Aerosol Radiative Feedbacks Research Articles

Winter haze amplification by aerosol hygroscopic growth over eastern Indo- Gangetic Plain

Frequent occurrences of widespread winter haze over Northern India largely appear to originate from exceedingly high concentrations of fine particulate matter from anthropogenic emissions. However, the underlying mechanisms driving winter haze in Northern India are not well understood. This study employed a synergy of satellite and reanalysis data from 2006 to 2021 to assess the role of hygroscopic growth of aerosol optical depth in winter haze over the eastern Indo-Gangetic Plain. A method has been developed to extract dry aerosol optical depth from ambient aerosol optical depth to elucidate the origin of winter haze. About 31% of severe haze episodes (aerosol optical depth > 0.85) occurring under ambient humidity conditions decrease to below 5% for dry conditions, indicating the critical role of particle hygroscopic growth. The change in radiative forcing at the top of the atmosphere due to hygroscopic growth is relatively small compared to that at the surface and in the atmosphere, indicating enhanced atmospheric warming. The ubiquitous winter haze over the eastern Indo-Gangetic Plain is exacerbated by hygroscopic growth under high anthropogenic aerosol emissions, further aggravated through aerosol-radiation feedback. These results will be valuable in devising haze forecasts, implementing effective mitigation policies, and representing aerosol hygroscopicity in climate models.

Elucidating the impacts of aerosol radiative effects for mitigating surface O3 and PM2.5 in Delhi, India during crop residue burning period

Atmospheric aerosol radiative effects regulate surface air pollution (O3 and PM2.5) via both the aerosol–photolysis effect (APE) and the aerosol–radiation feedback (ARF) on meteorology. Here, we elucidate the roles of APE and ARF on surface O3 and PM2.5 in the heavily polluted megacity, Delhi, India by using a regional model (WRF-Chem) with constraints from limited surface observations. While APE reduces surface O3 (by 6.1%) and PM2.5 concentrations (by 2.4% via impeding the secondary aerosol formations), ARF contributes to a 2.5% and 17.5% increase in surface O3 and PM2.5, respectively. The ARF from smoke enhances PM2.5 (by 8%), black carbon (by 10%), and primary organic aerosol (by 18%) during late autumn when crop residue burning is significant. The synergistic APE and ARF have a negligible impact on the total concentrations of O3 and PM2.5. Hence, the reduction of PM2.5 may lead to O3 escalation due to weakened APE. Sensitivity experiments indicate the need and effectiveness of reducing VOC emission for the co-benefits of mitigating both O3 and PM2.5 concentrations in Delhi.

Deciphering the seasonal dynamics of multifaceted aerosol-ozone interplay: Implications for air quality management in Eastern China

We employed an enhanced WRF-Chem to investigate the discrete mechanisms of aerosol-radiation-feedback (ARF), extinction-photochemistry (AEP), and heterogeneous-reactions (AHR) across different seasons in eastern China, aiming to assess the synergistic effects arising from the simultaneous operation of multiple processes on O3 and PM2.5. Our findings demonstrated that ARF fostered the accumulation of pollutants and moisture, initiating two distinct feedback mechanisms concerning O3. The elevation in the NO/NO2 ratio amplified O3 consumption. Increased near-surface moisture diminished upper-level cloud formation, thereby enhancing photolysis rates and O3 photochemical production. The pronounced impact of heightened NO/NO2 on O3 led to a decrease of 0.1–2.7 ppb. When decoupled from ARF, AEP led to a more significant reduction in photolysis rates, resulting in declines in both O3 and PM2.5, except for an anomalous increase observed in summer, with O3 increasing by 1.6 ppb and PM2.5 by 2.5 μg m−3. The heterogeneous absorption of hydroxides in spring, autumn, and winter predominantly governed the AHR-induced variation of O3, leading to a decrease in O3 by 0.7–1 ppb. Conversely, O3 variations in summer were primarily dictated by O3-sensitive chemistry, with heterogeneous absorption of NOy catalyzing a decrease of 2.4 ppb in O3. Furthermore, AHR accentuated PM2.5 by facilitating the formation of fine sulfates and ammonium while impeding nitrate formation. In summer, the collective impact of ARF, AEP, and AHR (ALL) led to a substantial reduction of 6.2 ppb in O3, alleviating the secondary oxidation of PM2.5 and leading to a decrease of 0.3 μg m−3 in PM2.5. Conversely, albeit aerosol substantially depleted O3 by 0.4–4 ppb through their interactions except for summer, aerosol feedback on PM2.5 was more pronounced, resulting in a significant increase of 1.7–6.1 μg m−3 in PM2.5. Our study underscored the seasonal disparities in the ramifications of multifaceted aerosol-ozone interplay on air quality.

