Suppression of convective precipitation by elevated man-made aerosols is responsible for large-scale droughts in north China

It has been proposed that the summer “South Flood–North Drought” (SFND) pattern observed in China over recent decades is caused by the relative impacts of global warming, aerosol loading, and natural variability on regional rainfall (1⇓–3). This conclusion is supported by a recent study by Day et al. (4) in which the SFND is attributed to the changes in the frequency of frontal rain events. Using a technique called the Frontal Rain Event Detection Algorithm for the observations during 1951–2007, decadal changes in the amount and distribution of rainfall in eastern China were found to be overwhelmingly due to changes in frontal rainfall (4). Day et al. conclude that frontal rainfall was envisioned as the product of large-scale frontal convergence and the nonfrontal rainfall was because of local convection, orographic rainfall, and typhoon rainfall. The authors further imply that the shifts in frequency and latitude of frontal rainfall over the recent decades in eastern China reflect changes in large-scale atmospheric circulation

Suppression of convective precipitation by elevated man-made aerosols is responsible for large-scale droughts in north China

It has been proposed that the summer “South Flood–North Drought” (SFND) pattern observed in China over recent decades is caused by the relative impacts of global warming, aerosol loading, and natural variability on regional rainfall (1⇓–3). This conclusion is supported by a recent study by Day et al. (4) in which the SFND is attributed to the changes in the frequency of frontal rain events. Using a technique called the Frontal Rain Event Detection Algorithm for the observations during 1951–2007, decadal changes in the amount and distribution of rainfall in eastern China were found to be overwhelmingly due to changes in frontal rainfall (4). Day et al. conclude that frontal rainfall was envisioned as the product of large-scale frontal convergence and the nonfrontal rainfall was because of local convection, orographic rainfall, and typhoon rainfall. The authors further imply that the shifts in frequency and latitude of frontal rainfall over the recent decades in eastern China reflect changes in large-scale atmospheric circulation

Aerosol-driven droplet concentrations dominate coverage and water of oceanic low level clouds

A lack of reliable estimates of cloud condensation nuclei (CCN) aerosols over oceans has severely limited our ability to quantify their effects on cloud properties and extent of cooling by reflecting solar radiation—a key uncertainty in anthropogenic climate forcing. We introduce a methodology for ascribing cloud properties to CCN and isolating the aerosol effects from meteorological effects. Its application showed that for a given meteorology, CCN explains three-fourths of the variability in the radiative cooling effect of clouds, mainly through affecting shallow cloud cover and water path. This reveals a much greater sensitivity of cloud radiative forcing to CCN than previously reported, which means too much cooling if incorporated into present climate models. This suggests the existence of compensating aerosol warming effects yet to be discovered, possibly through deep clouds

City-level air quality improvement in the Beijing-Tianjin-Hebei region from 2016/17 to 2017/18 heating seasons: Attributions and process analysis

With the implementation of clean air strategies, PM_(2.5) pollution abatement has been observed in the “2 + 26” cities in the Beijing-Tianjin-Hebei (BTH) region (referred to as the BTH2+26) and their surrounding areas. To identify the drivers for PM_(2.5) concentration decreases in the BTH2+26 cites from the 2016/17 heating season (HS1617) to the 2017/18 heating season (HS1718), we investigated the contributions of meteorological conditions and emission-reduction measures by Community Multi-Scale Air Quality (CMAQ) model simulations. The source apportionments of five sector sources (i.e., agriculture, industry, power plants, traffic and residential), and regional sources (i.e., local, within-BTH: other cities within the BTH2+26 cities, outside-BTH, and boundary conditions (BCON)) to the PM_(2.5) decreases in the BTH2+26 cities were estimated with the Integrated Source Apportionment Method (ISAM). Mean PM_(2.5) concentrations in the BTH2+26 cities substantially decreased from 77.4 to 152.5 μg m⁻³ in HS1617 to 52.9–101.9 μg m⁻³ in HS1718, with the numbers of heavy haze (daily PM_(2.5) ≥150 μg m⁻³) days decreasing from 17-77 to 5–30 days. The model simulation results indicated that the PM_(2.5) concentration decreases in most of the BTH2+26 cities were attributed to emission reductions (0.4–55.0 μg m⁻³, 2.3–81.6% of total), but the favorable meteorological conditions also played important roles (1.9–25.4 μg m⁻³, 18.4–97.7%). Residential sources dominated the PM_(2.5) reductions, leading to decreases in average PM_(2.5) concentrations by more than 30 μg m⁻³ in severely polluted cities (i.e., Shijiazhuang, Baoding, Xingtai, and Beijing). Regional source analyses showed that both local and within-BTH sources were significant contributors to PM_(2.5) concentrations for most cities. Emission controls in local and within-BTH sources in HS1718 decreased the average PM_(2.5) concentrations by 0.1–47.2 μg m⁻³ and 0.3–22.1 μg m⁻³, respectively, relative to those in HS1617. Here we demonstrate that a combination of favorable meteorological conditions and anthropogenic emission reductions contributed to the improvement of air quality from HS1617 to HS1718 in the BTH2+26 cities

