Open biomass burning (OBB) has significant impacts on air pollution, climate change and potential human health. OBB has gathered wide attention but with little focus on the annual variation of pollutant emission. Central and eastern China (CEC) is one of the most polluted regions in China. This study aims to provide a state-of-the-art estimation of the pollutant emissions from OBB in CEC from 2003 to 2015, by adopting the satellite observation dataset – the burned area product (MCD64Al) and the active fire product (MCD14 ML) – along with local biomass data (updated biomass loading data and high-resolution vegetation data) and local emission factors. The successful adoption of the double satellite dataset for long-term estimation of pollutants from OBB with a high spatial resolution can support the assessing of OBB on regional air quality, especially for harvest periods or dry seasons. It is also useful to evaluate the effects of annual OBB management policies in different regions. Here, monthly emissions of pollutants were estimated and allocated into a 1×1 km spatial grid for four types of OBB including grassland, shrubland, forest and cropland. From 2003 to 2015, the emissions from forest, shrubland and grassland fire burning had an annual fluctuation, whereas the emissions from crop straw burning steadily increased. The cumulative emissions of organic carbon (OC), elemental carbon (EC), methane (CH₄), nitric oxide (NO_x), non-methane volatile organic compounds (NMVOCs), sulfur dioxide (SO₂), ammonia (NH₃), carbon monoxide (CO), carbon dioxide (CO₂) and fine particles (PM_2.5) were 3.64×10³, 2.87×10², 3.05×10³, 1.82×10³, 6.4×10³, 2.12×10², 4.67×10², 4.59×10⁴, 9.39×10⁵ and 4.13×10³ Gg in these years, respectively. Crop straw burning was the largest contributor for all pollutant emissions, by 84 %–96 %. For the forest, shrubland and grassland fire burning, forest fire burning emissions contributed the most, and emissions from grassland fire were negligible due to little grass coverage in this region. High pollutant emissions concentrated in the connection area of Shandong, Henan, Jiangsu and Anhui, with emission intensity higher than 100 tons per square kilometer, which was related to the frequent agricultural activities in these regions. Peak emission of pollutants occurred during summer and autumn harvest periods including May, June, September and October, during which ∼ 50 % of the total pollutant emissions were emitted in these months. This study highlights the importance of controlling the crop straw burning emissions. From December to March, the crop residue burning emissions decreased, while the emissions from forest, shrubland and grassland exhibited their highest values, leading to another small peak in emissions of pollutants. Obvious regional differences in seasonal variations of OBB were observed due to different local biomass types and environmental conditions. Rural population, agricultural output, economic levels, local burning habits, social customs and management policies were all influencing factors for OBB emissions.

Black carbon (BC), which is formed from the incomplete combustion of fuel sources (mainly fossil fuel, biofuel and open biomass burning), is a chemically inert optical absorber in the atmosphere. It has significant impacts on global climate, regional air quality and human health. During transportation, its physical and chemical characteristics as well as its sources change dramatically. To investigate the properties of BC (i.e., mass concentration, sources and optical properties) during intra-regional transport between the southern edge of the North China Plain (SE-NCP) and central China (CC), simultaneous BC observations were conducted in a megacity (Wuhan – WH) in CC, in three borderline cities (Xiangyang – XY, Suixian – SX and Hong'an – HA; from west to east) between the SE-NCP and CC, and in a city (Luohe – LH) in the SE-NCP during typical winter haze episodes. Using an Aethalometer, the highest equivalent BC (eBC) mass concentrations and the highest aerosol absorption coefficients (σ_abs) were found in LH in the SE-NCP, followed by the borderline cities (XY, SX and HA) and WH. The levels, sources, optical properties (i.e., σ_abs and absorption Ångström exponent, AAE) and geographic origins of eBC were different between clean and polluted periods. Compared with clean days, higher eBC levels (26.4 %–163 % higher) and σ_abs (18.2 %–236 % higher) were found during pollution episodes due to the increased combustion of fossil fuels (increased by 51.1 %–277 %), which was supported by the decreased AAE values (decreased by 7.40 %–12.7 %). The conditional bivariate probability function (CBPF) and concentration-weighted trajectory (CWT) results showed that the geographic origins of biomass burning (BC_bb) and fossil fuel (BC_ff) combustion-derived BC were different. Air parcels from the south dominated for border sites during clean days, with contributions of 46.0 %–58.2 %, whereas trajectories from the northeast showed higher contributions (37.5 %–51.2 %) during pollution episodes. At the SE-NCP site (LH), transboundary influences from the south (CC) exhibited a more frequent impact (with air parcels from this direction comprising 47.8 % of all parcels) on the ambient eBC levels during pollution episodes. At WH, eBC was mainly from the northeast transport route throughout the observation period. Two transportation cases showed that the mass concentrations of eBC, BC_ff and σ_abs all increased, from upwind to downwind, whereas AAE decreased. This study highlights that intra-regional prevention and control for dominant sources at each specific site should be considered in order to improve the regional air quality.

