Aerosol Direct Radiative Forcing Research Articles

Aerosol radiative forcings induced by substantial changes in anthropogenic emissions in China from 2008 to 2016

Abstract. Anthropogenic emissions in China play an important role in altering the global radiation budget. Over the past decade, the strong clean-air policies in China have resulted in substantial reductions of anthropogenic emissions of sulfur dioxide (SO2) and primary particulate matter, and air quality in China has consequently improved. However, the resultant aerosol radiative forcings have been poorly understood. In this study, we used an advanced global climate model integrated with the latest localized emission inventory to quantify the aerosol radiative forcings by the changes of anthropogenic emissions in China between 2008 and 2016. By comparing with multiple observation datasets, our simulations reproduced the considerable reductions of sulfate and black carbon (BC) mass loadings reasonably well over eastern China (the key region subject to stringent emission controls) during the period and accordingly showed a clear decline in both aerosol optical depth and absorption aerosol optical depth. The results revealed a regional annual mean positive direct radiative forcing (DRF) of +0.29 W m−2 at the top of the atmosphere (TOA) due to the reduction of SO2 emissions. This positive aerosol radiative forcing was comprised of diminished sulfate scattering (+0.58 W m−2), enhanced nitrate radiative effects (−0.29 W m−2), and could be completely offset by the concurrent reduction of BC emissions that induced a negative BC DRF of −0.33 W m−2. Despite the small net aerosol DRF (−0.05 W m−2) at the TOA, aerosol–radiation interactions could explain the surface brightening in China over the past decade. The overall reductions in aerosol burdens and associated optical effects mainly from BC and sulfate enhanced the regional annual mean downward solar radiation flux at the surface by +1.0 W m−2 between 2008 and 2016. The enhancement was in general agreement with a long-term observational record of surface energy fluxes in China. We also estimated that aerosol effects on cloud radiative forcings may have played a dominant role in the net aerosol radiative forcings at the TOA in China and over the northern Pacific Ocean during the study period. This study will facilitate more informed assessment of climate responses to projected emissions in the future as well as to sudden changes in human activities (e.g., the COVID-19 lockdown).

Aerosol optical properties over Delhi during a dust event in summer 2014: plausible implications

Delhi experiences frequent dust storms during the pre-monsoon season. We studied one such event that occurred on May 30th, 2014 at New Delhi to understand the changes in the optical properties of aerosols. The present study utilizes the aerosol optical depth (AOD) from the ground and satellite measurements along with reanalysis data. The aerosol characteristics were examined during the pre-week and post-week of the dust storm event. The results show that AOD values reach ~ 1.0 in all the modes of measurements (i.e., sun photometer, MODIS, and MERRA-2) on the dust event day. MODIS-derived Aerosol Angstrom Exponent (AAE), which gradually decreases from 1.7 ± 0.3 to 1.1 ± 0.3, shows the presence of coarse mode particles during the event. The aerosol layer at an elevation from 1 to 2 km was observed using indigenously developed dual-polarization micro-pulse lidar (MPL) system during the dust event. The source of the dust during the storm was identified by the air mass back-trajectories analysis. The dust column mass density averaged 24 hourly at 850 hPa is 780 mg m−2 with a prevailing wind direction of west to east with wind speed of 11–13 ms−1. Shortwave aerosol direct radiative forcing (SWADRF) diurnally averaged at the top of the atmosphere (TOA), the atmosphere (ATM), and surface (SUR) are found to be − 10 W m−2, 27 W m−2, and − 37 W m−2, respectively. The heating rate was enhanced by 56% (from 0.16 to 0.25 K day−1) during the event in the whole atmospheric column. Visibility decreased by 31% during the dust storm day compared with its average value of non-dust storm days.

Optical Characteristics and Radiative Properties of Aerosols in Harbin, Heilongjiang Province during 2017

This study considers aerosol optical properties and direct radiative forcing over Harbin (126.63° E, 45.75° N), the highest latitude city in Northeast China, during 2017. Observations based on the CE-318 sun-photometer show that the annual mean values of the aerosol optical depth (AOD) at 500 nm and the Angstrom exponent (AE) at 440–870 nm over Harbin are respectively 0.26 ± 0.20 and 1.36 ± 0.26. Aerosol loading is the highest in the spring followed by winter, and the lowest loading is in autumn. AE440–870 is the highest in summer, second highest in winter, and lowest in autumn. The Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART) model is used to estimate the shortwave aerosol radiative forcing at the top of the atmosphere, on the Earth’s surface and in the atmosphere, and the annual mean values are −16.36 ± 18.42 Wm−2, −71.01 ± 27.37 Wm−2 and 54.65 ± 30.62 Wm−2, respectively, which indicate that aerosols cause climate effects of cooling the earth-atmosphere system, cooling the earth’s surface and heating the atmosphere. Four main aerosol types in Harbin are classified via AOD and AE. Specifically, clean continental, mixed type, biomass burning and urban industry, and desert dust aerosols accounted for 51%, 38%, 9%, and 2% of the total, respectively. Aerosol radiative forcing varies greatly in different seasons, and the aerosol load and type from different emission sources have an important influence on the seasonal variation of radiative forcing.