Weakened aerosol–radiation interaction exacerbating ozone pollution in eastern China since China's clean air actions

Abstract. Since China's clean air action, PM2.5 (particulate matter with an aerodynamic equivalent diameter of 2.5 µm or less) air quality has improved, while ozone (O3) pollution has become more severe. Here we apply a coupled meteorology–chemistry model (WRF-Chem: Weather Research and Forecasting model coupled to Chemistry v3.7.1) to quantify the responses of aerosol–radiation interaction (ARI) to anthropogenic emission reductions from 2013 to 2017, including aerosol–photolysis interaction (API) related to photolysis rate change and aerosol–radiation feedback (ARF) related to meteorological field change and their contributions to O3 increases over eastern China in summer and winter. Sensitivity experiments show that the decreased anthropogenic emissions play a more prominent role in the increased daily maximum 8 h average (MDA8) O3 in both summer (+1.96 ppb vs. +0.07 ppb) and winter (+3.56 ppb vs. −1.08 ppb) than the impacts of changed meteorological conditions in urban areas. The decreased PM2.5 caused by emission reductions can result in a weaker impact of ARI on O3 concentrations, which superimposes its effect on the worsened O3 air quality. The weakened ARI due to decreased anthropogenic emissions aggravates the summer (winter) O3 pollution by +0.81 ppb (+0.63 ppb), averaged over eastern China, with weakened API contributing 55.6 % (61.9 %) and ARF contributing 44.4 % (38.1 %), respectively. This superimposed effect is more significant for urban areas during summer (+1.77 ppb). Process analysis indicates that the enhanced chemical production is the dominant process for the increased O3 concentrations caused by weakened ARI in both summer and winter. This study innovatively reveals the adverse effect of weakened aerosol–radiation interaction due to decreased anthropogenic emissions on O3 air quality, indicating that more stringent coordinated air pollution control strategies should be implemented for significant improvements in future air quality.

The Tibetan Plateau space-based tropospheric aerosol climatology: 2007–2020

Abstract. A comprehensive and robust dataset of tropospheric aerosol properties is important for understanding the effects of aerosol–radiation feedback on the climate system and reducing the uncertainties of climate models. The “Third Pole” of Earth (Tibetan Plateau, TP) is highly challenging for obtaining long-term in situ aerosol data due to its harsh environmental conditions. Here, we provide the more reliable new vertical aerosol index (AI) parameter from the spaceborne-based lidar CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) on board CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations) for daytime and nighttime to investigate the aerosol's climatology over the TP region during 2007–2020. The calculated vertical AI was derived from the aerosol extinction coefficient (EC), which was rigorously quality-checked and validated for passive satellite sensors (MODIS) and ground-based lidar measurements. Generally, our results demonstrated that there was agreement of the AI dataset with the CALIOP and ground-based lidar. In addition, the results showed that, after removing the low-reliability aerosol target signal, the optimized data can obtain the aerosol characteristics with higher reliability. The data also reveal the patterns and concentrations of high-altitude vertical structure characteristics of the tropospheric aerosol over the TP. They will also help to update and make up the observational aerosol data in the TP. We encourage climate modelling groups to consider new analyses of the AI vertical patterns, comparing the more accurate datasets, with the potential to increase our understanding of the aerosol–cloud interaction (ACI) and aerosol–radiation interaction (ARI) and their climate effects. Data described in this work are available at https://doi.org/10.11888/Atmos.tpdc.300614 (Huang, 2023).

The EarthCARE mission – science and system overview

Abstract. The Earth Cloud, Aerosol and Radiation Explorer (EarthCARE) is a satellite mission implemented by the European Space Agency (ESA), in cooperation with the Japan Aerospace Exploration Agency (JAXA), to measure global profiles of aerosols, clouds and precipitation properties together with radiative fluxes and derived heating rates. The simultaneous measurements of the vertical structure and horizontal distribution of cloud and aerosol fields, together with outgoing radiation, will be used in particular to evaluate their representation in weather forecasting and climate models and to improve our understanding of cloud and aerosol radiative impact and feedback mechanisms. To achieve the objective, the goal is that a retrieved scene with footprint size of 10 km × 10 km is measured with sufficiently high resolution that the atmospheric vertical profile of short-wave (solar) and long-wave (thermal) flux can be reconstructed with an accuracy of 10 W m−2 at the top of the atmosphere. To optimise the performance of the two active instruments, the platform will fly at a relatively low altitude of 393 km, with an equatorial revisit time of 25 d. The scientific payload consists of four instruments: an atmospheric lidar, a cloud-profiling radar with Doppler capability, a multi-spectral imager and a broadband radiometer. Co-located measurements from these instruments are processed in the ground segment, which produces and distributes a wide range of science data products. As well as the Level 1 (L1) product of each instrument, a large number of multiple-instrument L2 products have been developed, in both Europe and Japan, benefiting from the data synergy. An end-to-end simulator and several test scenes have been developed that simulate EarthCARE observations and provide a development and test environment for L1 and L2 processors. Within this paper the EarthCARE observational requirements are addressed. An overview is given of the space segment with a detailed description of the four science instruments, demonstrating how the observational requirements will be met. Furthermore, the elements of the space segment and ground segment that are relevant for science data users are described and the data products are introduced.

