8h-NO2] ≤ 8.3). The ozone production and controlling effects of VOCs and NOx in different cities of China were compared according to their historical O3-NO2 evolution routes. The approach developed herein may have broad application potential for evaluating the efficiency of precursor controls and further mitigating O3 pollution, in particular, for regions where comprehensive photochemical studies are unavailable.

Investigating missing sources of glyoxal over China using a regional air quality model (RAMS-CMAQ).

Currently, modeling studies tend to significantly underestimate observed space-based glyoxal (CHOCHO) vertical column densities (VCDs), implying the existence of missing sources of glyoxal. Several recent studies suggest that the emissions of aromatic compounds and molar yields of glyoxal in the chemical mechanisms may both be underestimated, which can affect the simulated glyoxal concentrations. In this study, the influences of these two factors on glyoxal amounts over China were investigated using the RAMS-CMAQ modeling system for January and July 2014. Four sensitivity simulations were performed, and the results were compared to satellite observations. These results demonstrated significant impacts on glyoxal concentrations from these two factors. In case 1, where the emissions of aromatic compounds were increased three-fold, improvements to glyoxal VCDs were seen in high anthropogenic emissions regions. In case 2, where molar yields of glyoxal from isoprene were increased five-fold, the resulted concentrations in July were 3-5-fold higher, achieving closer agreement between the modeled and measured glyoxal VCDs. The combined changes from both cases 1 and 2 were applied in case 3, and the model succeeded in further reducing the underestimations of glyoxal VCDs. However, the results over most of the regions with pronounced anthropogenic emissions were still underestimated. So the molar yields of glyoxal from anthropogenic precursors were considered in case 4. With these additional mole yield changes (a two-fold increase), the improved concentrations agreed better with the measurements in regions of the lower reaches of the Yangtze River and Yellow River in January but not in July.

Oxidative potential and inflammatory impacts of source apportioned ambient air pollution in Beijing.

Air pollution exposure is associated with a range of adverse health impacts. Knowledge of the chemical components and sources of air pollution most responsible for these health effects could lead to an improved understanding of the mechanisms of such effects and more targeted risk reduction strategies. We measured daily ambient fine particulate matter (<2.5 µm in aerodynamic diameter; PM2.5) for 2 months in peri-urban and central Beijing, and assessed the contribution of its chemical components to the oxidative potential of ambient air pollution using the dithiothreitol (DTT) assay. The composition data were applied to a multivariate source apportionment model to determine the PM contributions of six sources or factors: a zinc factor, an aluminum factor, a lead point factor, a secondary source (e.g., SO4(2-), NO3(2-)), an iron source, and a soil dust source. Finally, we assessed the relationship between reactive oxygen species (ROS) activity-related PM sources and inflammatory responses in human bronchial epithelial cells. In peri-urban Beijing, the soil dust source accounted for the largest fraction (47%) of measured ROS variability. In central Beijing, a secondary source explained the greatest fraction (29%) of measured ROS variability. The ROS activities of PM collected in central Beijing were exponentially associated with in vivo inflammatory responses in epithelial cells (R2=0.65-0.89). We also observed a high correlation between three ROS-related PM sources (a lead point factor, a zinc factor, and a secondary source) and expression of an inflammatory marker (r=0.45-0.80). Our results suggest large differences in the contribution of different PM sources to ROS variability at the central versus peri-urban study sites in Beijing and that secondary sources may play an important role in PM2.5-related oxidative potential and inflammatory health impacts.

Modeling study on seasonal variation in aerosol extinction properties over China.

