: By analyzing the data of urban air pollutant measurements from 2013 to 2015 in Nanjing, East China, we found that the correlation coefficients between major atmospheric compound pollutants PM_2.5 and O₃ were respectively 0.40 in hot season (June, July and August) and −0.16 in cold season (December, January and February) with both passing the confidence level of 99%. This provides evidence for the inverse relations of ambient PM_2.5 and O₃ between cold and hot seasons in an urban area of East China. To understand the interaction of PM_2.5 and O₃ in air compound pollution, the underlying mechanisms on the inversion relations between cold and hot seasons were investigated from the seasonal variations in atmospheric oxidation and radiative forcing of PM_2.5 based on three-year environmental and meteorological data. The analyses showed that the augmentation of atmospheric oxidation could strengthen the production of secondary particles with the contribution up to 26.76% to ambient PM_2.5 levels. High O₃ concentrations in a strong oxidative air condition during hot season promoted the formation of secondary particles, which could result in a positive correlation between PM_2.5 and O₃ in hot season. In cold season with weak atmospheric oxidation, the enhanced PM_2.5 levels suppressed surface solar radiation, which could weaken O₃ production for decreasing ambient O₃ level with the low diurnal peaks. Under the high PM_2.5 level exceeding 115 μg·m⁻³, the surface O₃ concentration dropped to 12.7 μg·m⁻³ at noon with a significant inhibitory effect, leading to a negative correlation between PM_2.5 and O₃ in cold season. This observational study revealed the interaction of PM_2.5 and O₃ in air compound pollution for understanding the seasonal change of atmospheric environment.

Severe ozone (O₃) pollution episodes plague a few regions in eastern China at certain times of the year, e.g., the Yangtze River Delta (YRD). However, the formation mechanisms, including meteorological factors, contributing to these severe pollution events remain elusive. A severe summer smog stretched over the YRD region from 22 to 25 August 2016. This event displayed hourly surface O₃ concentrations that exceeded 300 µg m⁻³ on 25 August in Nanjing, an urban area in the western YRD. The weather pattern during this period was characterized by near-surface prevailing easterly winds and continuous high air temperatures. The formation mechanism responsible for this O₃ pollution episode over the YRD region, particularly the extreme values over the western YRD, was investigated using observation data and by running simulations with the Weather Research and Forecasting model with Chemistry (WRF-Chem). The results showed that the extremely high surface O₃ concentration in the western YRD area on 25 August was largely due to regional O₃ transport in the nocturnal residual layer (RL) and the diurnal change in the atmospheric boundary layer. On 24 August, high O₃ levels, with peak values of 220 µg m⁻³, occurred in the daytime mixing layer over the eastern YRD region. During nighttime from 24 to 25 August, a shallow stable boundary layer formed near the surface which decoupled the RL above it from the surface. Ozone in the decoupled RL remained quite constant, which resulted in an O₃-rich reservoir forming in this layer. This reservoir persisted due to the absence of O₃ consumption from nitrogen oxide (NO) titration or dry deposition during nighttime. The prevailing easterly winds in the lower troposphere governed the regional transport of this O₃-rich air mass in the nocturnal RL from the eastern to the western YRD. As the regional O₃ transport reached the RL over the western YRD, O₃ concentrations in the RL accumulated and rose to 200 µg m⁻³ over the western Nanjing site during the sunrise hours on 25 August. The development of the daytime convective boundary layer after sunrise resulted in the disappearance of the RL, as the vertical mixing in the convective boundary layer uniformly redistributed O₃ from the upper levels via the entrainment of O₃-rich RL air down to the O₃-poor air at the ground. This net downward transport flux reached up to 35 µg m⁻³ h⁻¹, and contributed a considerable surface O₃ accumulation, resulting in severe daytime O₃ pollution during the summer smog event on 25 August in the western YRD region. The mechanism of regional O₃ transport through the nocturnal RL revealed in this study has great implications regarding understanding O₃ pollution and air quality change.