Impacts of aerosol direct effects on PM2.5 and O3 respond to the reductions of different primary emissions in Beijing-Tianjin-Hebei and surrounding area

In recent years, the rapid pollution emission reductions significantly decreased the PM2.5, which weakened the aerosol radiation feedback effect and in turn influenced the changes of PM2.5 and O3. To promote our understanding of this effect, we used an online-coupled meteorology-chemistry model (WRF-Chem) to investigate the influence of aerosol direct effects (ADE) on PM2.5 and O3 responding to different pollutant emissions reduction for a summer PM2.5-O3 compound pollution month in Beijing-Tian-Hebei region (BTH) and surrounding area. We performed scenario simulations with turning on/off both aerosol influences on the meteorology (ADE_MET) and photolysis (ADE_PHO), and turning on only ADE_PHO with different reduction ratios for primary PM2.5 (100%), SO2 (80%) and NOx (40%) emissions. Our results showed that total reduction of primary PM2.5, SO2 and NOx emissions led to overall declines of surface PM2.5 in BTH and surrounding area. The SO2 and NOx emission reductions both furtherly decreased PM2.5 via the changed ADE, while the primary PM2.5 emission reduction increased the PM2.5 by its induced ADE. Different from PM2.5, O3 significantly increased in BTH after cutting the three types of emissions. The primary PM2.5 emission reduction led to O3 increase over the whole region mostly by the induced ADE, while the NOx emission reduction directly enhanced O3 in the plain areas mainly due to incompatible reduction of NOx and VOCs emissions. The O3 enhanced by emission-reduction-induced ADE accounted for 56.1% of the total increase. The ADE_PHO caused by PM2.5 primary emissions reduction was the dominant contribution to the total ADE induced increases of PM2.5 and O3, which enhanced the photolysis of gases and led to overall increase of oxidants that strengthened secondary aerosol formation, and also promoted the production of O3 via photochemical processes. The results implicate that with the continuously strengthened abatement of PM2.5 primary emissions, it may offset part of the reduction effect on PM2.5 decrease by the changed ADE, and also increase O3 over the whole region. The results also suggested that while controlling both PM2.5 and O3 in BTH in summer, not only the cooperative emission reduction of NOx and VOCs is essential, the induced ADE effect by PM2.5 primary emissions reduction should also be considered.

Inverse Modeling of the Initial Stage of the 1991 Pinatubo Volcanic Cloud Accounting for Radiative Feedback of Volcanic Ash

AbstractThe way volcanic clouds evolve is very sensitive to the initial spatial 3D distributions of volcanic materials, which are often unknown. In this study, we conducted inverse modeling of the Mt. Pinatubo cloud using total ozone mapping spectrometer 2D mapping of Aerosol Index and SO2 loading during the first three post‐eruption days to estimate the time‐dependent emissions profiles and initial 3D spatial distributions of volcanic ash and SO2. We account for aerosol radiative feedback and dynamic lofting of volcanic ash in the inversion calculations for the first time. This resulted in a lower ash injection height (by 1.5 km for ash) than without ash radiative feedback. The Pinatubo eruption ejected ≈77% of fine ash at 12–23 km, ≈65% of SO2 at 18–25 km. In contrast with previous studies, which suggested that all volcanic materials were emitted above the tropopause, a significant fraction of SO2 (5.1 of 15.5 Mt) and fine ash (37.2 of 66.5 Mt) were ejected in the troposphere, where SO2 quickly oxidized into sulfate aerosol that is short‐lived in the troposphere. This explains the early presence of sulfate aerosols in the plume and why the models can reproduce the observed volcanic aerosols' optical depth (AOD), assuming lower‐than‐observed SO2 emission in the stratosphere. Despite the quicker than in observations build‐up of sulfate AOD, in a month after the eruption, the evolution of the Pinatubo AOD simulated using the obtained ash and SO2 initial distributions converges with the available stratospheric aerosol and gas experiment observations.