To investigate the seasonal variation of aerosol optical depth (AOD), extinction coefficient (EXT), single scattering albedo (SSA) and the decomposed impacts from sulfate (SO4(2-)) and black carbon (BC) over China, numerical experiments are conducted from November 2007 to December 2008 by using WRF-Chem. Comparison of model results with measurements shows that model can reproduce the spatial distribution and seasonal variation of AOD and SSA. Over south China, AOD is largest in spring (0.6-1.2) and lowest in summer (0.2-0.6). Over north, northeast and east China, AOD is highest in summer while lowest in winter. The high value of EXT under 850 hPa which is the reflection of low visibility ranges from 0.4-0.8 km(-1) and the high value area shifts to north during winter, spring and summer, then back to south in autumn. SSA is 0.92-0.94 in winter and 0.94-0.96 for the other three seasons because of highest BC concentration in winter over south China. Over east China, SSA is highest (0.92-0.96) in summer, and 0.88-0.92 during winter, spring and autumn as the concentration of scattering aerosol is highest while BC concentration is lowest in summer over this region. Over north China, SSA is highest (0.9-0.94) in summer and lowest (0.82-0.86) in winter due to the significant variation of aerosol concentration. The SO4(2-) induced EXT increases about 5%-55% and the impacts of BC on EXT is much smaller (2%-10%). The SO4(2-)-induced increase in SSA is 0.01-0.08 and the BC-induced SSA decreases 0.02-0.18.

Nocturnal ozone enhancement in Shandong Province, China, in 2020-2022: Spatiotemporal distribution and formation mechanisms.

Nighttime ozone enhancement (NOE) can increase the oxidation capacity of the atmosphere by stimulating nitrate radical formation and subsequently facilitating the formation of secondary pollutants, thereby affecting air quality in the following days. Previous studies have demonstrated that when nocturnal ozone (O3) concentrations exceed 80 µg/m3, it leads to water loss and reduction of plant yields. In this study, the characteristics and mechanisms of NOE over Shandong Province as well as its 16 cities were analyzed based on observed hourly O3 concentrations from 2020 to 2022. The analysis results show that NOE predominantly occurred in the periods of 0:00-3:00 (41 %). The annual mean frequency of NOE events was ~64 days/year, approximately 4-7 days per month. The average concentration of nocturnal O3 peak (NOP) was ~72.6 µg/m3. Notably, high NOP was observed in the period from April to September with the maximum in June. Coastal cities experienced more NOE events. Typical NOE events characterized by high NOP concentrations in the coastal cities of QingDao, WeiHai and YanTai in June 2021 were selected for detailed analysis with a regional chemical transport model. The results showed that high levels of O3 in eastern coastal cities during NOE events primarily originate from horizontal transport over the sea, followed by vertical transport. During the daytime, O3 and its precursors are transported to the Yellow Sea by westerly winds, leading to the accumulation of O3 near the sea and coastline. Consequently, under the influence of prevailing winds, the movement of O3 pollution belts from the sea to land causes rapid increases in near-surface O3 levels. Meanwhile, vertical transport can also contribute to NOE in coastal areas. The high-level O3 in the upper atmosphere generally originates from long-distance transport and turbulent transport of O3 produced near the ground during the daytime. At night, the absence of chemicals that consume O3 in the upper air and descending air flow carries O3 to the near-surface. The impacts of other O3-depletion processes (such as dry deposition) on NOE are less pronounced than those of transport processes.

Assessment of surface ozone production in Qinghai, China with satellite-constrained VOCs and NOx emissions.

The implementation of the western development strategy of China and the migration of air pollutants from eastern China might lead to a rapid increase in volatile organic compounds (VOCs) concentrations in the Qinghai-Tibet Plateau (QTP) and an amplified role of Nitrogen Oxides (NOx) in ozone (O3) pollution. Qinghai province, situated on the northeast of the QTP, had fewer human activities compared to eastern China, while ozone pollution worsened over the years. To better capture recent emission trends and improve the accuracy of O3 simulation in Qinghai, this study proposed a top-down method, which combined the air quality modeling system RAMS-CMAQ, with formaldehyde (HCHO) and nitrogen dioxide (NO2) columns derived from TROPOMI as the constraints to improve the emission estimates of VOCs and NOx in July 2020, respectively. Through a series of sensitivity experiments, better quantified emission estimates of VOCs and NOx were obtained to be 1.33 and 0.34 Tg/yr, 2.5 and 2.1 times larger than the bottom-up ones. The results demonstrated the effectiveness of the top-down method and satellite observations constraints in improving VOCs and NOx emission estimates, resulting in a reduction in the differences between the observed and modeled HCHO and NO2 columns to 0.7 and 0.2 × 1015 molec/cm2, respectively. As a result, the simulated maximum daily 8-h average (MDA8) O3 concentrations increased from 58.9 to 81.6 µg/m3, which were closer to observations (85.4 µg/m3), the normalized mean bias (NMB) and normalized mean error (NME) values of hourly O3 concentrations changed from -24.7 % to -2.9 % and from 29.9 % to 22.3 %, respectively. This study showed the potential of top-down estimates to aide in the development of emission scenarios, which were critical for accurately simulating the O3 pollution and pollution control policy studies.