Investigation of quantitative predictions of precipitation amounts and forecasts of drought events are conducive to facilitating early drought warnings. However, there has been limited research into or modern statistical analyses of precipitation and drought over Northeast China, one of the most important grain production regions. Therefore, a case study at three meteorological sites which represent three different climate types was explored, and we used time series analysis of monthly precipitation and the grey theory methods for annual precipitation during 1967–2017. Wavelet transformation (WT), autoregressive integrated moving average (ARIMA) and long short-term memory (LSTM) methods were utilized to depict the time series, and a new hybrid model wavelet-ARIMA-LSTM (W-AL) of monthly precipitation time series was developed. In addition, GM (1, 1) and DGM (1, 1) of the China Z-Index (CZI) based on annual precipitation were introduced to forecast drought events, because grey system theory specializes in a small sample and results in poor information. The results revealed that (1) W-AL exhibited higher prediction accuracy in monthly precipitation forecasting than ARIMA and LSTM; (2) CZI values calculated through annual precipitation suggested that more slight drought events occurred in Changchun while moderate drought occurred more frequently in Linjiang and Qian Gorlos; (3) GM (1, 1) performed better than DGM (1, 1) in drought event forecasting.

Air pollutant emissions play a determinant role in deteriorating air quality. However, an uncertainty in emission inventories is still the key problem for modeling air pollution. In this study, an updated emission inventory of coal-fired power plants (UEIPP) based on online monitoring data in Jiangsu Province of East China for the year of 2012 was implemented in the widely used Multi-resolution Emission Inventory for China (MEIC). By employing the Weather Research and Forecasting model with Chemistry (WRF-Chem), two simulation experiments were executed to assess the atmospheric environment change by using the original MEIC emission inventory and the MEIC inventory with the UEIPP. A synthetic analysis shows that power plant emissions of PM_2.5, PM₁₀, SO₂, and NO_x were lower, and CO, black carbon (BC), organic carbon (OC) and NMVOCs (non-methane volatile organic compounds) were higher in UEIPP relative to those in MEIC, reflecting a large discrepancy in the power plant emissions over East China. In accordance with the changes in UEIPP, the modeled concentrations were reduced for SO₂ and NO₂, and increased for most areas of primary OC, BC, and CO. Interestingly, when the UEIPP was used, the atmospheric oxidizing capacity significantly reinforced. This was reflected by increased oxidizing agents, e.g., O₃ and OH, thus directly strengthening the chemical production from SO₂ and NO_x to sulfate and nitrate, respectively, which offset the reduction of primary PM_2.5 emissions especially on haze days. This study indicates the importance of updating air pollutant emission inventories in simulating the complex atmospheric environment changes with implications on air quality and environmental changes.

Weekly vertical profiles of ammonia (NH₃) were measured at 16 heights on the Beijing 325 m meteorological tower for 1 year from March 2016 to March 2017. The average NH₃ concentrations exceeded 4 µg m⁻³ at all heights with an overall average (±1σ) value of 13.3 (±4.8) µg m⁻³. The highest NH₃ concentrations along the vertical profiles mostly occurred from 32 to 63 m, decreasing both towards the surface and at higher altitudes. Significant decreases in NH₃ concentrations were only found at the top two heights (280 and 320 m). These results suggest an NH₃ rich atmosphere during all seasons in urban Beijing, from the ground to at least 320 m. The highest seasonal NH₃ concentrations across the profile were observed in summer (18.2 µg m⁻³) with high temperature, followed by spring (13.4 µg m⁻³), autumn (12.1 µg m⁻³) and winter (8.3 µg m⁻³). A significant vertical variation in the NH₃ concentration was only found in summer. Source region analyses suggest that air masses from intensive agricultural regions to the south contribute most to the high NH₃ concentrations in Beijing. Local sources such as traffic emissions also appear to be important contributors to atmospheric NH₃ in this urban environment.