Aerosol radiative feedback enhances particulate pollution over India: A process understanding

In this study, we investigate the role of aerosol direct radiative feedback on regional meteorological changes and subsequent distribution of air pollutant with the focus on particulate matter PM2.5 and ozone (O3) over India. WRF-Chem simulations have been applied for Baseline case with both aerosol direct and indirect effects and another simulation including only aerosol indirect effects. The aerosol direct radiative feedback on the meteorology and air quality were investigated by taking the differences between these two simulations. A comprehensive model evaluation showed the model had the capacity to reproduce the observations and well captured the temporal and spatial variation in meteorological and aerosol fields. We found that the reduction in incoming shortwave solar radiation, temperature at 2 m and planetary boundary layer height annually and across all the season due to aerosol radiation interaction (ARI). The concentration of PM2.5 and gaseous precursors like SO2 and NO2 showed enhancements, while O3 concentration mostly showed a reduction. Spatial and annual average PM2.5 concentration was increased by +6%, SO2 concentration was increased by +1.4%, NO2 concentration was increased by +3.4%, and O3 concentration was reduced by −1.4%. Largest regional increases of 40% in particulate pollution induced by ARI occurred over north-western Indian region. The increase in PM2.5 concentration was highly related to stabilisation induced through meteorological variables and increases in primary aerosol concentration. The decrease in O3 concentration was highly related increases in NO2 and SO2 concentration. In the most polluted regions of India, ARI significantly enhance surface-level particulate pollution and related premature mortality, of particular concern considering likely future increasing trends. This study highlights the importance of inclusion and better representations of ARI and for implementing effective mitigation plans.

Impact of atmospheric thermodynamic structures and aerosol radiation feedback on winter regional persistent heavy particulate pollution in the Sichuan-Chongqing region, China

Potential relationships among heavy air pollution, weather conditions, and meteorological effects are unclear and require further investigation, especially for areas with complex terrains, such as the Sichuan Basin (SCB), one of the most polluted regions in China. In this study, air pollution in the SCB was examined and 18 regional persistent heavy pollution events (RPHEs) were identified for the winters of 2014–2018. The average persistent period of the RPHEs was 8.89 days, and the number of affected cities was 17. Based on ground-based observations, CALIPSO satellite data, reanalysis data, and backward trajectory calculations, the synergistic effects of the thermodynamic structures, synoptic circulations and the radiative feedback of aerosols on the formation of RPHEs were revealed. The results can be summarized as follows: (1) An abnormal warming center, attributing to the warm southerly advection in the upper layer and the cold air dammed by the topography near the surface, always presented around 800–700 hPa to form a deep stable layer. (2) The diurnal variations in vertical motions triggered by the thermodynamic structures could regulate the pollution episodes. During the daytime, pollutants accumulated rapidly and thoroughly mixed under the control of sinking airflow from 800 hPa layer to the ground. At night, pollutants sometimes slowly diffused when weak ascending airflow appeared. (3) Forced by the stable layer and topography of the Tibetan Plateau, the local circulation was confined within SCB, resulting in the intensive mixing of local emissions and transport pollutants from other regions. This situation could be maintained for a long time with stable synoptic circulation in winter, leading to the formation of RPHEs. (4) The pollution episodes were featured with multi-layer pollutants above SCB according to the CALIPSO observations, including the local anthropogenic aerosols near the surface, dust aerosols originating from the Taklamakan Desert, and biomass burning aerosols from Southeast Asia. Solar absorption aerosols, including black carbon and dust above the region, could cause meteorological feedback, making the vertical layer more stable and enhancing the persistence and intensity of the pollution episodes. This study highlights the appreciable effects of synoptic circulations on the vertical thermodynamic structures of the atmosphere and air quality, and raises the understanding of the environmental and climate impacts of RPHEs in complex terrains.