Whether the urban heat island (UHI) is affected by air pollution in urban areas has attracted much attention. By analyzing the observation data of automatic weather stations and environmental monitoring stations in Beijing from 2016 to 2018, we found a seasonally dependent interlink of the UHI intensity (UHII) and PM_2.5 concentration in urban areas. PM_2.5 pollution weakens the UHII in summer and winter night, but strengthens it during winter daytime. The correlation between the UHI and PM_2.5 concentration has been regulated by the interaction of aerosol with radiation, evaporation and planetary boundary layer (PBL) height. The former two change the surface energy balance via sensible and latent heat fluxes, while the latter affects atmospheric stability and energy exchange. In summer daytime, aerosol‐radiation interaction plays an important role, and the energy balance in urban areas is more sensitive to PM_2.5 concentration than in rural areas, thereby weakening UHII. In winter daytime, aerosol‐PBL interaction is dominant, because aerosols lower the PBL height and stabilize atmosphere, weaken the heat exchange with the surrounding, with more heat accumulated in the urban areas and the increased UHII. Changes in evaporation and radiation strengthen the relationship. At night, the change of UHII more depends on the energy stored in the urban canopy. Aerosols effectively reduce the incident energy during daytime, and the long‐wave radiation from the buildings of urban canopy at night becomes less, leading to a weakened UHII. Our analysis results can improve the understanding of climate‐aerosols interaction in megacities like Beijing.

Previous studies indicated that numbers of meso‐micro scale severe convective weather exhibited a significant decrease over China's mainland in the past decades. However, the possible mechanism of the decrease has not been well understood. This paper analyzes the changes in frequency of thunderstorm and lightning days using an updated data set of denser national observational stations and theirs links to the atmospheric circulation and regional environmental factors. The results confirm the highly significant decreasing trend of frequency of thunderstorm and lightning days during 1961–2013, with the decreasing rates of −2.6 days/10 years and −6.5 days/10 years in the warm season, respectively. The decrease of geopotential height difference between south and north, the weakening of westerly jet strength at 200 hPa and the decline of zonal wind speed of high level are found to be the direct reasons for the wide‐spread decrease of thunderstorm and lightning days across the country. Meanwhile, there is a significant positive correlation between thunderstorm days and convective available potential energy (CAPE), indicating a reduction of potential atmospheric instability accompanying the decline of thunderstorm days especially in the south of China. The decrease in the relative humidity at the lower troposphere, the weakening of 0–6 km vertical wind shear and the accompanying slackening of actual convective activity, may have also contributed to the downward trends of the severe convective weather. Our analysis shows a strong link of the decreasing thunderstorm and lightning days to the long‐term changes of the atmospheric circulation and environmental factors. Further investigation is needed to examine what have driven the changes of the environmental factors over the time period.

Landfalling tropical cyclones (LTCs) is one of the most serious meteorological disasters in China due to the provided severe wind and heavy rainfall. Tropical cyclone–induced rainfall in China has been proved to decrease in recent decades. However, how landfalling tropical cyclone–induced extreme rainfall (LTCER) has changed across China, as well as the relationship between LTCER and LTCs remains poorly understood. Accordingly, the spatiotemporal distribution characteristics and long-term changes of LTCER over mainland China during the past 60 years were investigated, by using an Objective Synoptic Analysis Technique to identify LTCER. Mid and high latitudes are exposed to a greater risk of extreme rainfall from northward-moving landfalling tropical cyclones (LTCs). Meanwhile, LTCER tends to increase from 1960 to 2019 across mainland China (characterized by a decrease from 1960 to 1989 and an increase from 1990 to 2019). The LTCER trend exhibits a large spatial difference, with an increase near and to the north of 30°N, but no significant change to the south of 30°N. Moreover, the central latitude of the LTCER zone to the north of 30°N has shifted significantly southwards, while that to the south of 30°N has shifted north. Further analysis revealed that the average latitude of the LTC intensity centers to the north/south of 30°N exhibits the same shift to that of LTCER, indicating that the shift of LTCER has mainly been imposed by the migration of LTCs.