Effect of COVID-19 shutdown on aerosol direct radiative forcing over the Indo-Gangetic Plain outflow region of the Bay of Bengal

The Indo Gangetic Plain (IGP), one of the most densely populated regions of the world, is a global hotspot of anthropogenic aerosol emissions. In the pre-monsoon season (March–May), the strong westerlies carry transported dust aerosols along with anthropogenic aerosols onto the Bay of Bengal (BoB). The outflow from IGP modulates the aerosol loading and the aerosol direct radiative forcing (ADRF) over the BoB. The quantification of the anthropogenic aerosol impact on the radiative forcing over the outflow region remains inadequate. The enforced shutdown amid the COVID-19 pandemic eased the anthropogenic activities across the country, which helped to examine the magnitude and variability of aerosol loading and subsequent changes in ADRF over IGP and the outflow region of the BoB. Wind trajectory analysis illustrates that the ADRF over the BoB is greater during the days when the winds originated from the IGP region (at the surface −54.2 ± 6.4 W m−2, at the top of the atmosphere, −26.9 ± 3.4 W m−2 and on the atmosphere, 27.0 ± 3.1 W m−2) compared to the seasonal average (−46.3 ± 7.1 W m−2, −24.9 ± 4.0 W m−2 and 20.6 ± 3.2 W m−2, respectively). This indicates that anthropogenic aerosols emission from IGP can contribute an additional 31% of the atmospheric ADRF over the IGP outflow region of the BoB. The reduced aerosol loading during the shutdown period resulted in a reduction of ADRF at the surface, at the top of the atmosphere, and on the atmosphere over the IGP outflow region of the BoB by 22.0 ± 3.1%, 20.9 ± 3.4% and 23.2 ± 3.3%, respectively. This resultant 20–25% reduction in ADRF over the IGP outflow region of BOB matches well with 10–25% reduction in aerosol optical depth (AOD) over the IGP during the shutdown period showing a robust coupling between IGP aerosol emissions and ADRF over the BoB.

Direct radiative forcing of biomass burning aerosols from the extensive Australian wildfires in 2019–2020

In 2019, an unusually strong positive Indian Ocean Dipole spawned hot and dry weather in southeastern Australia, which promoted devastating wildfires in the period from September 2019 to February 2020. The fires produced large plumes of biomass burning aerosols that prevented sunlight from reaching the Earth’s surface, and in this way elicited regional radiative cooling. We estimated the direct aerosol radiative forcing (ARF) resulting from these wildfires, based on Moderate Resolution Imaging Spectroradiometer space-based data and an empirical relationship from AErosol RObotic NETwork ground-based data collected in biomass-burning regions. The wildfire-derived air pollution was associated with an aerosol optical thickness of >0.3 in Victoria and a strongly negative ARF of between −14.8 and −17.7 W m−2, which decreased the surface air temperature by about 3.7 °C–4.4 °C. This is of the same order of magnitude as the radiative cooling from volcanic eruptions. Although the atmospheric lifetime of biomass-burning aerosols is relatively short (about a week), the Australian wildfire pollution plumes extended across the Pacific Ocean to South America. Since climate change is expected to lead to more frequent and increasingly intense fires in many regions worldwide, the consequent biomass burning aerosols may become a significant radiative forcing factor, which will need to be accounted for in climate model projections for the future.