Impacts of aerosol–photolysis interaction and aerosol–radiation feedback on surface-layer ozone in North China during multi-pollutant air pollution episodes

Abstract. We examined the impacts of aerosol–radiation interactions, including the effects of aerosol–photolysis interaction (API) and aerosol–radiation feedback (ARF), on surface-layer ozone (O3) concentrations during four multi-pollutant air pollution episodes characterized by high O3 and PM2.5 levels during 28 July to 3 August 2014 (Episode1), 8–13 July 2015 (Episode2), 5–11 June 2016 (Episode3), and 28 June to 3 July 2017 (Episode4) in North China, by using the Weather Research and Forecasting with Chemistry (WRF-Chem) model embedded with an integrated process analysis scheme. Our results show that API and ARF reduced the daytime shortwave radiative fluxes at the surface by 92.4–102.9 W m−2 and increased daytime shortwave radiative fluxes in the atmosphere by 72.8–85.2 W m−2, as the values were averaged over the complex air pollution areas (CAPAs) in each of the four episodes. As a result, the stabilized atmosphere decreased the daytime planetary boundary layer height and 10 m wind speed by 129.0–249.0 m and 0.05–0.15 m s−1, respectively, in CAPAs in the four episodes. Aerosols were simulated to reduce the daytime near-surface photolysis rates of J[NO2] and J[O1D] by 1.8 × 10−3–2.0 × 10−3 and 5.7 × 10−6–6.4 × 10−6 s−1, respectively, in CAPAs in the four episodes. All of the four episodes show the same conclusion, which is that the reduction in O3 by API is larger than that by ARF. API (ARF) was simulated to change daytime surface-layer O3 concentrations by −8.5 ppb (parts per billion; −2.9 ppb), −10.3 ppb (−1.0 ppb), −9.1 ppb (−0.9 ppb), and −11.4 ppb (+0.7 ppb) in CAPAs of the four episodes, respectively. Process analysis indicated that the weakened O3 chemical production made the greatest contribution to API effect, while the reduced vertical mixing was the key process for ARF effect. Our conclusions suggest that future PM2.5 reductions may lead to O3 increases due to the weakened aerosol–radiation interactions, which should be considered in air quality planning.

Modeling study of aerosol-meteorology feedback during winter haze events over the north China plain

An online-coupled regional atmospheric chemistry/aerosol-climate model RIEMS-Chem was utilized to investigate the aerosol radiative feedback effect during winter hazes over the North China Plain. Three severe winter haze events (8–January 15, 2013, 20–February 26, 2014, and 15–December 22, 2016) were studied. Model comparison against observations showed a reduction of absolute model bias by 16–21% for PM2.5 when aerosol radiative effect (ARE) was included. Due to ARE, the period- and regional-mean changes in 2 m air temperature (T2), planetary boundary layer height (PBLH), and PM2.5 concentration were −0.4∼-1.8 °C (−14%∼-60%), −30.9∼-183.6 m (−12%∼-31%), and 37.7–61.1 μg/m3 (26%–47%), respectively. The magnitudes of changes in downward solar radiation (SWD), T2, and PBLH were proportional to AOD (aerosol optical depth) and PM2.5 column level but not to surface PM2.5 level. The response of PM2.5 to aerosol radiative feedback was dependent on both PM2.5 level and ARE. The average Integrated Process Rate (IPR) for PM2.5 in Beijing was estimated to be 11.9 μg/m3/hr during the growth stage of the three severe hazes. Aerosol radiative feedback caused a net increase of IPR for PM2.5 by 2.1–9.5 μg/m3/hr during the growth stages, mostly contributed by chemical processes. Linear relationship can be derived for the PM2.5-ΔPM2.5 data pairs over the study region. The high correlation coefficient (0.95–0.96) suggested PM2.5 could be an indicator of the feedback-induced ΔPM2.5. A threshold PM2.5 concentration of 75 μg/m3 was found above which more than 10% PM2.5 change would occur due to aerosol radiative feedback, but this value differed among haze events and cities.

Aerosol influence on the pre-monsoon rainfall mechanisms over North-East India: A WRF-Chem study

The importance of aerosols and absorbing aerosols in influencing weather is well recognized. This study is carried out over North-East (NE) India during the pre-monsoon season, during which the region experiences the highest atmospheric heating due to high loading of absorbing aerosols. The region, also a biodiversity hotspot, has observed a decreasing rainfall trend in this season. Previous aerosol-rainfall studies over India have mostly focused on the monsoon season, but pre-monsoon rainfall also is an important rainfall component for this region. This study includes the results of a modelling study carried out with WRF-Chem model to investigate how aerosols impact the pre-monsoon rainfall and their mechanisms over this region. The influence of black carbon (BC) on radiation, clouds, rainfall and rainfall characteristics has also been investigated. The study shows that aerosols induce changes in cloud amounts which relates to rainfall changes. The semi-direct effect was found to be responsible for cloud reduction below 2 km altitude, thus affecting mainly lower-level clouds. Aerosol radiative feedbacks affected rainfall according to the height of the terrain in the region and tended to increase and decrease rainfall in general over low and high-terrain areas, respectively. Aerosol effects suppressed rainfall for more duration than it caused enhancement. The effect of increasing black carbon emissions showed a consistent increase in moisture in the region but also a consistent reduction in clouds, the changes in the spatial distribution of which was also related to the rainfall changes. BC caused suppression of lower intensity and enhancement of higher intensity rainfall. These results help to understand better the effects of aerosols on pre-monsoon rainfall distributions and the mechanisms involved.