Mapping high-resolution energy consumption CO2 emissions in China by integrating nighttime lights and point source locations.

High-resolution CO2 emission inventories are essential to accurately assess spatiotemporal patterns of carbon emissions, analyze factors affecting carbon emissions, and develop sound emission reduction policies. The top-down approach is often used to map CO2 emissions from energy consumption due to its simplicity. However, the spatial proxy variables commonly used in this method, such as nighttime light (NL), land use, and population, are difficult to reflect the spatial distribution of CO2 emissions from large point sources. Therefore, this study uses the active fire product provided by Visible Infrared Imaging Radiometer Suite (VIIRS) sensors on Suomi National Polar-Orbiting Partnership (Suomi-NPP) satellite to extract the location of industrial heat sources in China, and then develops an improved CO2 emission estimation model by integrating industrial heat sources, Global Energy Monitor (GEM) power plant location and nighttime lights. The model is used to map CO2 emissions from energy consumption at a resolution of 1 km*1 km from 2012 to 2019 in China. It is found that the overall accuracy of the model is greatly improved at the provincial level, the R2 value is >0.75, and RMSE is distributed in 40-110 Mt. At the grid level, the improved model allocates more carbon emissions to the grid where the point source is located, which makes the spatial distribution of CO2 emissions more reasonable.

High-resolution atmospheric mercury emission from open biomass burning in China: Integration of localized emission factors and multi-source finer resolution remote sensing data.

Mercury (Hg) emissions from open biomass burning represent one of the largest Hg inputs to the atmosphere, with considerable effects on the atmospheric Hg budget. However, there is currently large uncertainty in the inventory of Hg emissions from open biomass burning in China due to limitations on the coarse resolution of burned area products, rough biomass data, and the unavailability of suitable emission factors (EFs). In this study, we developed high tempo-spatial resolution (30 m) and long time-series (2000-2019) atmospheric Hg emission inventories from open biomass burning using the Global Annual Burned Area Map (GABAM) product, high-resolution biomass map, Landsat-based tree cover datasets as well as local EFs in China. The results showed that the average annual Hg emission from open biomass burning in China amounted to 172.6 kg during 2000-2019, with a range of 63-398.5 kg. The largest Hg emissions were found in cropland (72%), followed by forest (25.9%), and grassland (2.1%). On a regional level, Northeast China (NE) and Southwest China (SW) were the two main contributors, together accounting for more than 60% of total Hg emissions. The temporal distribution of Hg emissions showed that the peaks occurred in 2003 and 2014. This is a comprehensive estimation of Hg emissions from open biomass burning in China by integrating various high-resolution remotely sensed data and nationwide localized EFs, which has important implications for understanding the role of open biomass burning in China in regional and global atmospheric Hg budget.

Increases in ozone-related mortality in China over 2013-2030 attributed to historical ozone deterioration and future population aging.