Uncertainty in Observational Estimates of the Aerosol Direct Radiative Effect and Forcing

AbstractA lower bound on the uncertainty in observational estimates of the aerosol direct radiative effect (DRE; the direct interaction with solar radiation by all aerosols) and the aerosol direct radiative forcing [DRF; the radiative effect of just anthropogenic aerosols (RFari)] is quantified by making the optimistic assumption that global aerosol observations can be made with the accuracy found in the Aerosol Robotic Network (AERONET) sun photometer retrievals. The global-mean all-sky aerosol DRE uncertainty was found to be 1.1 W m−2 (one standard deviation). The global-mean all-sky aerosol DRF (RFari) uncertainty was determined to be 0.31 W m−2. The total uncertainty in both quantities is dominated by contributions from the aerosol single scattering albedo uncertainty. These uncertainty estimates were compared to a literature survey of mostly satellite-based aerosol DRE/DRF values. Comparisons to previous studies reveal that most have significantly underestimated the aerosol DRE uncertainty. Past estimates of the aerosol DRF uncertainty are smaller (on average) than our optimistic observational estimates, including the aerosol DRF uncertainty given in the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5). This disconnect between our observation-based uncertainty and that found in past aerosol DRF studies that rely, at least in part, on modeling is discussed. Also quantified is a potential reduction in the current observational uncertainty possible with a future generation of satellite observations that would leverage aerosol typing and more refined vertical information.

도시 및 청정지역에서 RCP 기후변화시나리오의 상대습도 변화에 따른 에어로졸 화학성분별 미래 직접적 복사강제력 추정

도시 및 청정지역에서 RCP 기후변화시나리오의 상대습도 변화에 따른 에어로졸 화학성분별 미래 직접적 복사강제력 추정

The impact of different aerosol properties and types on direct aerosol radiative forcing and efficiency using AERONET version 3

In this study, the results from a classification method are used to determine how aerosol type influence the radiative balance of the Earth's climate. Quality-assured data from the AErosol RObotic NETwork (AERONET) Level 2.0 (L2) Version 3 (V3) inversion product are used during the 2008–2017 period. The study includes the Arabian Peninsula (AP), Atlantic (AT), Middle East North Africa (MENA), South Europe (SE) and Central and East Europe (CEE). The impacts of Solar Zenith Angle (SZA), Surface Albedo (SA), Aerosol Optical Depth (AOD) and Single Scattering Albedo (SSA) on Direct Aerosol Radiative Forcing (DARF) and Efficiency (DARFeff) at the bottom and top of atmosphere (BOA, TOA) are investigated. Fine slightly absorbing particles show the highest positive gradient of DARFeffBOA with SA and the highest negative gradient of DARFefTOA with SZA. The mixed absorbing particles provide the highest alteration for DARF at the BOA with the increase of AOD. The analysis of aerosols absorptivity is performed by dividing SSA into six-subgroups. Coarse absorbing particles provide the highest (in magnitude) DARFeff values at the TOA under absorbing aerosol conditions (SSA < 0.89), whereas similar behavior is revealed by the fine absorbing particles for DARFeff values at the BOA. Furthermore, we analyze the long-term averages of RF metrics for all aerosol types among the regions of our study. At the TOA, fine non-absorbing particles show the highest absolute values for DARFeff (from −75 W m−2 to −79 W m−2) and DARF (from −38 W m−2 to −48 W m−2). At the BOA, coarse and mixed absorbing clusters indicate the highest absolute values for DARF (from −66 W m−2 to −79 W m−2) and DARFeff (from −135 W m−2 to −149 W m−2), respectively. The presented results could be used for the validation and improvement of the performance of remote sensing algorithms and models that assess the aerosol effect in radiative balance.

Aerosol Direct Radiative Effects over China Based on Long-Term Observations within the Sun–Sky Radiometer Observation Network (SONET)

To investigate aerosol radiative effects, the Sun–Sky Radiometer Observation Network (SONET) has performed long-term observations of columnar atmospheric aerosol properties at 20 distributed stations around China. The aerosol direct radiative forcing (RF) and efficiency (RFE, the rate at which the atmosphere is forced per unit of aerosol optical depth) were estimated using radiative transfer model simulations based on the ground-based observations dating back to 2009. Results of multi-year monthly average RF illustrated that: the dust-dominant aerosol population at arid and semi-arid sites exerted moderate cooling effects (−8.0~−31.2 W/m2) at the top and bottom of atmosphere (TOA and BOA); RF at continental background site was very weak (−0.8~−2.9 W/m2); fine-mode dominant aerosols at urban and suburban sites exerted moderate cooling effects (−9.3~−24.1 W/m2) at TOA but more significant cooling effects (−19.4~−50.6 W/m2) at BOA; RF at coastal sites was comparable with values of urban sites (−5.5~−19.5 W/m2 at TOA, and −15.6~−44.6 W/m2 at BOA), owing to combined influences by marine and urban–industrial aerosols. Differences between RFE at TOA and BOA indicated that coarse-mode dominant aerosols at arid, semi-arid, and continental background sites were less efficient to warm the atmosphere; but fine-mode dominant aerosols at urban, suburban, and coastal sites were shown to be more efficient to heat the atmosphere.