Aerosol-modulated heat stress in the present and future climate of India

Heat stress is one of the leading natural causes of mortality in India. Aerosols can potentially impact heat stress by modulating the meteorological conditions via radiative feedback. However, a quantitative understanding of such an impact is lacking. Here, using a chemical transport model, Weather Research Forecasting model coupled with chemistry, we show that high aerosol loading in India was able to mask the heat stress (quantified by the wet bulb globe temperature (WBGT)) by 0.3 °C–1.5 °C in 2010 with a regional heterogeneity across the major climate zones in India. However, the cooling effect of aerosol direct radiative forcing is partially compensated by an increase in humidity. To understand the potential impact of air quality improvement (i.e. reducing aerosol load) on heat stress in the future, WBGT was projected for 2030 under two contrasting aerosol emission pathways. We found that heat stress would increase by >0.75 °C in all the climate zones in India except in the montane zone under the RCP4.5 scenario with a bigger margin of increase in the mitigation emission pathway relative to the baseline emission pathway. On the contrary, under the RCP8.5 scenario, heat stress is projected to increase in limited regions, such as the tropical wet and dry, north-eastern part of the humid sub-tropical, tropical wet, and semi-arid climate zone in peninsular India. Our results demonstrate that aerosols modulate heat stress and, therefore, the heat stress projections in India and anywhere else with high aerosol loading should consider aerosol radiative feedback.

Vertical Distribution of Aerosols during Deep-Convective Event in the Himalaya Using WRF-Chem Model at Convection Permitting Scale

The Himalayan region is facing frequent cloud bursts and flood events during the summer monsoon season. The Kedarnath flooding of 2013 was one of the most devastating recent events, which claimed thousands of human lives, heavy infrastructure, and economic losses. Previous research reported that the combination of fast-moving monsoon, pre-existing westerlies, and orographic uplifting were the major reasons for the observed cloud burst over Kedarnath. Our study illustrates the vertical distribution of aerosols during this event and its possible role using the Weather Research and Forecasting model coupled with chemistry (WRF-Chem) simulations. Model performance evaluation shows that simulations can capture the spatial and temporal patterns of observed precipitation during this event. Model simulation at 25 km and 4 km horizontal grid resolution, without any changes in physical parameterization, shows a very minimal difference in precipitation. Simulation at convection-permitting scale shows detailed information related to parcel motion compared to coarser resolution. This indicates that the parameterization at different resolutions needs to be further examined for a better outcome. The modeled result shows changes of up to 20–50% in the rainfall over the area near Kedarnath due to the presence of aerosols. Simulation at both resolutions shows the significant vertical transport of natural (increases by 50%+) and anthropogenic aerosols (increases by 200%+) during the convective event, which leads to significant changes in cloud properties, rain concentration, and ice concentration in the presence of these aerosols. Simulations can detect changes in important instability indices such as convective available potential energy (CAPE), convective inhibition energy (CIN), vorticity, etc., near Kedarnath due to aerosol–radiation feedback.

Enhanced PM2.5 Decreases and O3 Increases in China During COVID-19 Lockdown by Aerosol-Radiation Feedback.

We apply an online‐coupled meteorology‐chemistry model (WRF‐Chem) embedded with an improved process analysis to examine aerosol‐radiation feedback (ARF) impacts on effectiveness of emission control due to Coronavirus Disease 2019 (COVID‐19) lockdown over North China Plain. Emission reduction alone induces PM2.5 decrease by 16.3 μg m−3 and O3 increase by 10.2 ppbv during COVID‐19 lockdown. The ARF enhances PM2.5 decrease by 2.7 μg m−3 (16.6%) and O3 increase by 0.8 ppbv (7.8%). The ARF‐induced enhancement of PM2.5 decline is mostly attributed to aerosol chemistry process, while enhancement of O3 rise is ascribed to physical advection and vertical mixing processes. A set of sensitivity experiments with emission reductions in different degrees indicate that the ARF‐induced enhancements of PM2.5 declines (O3 rises) follow a robust linear relationship with the emission‐reduction‐induced PM2.5 decreases. The fitted relationship has an important implication for assessing the effectiveness of emission abatement at any extent.