We systematically examine historical and future changes in premature respiratory mortalities attributable to ozone (O3) exposure (O3-mortality) in China and identify the leading cause of respective change for the first time. The historical assessment for 2013-2019 is based on gridded O3 concentrations generated by a multi-source-data-fusion algorithm; the future prediction for 2019-2030 uses gridded O3 concentrations projected by four Coupled Model Intercomparison Project Phase 6 (CMIP6) models under three Shared Socioeconomic Pathways (SSP) scenarios. During 2013-2019, national annual O3-mortality is 176.3 thousand (95%CI: 123.5-224.0 thousand) averaged over 2013-2019 with an increasing trend of 14.1 thousand yr-1 (95%CI: 10.2-17.4 thousand yr-1); sensitivity experiments show that the O3-mortality varies at a rate of +12.7 (95%CI: 9.2-15.6), +5.8 (95%CI: 4.0-7.4), +1.0 (95%CI: 0.7-1.2), -5.4 (95%CI: -6.9 to -3.7) thousand yr-1, owing to changes in O3 concentration, population age structure, population size, mortality rate for respiratory disease, respectively. The deterioration of O3 air quality, shown as significant increase in O3 concentration, is identified as the primary factor which contributes 90.1 % of 2013-2019 O3-mortality rise. Compared with O3-mortality estimated in this study, the widely-used O3-mortality assessment method based on urban-site-dominant O3 measurements generates close national O3-mortality but overestimates (underestimates) provincial O3-mortality in coastal (central) provinces. From 2019 to 2030, national O3-mortality is projected to increase by 50.4-103.7 thousand under different SSP scenarios. The change in age structure (i.e. population aging) alone will result in significant O3-mortality rises of 137.9-160.5 thousand. Compared with 2013-2019 rapid O3 increase (+2.5 µg m-3 yr-1 at national level), O3 concentrations are projected to increase at a lower rate (+0.4 µg m-3 yr-1 in SSP5-8.5) or even decrease (-0.7 µg m-3 yr-1 in SSP1-2.6) from 2019 to 2030. Therefore, population aging, in place of O3 air quality deterioration, will become the leading cause of future O3-mortality rises during the coming decade.

Mercury and cadmium contamination in traffic soil of Beijing, China.

The concentrations of mercury and cadmium in surface soil along the three ring road with high traffic density from Beijing, China, were investigated. The concentrations of cadmium and mercury in urban soils were in the range of 0.086-1.59 and 0.16-3.68 mg kg(-1), which exceeded the upper limits of potential toxic elements concentration for agricultural soils of China (PTE-MPC). The metal elements of cadmium and mercury showed similar spatial distribution and seasonal variations. Concentrations of cadmium and mercury in surface soil along the three ring road were significantly higher than previous reported values, which was attributed to the increased traffic density in Beijing. Results from this study demonstrated that the soil of the three ring road had endured severe cadmium and mercury pollution.

Sensitivity analysis of surface ozone to emission controls in Beijing and its neighboring area during the 2008 Olympic Games.

The regional air quality modeling system RAMS (regional atmospheric modeling system)-CMAQ (community multi-scale air quality modeling system) is applied to analyze temporal and spatial variations in surface ozone concentration over Beijing and its surrounding region from July to October 2008. Comparison of simulated and observed meteorological elements and concentration of nitrogen oxides (NOx) and ozone at one urban site and three rural sites during Olympic Games show that model can generally reproduce the main observed feature of wind, temperature and ozone, but NOx concentration is overestimated. Although ozone concentration decreased during Olympics, high ozone episodes occurred on 24 July and 24 August with concentration of 360 and 245 microg/m3 at Aoyuncun site, respectively. The analysis of sensitive test, with and without emission controls, shows that emission controls could reduce ozone concentration in the afternoon when ozone concentration was highest but increase it at night and in the morning. The evolution of the weather system during the ozone episodes (24 July and 24 August) indicates that hot and dry air and a stable weak pressure field intensified the production of ozone and allowed it to accumulate. Process analysis at the urban site and rural site shows that under favorable weather condition on 24 August, horizontal transport was the main contributor of the rural place and the pollution from the higher layer would be transported to the surface layer. On 24 July, as the wind velocity was smaller, the impact of transport on the rural place was not obvious.

OMI formaldehyde column constrained emissions of reactive volatile organic compounds over the Pearl River Delta region of China.

In recent years, surface ozone (O3) concentration was high and became the primary air pollutant in the Pearl River Delta (PRD) region. However, as precursors of tropospheric O3, the emissions of reactive nonmethane volatile organic compounds (NMVOCs) were reported to have large uncertainties. Here, combined with the simulated formaldehyde (HCHO) columns from the RAMS-CMAQ modeling system, formaldehyde (HCHO) columns derived from the Ozone Monitoring Instrument (OMI) were used as the constraints to improve the emission estimates of the reactive NMVOCs through the linear regression method over the PRD region in March of 2017. The observed highest HCHO concentration was 2-4 times as high as the original simulated results over the PRD region mostly due to the underestimation in the reactive NMVOC emissions, especially the anthropogenic sources. With the regression coefficients calculated through five sensitivity simulation cases as well as the observed HCHO column, the better quantified emissions of reactive NMVOCs were obtained over the PRD region. It showed that the total emissions of reactive NMVOCs were improved by a factor of 2.1. The emissions derived from anthropogenic, biomass burning and biogenic sources increased from 0.0329, 4.69 × 10-4 and 0.0524 Tg/month to 0.0959, 0.0215 and 0.0620 Tg/month, respectively. As a result, the difference between the observed and modeled high HCHO column decreased to 1-2.5 times, which may be dominated by the enhanced reactive NMVOC emissions derived from anthropogenic sources. Besides, the great improvement in the emissions of reactive NMVOCs contributed to an increase of 20-40 µg/m3 in the maximum daily 8-h average (MDA8) O3 concentration over the PRD region.