Aerosol-driven droplet concentrations dominate coverage and water of oceanic low level clouds

A lack of reliable estimates of cloud condensation nuclei (CCN) aerosols over oceans has severely limited our ability to quantify their effects on cloud properties and extent of cooling by reflecting solar radiation—a key uncertainty in anthropogenic climate forcing. We introduce a methodology for ascribing cloud properties to CCN and isolating the aerosol effects from meteorological effects. Its application showed that for a given meteorology, CCN explains three-fourths of the variability in the radiative cooling effect of clouds, mainly through affecting shallow cloud cover and water path. This reveals a much greater sensitivity of cloud radiative forcing to CCN than previously reported, which means too much cooling if incorporated into present climate models. This suggests the existence of compensating aerosol warming effects yet to be discovered, possibly through deep clouds

Significant wintertime PM_(2.5) mitigation in the Yangtze River Delta, China, from 2016 to 2019: observational constraints on anthropogenic emission controls

Ambient fine particulate matter (PM_(2.5)) mitigation relies strongly on anthropogenic emission control measures, the actual effectiveness of which is challenging to pinpoint owing to the complex synergies between anthropogenic emissions and meteorology. Here, observational constraints on model simulations allow us to derive not only reliable PM_(2.5) evolution but also accurate meteorological fields. On this basis, we isolate meteorological factors to achieve reliable estimates of surface PM_(2.5) responses to both long-term and emergency emission control measures from 2016 to 2019 over the Yangtze River Delta (YRD), China. The results show that long-term emission control strategies play a crucial role in curbing PM_(2.5) levels, especially in the megacities and other areas with abundant anthropogenic emissions. The G20 summit hosted in Hangzhou in 2016 provides a unique and ideal opportunity involving the most stringent, even unsustainable, emergency emission control measures. These emergency measures lead to the largest decrease (∼ 35 µg m⁻³, ∼ 59 %) in PM_(2.5) concentrations in Hangzhou. The hotspots also emerge in megacities, especially in Shanghai (32 µg m⁻³, 51 %), Nanjing (27 µg m⁻³, 55 %), and Hefei (24 µg m⁻³, 44 %) because of the emergency measures. Compared to the long-term policies from 2016 to 2019, the emergency emission control measures implemented during the G20 Summit achieve more significant decreases in PM_(2.5) concentrations (17 µg m⁻³ and 41 %) over most of the whole domain, especially in Hangzhou (24 µg m⁻³, 48 %) and Shanghai (21 µg m⁻³, 45 %). By extrapolation, we derive insight into the magnitude and spatial distribution of PM_(2.5) mitigation potential across the YRD, revealing significantly additional room for curbing PM_(2.5) levels

High reduction of ozone and particulate matter during the 2016 G-20 summit in Hangzhou by forced emission controls of industry and traffic

Many regions in China experience air pollution episodes because of the rapid urbanization and industrialization over the past decades. Here we analyzed the effect of emission controls implemented during the G-20 2016 Hangzhou summit on air quality. Emission controls included a forced closure of highly polluting industries, and limiting traffic and construction emissions in the cities and surroundings. Particles with aerodynamic diameter lower than 2.5 μm (PM_(2.5)) and ozone (O_3) were measured. We also simulated air quality using a forecast system consisting of the two-way coupled Weather Research and Forecast and Community Multi-scale Air Quality (WRF-CMAQ) model. Results show PM_(2.5) and ozone levels in Hangzhou during the G-20 Summit were considerably lower than previous to the G-20 Summit. The predicted concentrations of ozone were reduced by 25.4%, whereas the predicted concentrations of PM_(2.5) were reduced by 56%

Predicted impact of thermal power generation emission control measures in the Beijing-Tianjin-Hebei region on air pollution over Beijing, China

Widespread economic growth in China has led to increasing episodes of severe air pollution, especially in major urban areas. Thermal power plants represent a particularly important class of emissions. Here we present an evaluation of the predicted effectiveness of a series of recently proposed thermal power plant emission controls in the Beijing-Tianjin-Hebei (BTH) region on air quality over Beijing using the Community Multiscale Air Quality(CMAQ) atmospheric chemical transport model to predict CO, SO_2, NO_2, PM_(2.5), and PM_(10) levels. A baseline simulation of the hypothetical removal of all thermal power plants in the BTH region is predicted to lead to 38%, 23%, 23%, 24%, and 24% reductions in current annual mean levels of CO, SO_2, NO_2, PM_(2.5), and PM_(10) in Beijing, respectively. Similar percentage reductions are predicted in the major cities in the BTH region. Simulations of the air quality impact of six proposed thermal power plant emission reduction strategies over the BTH region provide an estimate of the potential improvement in air quality in the Beijing metropolitan area, as a function of the time of year