Aerosol optical properties of haze episodes in eastern China based on remote-sensing observations and WRF-Chem simulations

The ability for remote-sensing sensors and global/regional models to describe aerosol optical properties (AOPs) is critical to reducing the uncertainty in aerosol radiative forcing associated with climate change, and improving model prediction accuracy. In this study, remote-sensing observations and WRF-Chem simulations were used to evaluate AOPs over Eastern China during a severe winter haze event, in terms of aerosol optical depth (AOD), Ångström exponent (AE) and aerosol extinction profiles (AEP). This study also characterizes whether the inclusion of aerosol radiative feedbacks (ARFs) may improve the ability of WRF-Chem to acquire AOPs during haze episodes. Three simulation scenarios were considered: the non-radiation feedback (NRF), aerosol direct effect (ADE), as well as combined ADE and aerosol indirect effect (ADE + AIE). The results indicate that the satellite AOD products could represent the spatiotemporal distribution characteristics of the haze event. The AOD retrieved by the MODIS C6.1 DB algorithm was highly consistent with ground-based observations. A comparison between simulations and observations demonstrated that WRF-Chem, including the ADE or ADE + AIE scheme, may improve AOPs simulation in heavily polluted areas. The most significant improvements occurred in the Sichuan basin (SB) and North China Plain (NCP) (AOD = 50–70%; AE = 10–20%). In particular, the ADE + AIE scheme was the most obvious for AOD improvement, followed by AE. The AEP was also qualitatively analyzed through simulations and observations, and the result showed that the model overestimates aerosol extinction coefficients in NCP. However, the model is still able to represent the entire AEP contour over East China. Similar to AOD and AE, the model can also improve AEP simulation by adding ADE or ADE + AIE scheme, especially in SB. In summary, it is found that the ability of WRF-Chem to simulate the AOPs was significantly enhanced, particularly for regions loading with heavy aerosol during haze events in Eastern China.

Drivers of the rapid rise and daily-based accumulation in PM1

Submicron particle matter (PM1) that rapidly reaches exceedingly high levels in several or more hours in the North China Plain (NCP) has been threating~400 million individuals' health for decades. The precise cause of the rapid rise in PM1 remains uncertain. Based on sophisticated measurements in PM1 characterizations and corresponding boundary-layer (BL) meteorology in the NCP, it demonstrates that this rising is mainly driven by BL meteorological variability. Large increases in near-ground inversions and decreases in vertical heat/momentum fluxes during the day-night transition result in a significant reduction in mixing space. The PM1 that is vertically distributed before accumulates at the near-ground and then experiences a rapid rise. Besides meteorological variability, a part of the rise in organics is ascribed to an increase of coal combustion at midnight. The daily-based accumulation of PM1 is attributed to day-to-day vertical meteorological variability, particularly diminishing mixing layer height exacerbated by aerosol-radiation feedback. Resolved by a multiple linear regression model, BL meteorological variability can explain 71% variances of PM1. In contrast, secondary chemical reactions facilitate the daily-based accumulation of PM1 rather than the rapid rise. Our results show that BL meteorological variability plays a dominant role in PM1 rising and day-to-day accumulation, which is crucial for understanding the mechanism of heavy pollution formation.

Impacts of aerosol–radiation interaction on meteorological forecasts over northern China by offline coupling of the WRF-Chem-simulated aerosol optical depth into WRF: a case study during a heavy pollution event