Model analysis of vertical exchange of boundary layer ozone and its impact on surface air quality over the North China Plain.

In addition to photochemical production and horizontal regional transport, surface O3 concentration can also likely be affected by vertical transport, which is not well known so far. The process analysis was conducted by using the Regional Atmospheric Modeling System Community Multiscale Air Quality (RAMS-CMAQ) model to investigate photochemical production and the vertical transport mechanism of boundary-layer O3 during a typical O3 pollution episode in the North China Plain (NCP), and further quantify the contribution of vertical transport to surface O3. The diurnal variations of vertical budgets of O3 and NO2 in the boundary layer at multiple sites showed that there were substantial differences in the vertical distribution of O3 production and transport between urban and suburban/rural areas. In urban areas, surface O3 is consumed by titration reaction to generate NO2, which is then transported to the upper boundary layer and produces O3 by photochemical reaction. With the development of the boundary layer, the upper-layer O3 stored in the residual layer at nighttime can be transported vertically to the surface as the turbulent diffusion intensifies the next morning. While in suburban and rural areas, the vertical transport is relatively weaker because the photochemical formation of O3 occurs in the whole boundary layer, although it decreases slightly with the altitude. Model simulation showed that 20.6-27.9% of urban surface O3 changes in the morning (09:00-10:00 LST) was attributable to the downward transport from the residual layer, while it is 15.0-22.1% at suburban site. The vertical transport from above the boundary layer contributed 24.0-63.6% to daytime urban surface O3 changes, which was weak in suburban areas. Differences and similarities in O3 formation and transport mechanism in urban and suburban regions revealed here highlight the importance of earlier control and regional collaboration.

Simulated seasonal variations in wet acid depositions over East Asia.

The air quality modeling system Regional Atmospheric Modeling System-Community Multi-scale Air Quality (RAMS-CMAQ) was applied to analyze temporospatial variations in wet acid deposition over East Asia in 2005, and model results obtained on a monthly basis were evaluated against extensive observations, including precipitation amounts at 704 stations and SO4(2-), NO3-, and NH4+ concentrations in the atmosphere and rainwater at 18 EANET (the Acid Deposition Monitoring Network in East Asia) stations. The comparison shows that the modeling system can reasonably reproduce seasonal precipitation patterns, especially the extensive area of dry conditions in northeast China and north China and the major precipitation zones. For ambient concentrations and wet depositions, the simulated results are in reasonable agreement (within a factor of 2) with observations in most cases, and the major observed features are mostly well reproduced. The analysis of modeled wet deposition distributions indicates that East Asia experiences noticeable variations in its wet deposition patterns throughout the year. In winter, southern China and the coastal areas of the Japan Sea report higher S04(2-) and NO3- wet depositions. In spring, elevated SO4(2-) and NO3-wet depositions are found in northeastern China, southern China, and around the Yangtze River. In summer, a remarkable rise in precipitation in northeastern China, the valleys of the Huaihe and Yangtze rivers, Korea, and Japan leads to a noticeable increase in SO4(2-) and NO3- wet depositions, whereas in autumn, higher SO4(2-) and NO3-wet depositions are found around Sichuan Province. Meanwhile, due to the high emission of SO2, high wet depositions of SO4(2-) are found throughout the entire year in the area surrounding Sichuan Province. There is a tendency toward decreasing NO3- concentrations in rainwater from China through Korea to Japan in both observed and simulated results, which is a consequence of the influence of the continental outflow from Eurasia. The same tendency is not found for SO4(2-).

Visual range trends in the Yangtze River Delta Region of China, 1981-2005.

Visual range (VR) data from 1981 to 2005 were examined for 20 meteorological monitoring sites in the Yangtze River Delta Region of China. Cumulative percentile analysis was used to construct VR trend. The 25-yr average domain-average 50% VR was approximately 21.9 +/- 1.9 km. Domain-average 50% VR decreased from 1981 to 2005 with a trend of -2.41 km/decade. The worst 20% and 50% and best 20% VR and variation rates for the 20 sites were analyzed. The 50% VR of the town, county-level city, and prefecture-level city sites were 24.1, 21.5, and 19.4 km, respectively. The best 20% VR decreased fastest with a rate of -3.5 km/decade. Regional median VR decreased from the coastal sites to the inland sites. Ridit analysis and cumulative percentile were adopted to study the VR variation properties between economically developed areas (e.g., Nanjing and Hangzhou) and remote areas (e.g., Lvsi). The two analyses show that VR decreased in Nanjing and Hangzhou but remained constant in Lvsi from 1981 to 2005.

Model analysis of meteorology and emission impacts on springtime surface ozone in Shandong.

Air quality observations showed that surface ozone (O3) concentrations over Shandong increased significantly in springtime in recent years, especially 2017. The observed 90th percentile of hourly O3 concentrations (O3-h_90) in May increased from 148.4 µg/m3 in 2016 to 176.2 µg/m3 in 2017. To investigate the reasons of significant increase of O3 in spring of 2017, seven sensitivity cases were performed with the RAMS-CMAQ modeling system to identify the impacts of meteorological conditions (M) and emissions (E) on O3 concentrations in May of Shandong during the time period 2016-2018. The regional O3-h_90 in May of Shandong were 103.0, 120.3 and 86.3 µg/m3 in 2016, 2017 and 2018, respectively. It was found that the positive effects from favorable meteorology were the dominant reasons that resulted in the high O3 concentration in May 2017. When compared to 2017 standard simulation (17E17M), the differences of meteorological conditions led to the decrease of 17.5 and 33.8 µg/m3 in regional O3-h_90 of May in 2016 (17E16M) and 2018(17E18M), while small changes (0.6 and - 0.3 µg/m3) appeared in that of May 2016 (16E17M) and 2018(18E17M) due to emission differences. Since there were few differences in the wind speeds of May between three years, the higher temperature and lower relative humidity significantly contributed to O3 formation in May 2017 compared to May of 2016 and 2018. Besides, the amount of cloud fraction (CF), which has an indirect influence on the surface temperature and photochemical production of ozone by its impacts on the insolation, was the least in May 2017 compared to that in May 2016 and 2018 over Shandong. The distributions of changes in CF had obvious negative correlations with that of O3 vertical column concentrations and temperature. Thus, the fewer cloud fraction may play a key role in O3 formation of May 2017.

[Impact of Pollutant Emission Reduction on Air Quality During the COVID-19 Pandemic Control in Early 2020 Based on RAMS-CMAQ].

In this work, the relationships between air quality and pollutant emissions were investigated during the COVID-19 pandemic in Shandong Province. During the quarantine period (from January 24 to February 7, 2020), the concentrations of atmospheric pollutants decreased significantly relative to the period before controls were imposed (from January 15 to 23, 2020). Specifically, except for an increase in the concentration of O3, concentrations of PM10, PM2.5, NO2, SO2, and CO decreased for 72.6 µg·m-3 (45.86%), 47.4 µg·m-3(41.24%), 25.6 µg·m-3 (58.00%), 3.0 µg·m-3 (17.71%), and 0.5 mg·m-3 (31.40%), respectively. RAMS-CMAQ simulation showed that meteorological diffusion had an essential role in improving air quality. Influenced by meteorological factors, emissions of PM10, PM2.5, NO2, SO2, and CO were reduced 26.04%, 33.03%, 28.35%, 43.27%, and 23.29%, respectively. Furthermore, the concentrations of PM10, PM2.5, NO2, SO2, and CO were reduced by 19.82%, 8.21%, 29.65%, -25.56%, and 8.12%, respectively, due to pollution emissions reductions during the quarantine period. O3 concentrations increased by 20.51% during quarantine, caused by both meteorological factors (10.47%) and human activities (10.04%). These results indicate that primary pollutants were more sensitive to emissions reductions; however, secondary pollutants demonstrated a lagged response the emissions reduction and were significantly affected by meteorological factors. The linear relationship between ozone and the emissions reduction was not significant, and was inverse overall. Further investigation are now required on the impact of emissions reduction on ozone pollution control.