Abstract. To facilitate the future inclusion of aerosol–radiation interactions in the regional operational numerical weather prediction (NWP) system RMAPS-ST (adapted from Weather Research and Forecasting, WRF) at the Institute of Urban Meteorology (IUM), China Meteorological Administration (CMA), the impacts of aerosol–radiation interactions on the forecast of surface radiation and meteorological parameters during a heavy pollution event (6–10 December 2015) over northern China were investigated. The aerosol information was simulated by RMAPS-Chem (adapted from the WRF model coupled with Chemistry, WRF-Chem) and then offline-coupled into the Rapid Radiative Transfer Model for General Circulation Models (RRTMG) radiation scheme of WRF to enable the aerosol–radiation feedback in the forecast. To ensure the accuracy of the high-frequency (hourly) updated aerosol optical depth (AOD) field, the temporal and spatial variations of simulated AOD and aerosol extinction coefficient at 550 nm were evaluated against in situ and satellite observations. Comparisons with in situ and Moderate Resolution Imaging Spectroradiometer (MODIS), AErosol Robotic NETwork (AERONET), and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite observations showed that the model could reproduce the spatial and vertical distribution as well as the temporal variation of the polluted episode. Further comparison of PM2.5 with in situ observation showed WRF-Chem reasonably captured the PM2.5 field in terms of spatial distribution and magnitude, with the correlation coefficients of 0.85, 0.89, 0.76, 0.92 and 0.77 in Beijing, Shijiazhuang, Tianjin, Hebei and Henan, respectively. Forecasts with and without the aerosol information were conducted further, and the differences of surface radiation, energy budget and meteorological parameters were evaluated against surface and sounding observations. The offline-coupling simulation (with aerosol–radiation interaction active) showed a remarkable decrease in downward shortwave (SW) radiation reaching the surface, thus helping to reduce the overestimated SW radiation during the daytime. The simulated surface radiation budget was also improved, with the biases of net surface radiation decreased by 85.3 %, 50.0 %, 35.4 % and 44.1 % during the daytime in Beijing, Tianjin, Taiyuan and Jinan respectively, accompanied by the reduction of sensible (16.1 W m−2, 18.5 %) and latent (6.8 W m−2, 13.4 %) heat fluxes emitted by the surface around noon. In addition, the cooling of 2 m temperature (∼0.40 ∘C) and the decrease in horizontal wind speed near the surface (∼0.08 m s−1) caused by the aerosol–radiation interaction over northern China helped to reduce the bias by ∼73.9 % and ∼7.8 % respectively, particularly during the daytime. Further comparisons indicated that the simulation-implemented AOD could better capture the vertical structure of atmospheric wind. Accompanied with the lower planetary boundary layer and the increased atmospheric stability, both U and V wind at 850 hPa showed convergences which were unfavorable for pollutant dispersion. Since RMPAS-ST provides meteorological initial conditions for RMAPS-Chem, the changes of meteorology introduced by aerosol–radiation interaction would routinely impact the simulations of pollutants. To verify the statistical significance of the results, we further conducted the 24 h forecasts for a longer period lasting 27 d (13 January–8 February 2017), with no AOD field (NoAero) and WRF-Chem-simulated hourly AOD fields (Aero) included, as well as a constant AOD value of 0.12 (ClimAero). The 1-month results were statistically significant and indicated that the mean RMSE of 2 m temperature (wind speed at 10 m) in Aero and ClimAero relative to NoAero was reduced by 4.0 % (1.9 %) and 1.2 % (1.6 %). More detailed evaluations and analysis will be addressed in a future article. These results demonstrated the influence of aerosol–radiation interactions on the improvement of predictive accuracy and the potential prospects to offline coupling of near-real-time aerosol information in regional RMAPS-ST NWP in northern China.

Impacts of PBL schemes on PM2.5 simulation and their responses to aerosol-radiation feedback in GRAPES_CUACE model during severe haze episodes in Jing-Jin-Ji, China

Using the online-coupled Global/Regional Assimilation and Prediction System coupled with the Chinese Unified Atmospheric Chemistry Environment (GRAPES_CUACE) model, numerical experiments are conducted to compare the impacts of three planetary boundary layer (PBL) schemes—Medium Range Forecast (MRF), Yonsei University (YSU), and Mellor-Yamada-Janjic (MYJ)—on PM2.5 simulations and their responses to aerosol-radiation feedback (ARF), under different terrain and weather conditions. The baseline experiments without ARF with all three PBL schemes can reproduce the spatial and temporal distribution characteristics of surface PM2.5 on the whole and show that the model can capture the PM2.5 variation better in unstable conditions (clean days) than in stable conditions (pollution days). The three PBL schemes show differences in the simulations of meteorological factors, e.g., the MRF scheme produces the best 10-m wind speed, the MYJ scheme produces the best 2-m temperature, and the YSU scheme produces the lowest PBL height (PBLH) and vertical diffusivity. Consequently, there is no significant difference among the three PBL schemes in surface PM2.5 simulations due to the combined action of the surface and the PBL structure. However, all the baseline experiments massively underestimate PM2.5 during severe pollution days. The online computing of ARF in the three PBL schemes can substantially improve the underestimation of PM2.5 by reducing sensible heat fluxes, 2-m temperature, friction velocity, and the PBL height. More importantly, the response intensity of three PBL schemes to ARF is very different. For PM2.5 simulations, MYJ has the weakest response, whereas MRF and YSU have similar responses. During the heavy pollution days with more intense aerosol-radiation effect, the increase of PM2.5 induced by ARF in MRF and YSU is about twice that in MYJ, making the PM2.5 simulated by MRF and YSU more consistent with the observations. This result shows the ARF dependence on the PBL processes and suggests the importance of PBL-ARF interaction to the PM2.5 numerical prediction during haze episodes.