Observation of nitrous acid (HONO) in Beijing, China: Seasonal variation, nocturnal formation and daytime budget.

Seasonal characteristics of atmospheric nitrous acid (HONO) were investigated with high time-resolution field measurements at an urban site of Beijing in four select months (representing four different seasons) from September 2015 to July 2016. The HONO concentrations displayed a pronounced seasonal profile with a maximum in autumn (2.27±1.82ppb) and a minimum in winter (1.05±0.89ppb). Significant diurnal cycles were also observed during the whole campaign. We found that the nighttime build-up of HONO was attributed to the heterogeneous conversion of NO2 on wet surface. The calculated NO2 to HONO conversion frequencies varied from 0.005h-1 in spring to 0.010h-1 in summer, with an average value of 0.008h-1. The seasonality of these conversion frequencies was closely related to the RH levels in different seasons. During daytime, large additional HONO sources were calculated. The noontime additional source was the highest in autumn 3.82ppbh-1, followed by summer 3.05ppbh-1, spring 2.63ppbh-1 and winter 1.30ppbh-1. Correlation studies between the additional HONO source and related parameters demonstrated that the controlling processes responsible for HONO daytime formation varied in different seasons, and that the photo-enhanced formation on wet surface or the photolysis of adsorbed nitric acid and nitrate could be potential HONO sources in Beijing.

Temporal variability of ammonia emission potentials for six plant species in an evergreen subtropical forest in southwest China.

The temporal variability of leaf ammonia (NH3) emission potentials (the ratio of leaf tissue ammonium to proton concentration) and nitrogen (N) pools of six dominant plant species were investigated at the Tieshanping (TSP) forested catchment, southwest China. The results showed that the NH3 emission potentials and N pools presented small variations among seasons, which were mainly controlled by plant species and the leaf age. Also, high emission potential in one species did not correspond to high tissue N content. Specifically, the Chinese fir (Cunninghamia lanceolata) had higher NH3 emission potential (mean: 46.2) but lower N content (mean: 1.6% of Dw). The leaf privet (Ligustrum quihoui Carr.) was with the moderate emission potential (15) and the highest N content (2.7% of Dw) on average, which for the Masson pine (Pinus massoniana) were both low. Overall, the emission potentials of the six species were too low (<200) to build up a sufficiently high NH3 partial pressure in the leaves. Therefore, the Masson pine dominant subtropical forest at TSP acts as a sink for the atmospheric NH3, indicating that using the N flux in throughfall only may significantly underestimate the N income of the ecosystem. The results are informative for future modeling of plant-atmosphere NH3 exchange and estimating N budget in local or regional scales.

Large-scale transport of PM2.5 in the lower troposphere during winter cold surges in China.

A comprehensive investigation using the air quality network and meteorological data of China in 2015 showed that PM2.5 driven by cold surges from the ground level could travel up to 2000 km from northern to southern China within two days. Air pollution is more severe and prominent during the winter in north China due to seasonal variations in energy usage, trade wind movements, and industrial emissions. In February 2015, two cold surges traveling from north China caused a temporary increase in the concentration of PM2.5 in Shanghai. Subsequently, the concentration of PM2.5 in Xiamen increased to a high of 80 µg/m3, which is double the average PM2.5 concentration in Xiamen during the winter. This finding is a new long-range transport mechanism comparing to the well-established mechanism, with long-range transport more likely to occur in the upper troposphere than at lower levels. These observations were validated by results from the back trajectory analysis and the RAMS- CMAQ model. While wind speed was found to be a major facilitator in transporting PM2.5 from Beijing to Xiamen, more investigation is required to understand the complex relationship between wind speed and PM2.5 and how it moderates air quality in Beijing, Shanghai, and Xiamen.