Simulation of long-term direct aerosol radiative forcing over the arctic within the framework of the iAREA project

This paper presents the climatology of aerosol optical properties and radiative forcing over the Arctic obtained within the framework of the iAREA (impact of absorbing aerosols on radiative forcing in the European Arctic) project. The presented data were obtained from the Navy Aerosol Analysis and Prediction System (NAAPS) and the Fu-Liou radiative transfer model. NAAPS was used to simulate particle concentration and aerosol optical depth (AOD) at 1 ° × 1 ° spatial resolution. Direct aerosol radiative forcing (ARF) was calculated for clear-sky and all-sky conditions based on NAAPS reanalysis (with AOD assimilation) and satellite observations of surface and cloud properties. Long-term data (2003–2015) from NAAPS show that anthropogenic and biogenic aerosol, as well as sea salt, make the most important contribution to total AOD (35 and 30%, respectively). However, smoke (15%) and mineral dust (20%) cannot be neglected, especially during spring and summer. Results of numerical simulations indicate mean shortwave (SW) ARF for the whole Arctic (>70.5oN) at the Earth's surface to be −4 W/m2 for clear-sky and −1.3 W/m2 for all-sky conditions, and at top of the atmosphere (TOA) −1.3 W/m2 and -0.4 W/m2, respectively. TOA ARF for anthropogenic and biogenic particles is only −0.1 W/m2 for clear-sky and almost zero for all-sky conditions. For smoke and dust particles, SW ARF is very similar for both Earth's surface and TOA, as well as for clear-sky and all-sky conditions. For sea salt, SW ARF is the same at the surface and at TOA: 0.6 W/m2 for clear-sky and −0.3 W/m2 for all-sky conditions, because of negligible solar absorption. Cloud cover reduces surface cooling (direct clear-sky SW ARF) by a factor of 40% and shifts TOA SW ARF towards positive values.

Long-term variation in aerosol optical properties and their climatic implications over major cities of Pakistan

The present study investigates long-term variation in aerosol optical properties (AOP) and their associated climatic implications over selected cities of Chitral, Gwadar, Karachi, Lahore, Peshawar and Quetta in Pakistan for the period 2005–2018. For this purpose, aerosol optical depth (AOD) and aerosol index (AI) are retrieved from Moderate Resolution Imaging Spectroradiometer (MODIS) and Ozone Monitoring Instrument (OMI). Results revealed annual increasing trend of AOD with maximum values in summer during the study period over all study regions, except for Chitral. Similar annual trend was observed for AI but with minimum values in summer. Further, temperature and relative humidity (RH) showed significant relationships with AOD along with evidences of precipitation influences. In addition, the trajectory analysis of Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) Model confirmed the arrival of both short and long-range air masses to the receptor sites. Similarly, the direct aerosol radiative forcing (DARF) and direct aerosol radiative forcing efficiency (DARFE) were calculated using Clouds and the Earth's Radiant Energy System (CERES) data. The multiyear mean monthly atmospheric DARF values of 8.80, 16.32, 10.74, 21.64, 9.10 and 10.94 W/m2 were observed over Chitral, Gwadar, Karachi, Lahore, Peshawar and Quetta, respectively. Consequently, the maximum heating rate (HR) of 0.48 K/day (Chitral), 0.93 K/day (Gwadar), 0.51 K/day (Karachi), 1.23 K/day (Lahore), 0.44 K/day (Peshawar) and 0.56 K/day (Quetta) showed a net warming effect during 2005–2018. These results can give an insight into aerosol concentration and can form the basis for aerosol-induced climatic implications in the study area.

Weather Effects of Aerosols in the Global Forecast Model

The weather effects of aerosol types were investigated using well-posed aerosol climatology through the aerosol sensitivity test of thermodynamic and hydrometeor fields, and the weather forecast performances in July of 2017. The largest aerosol direct radiative forcing (ADRF) in July was due to dust aerosols at the surface and atmosphere, and sulfate at the top of the atmosphere (TOA), respectively. The ADRF of total aerosols had unilateral tendencies in thermodynamic and hydrometeor fields. The contribution of individual aerosols was linearly additive to those of total aerosols in the heat fluxes, heating rates, humidity, and convective precipitation. However, no such linearity existed in temperature, geopotential height, cloud liquid or ice contents, and large-scale precipitation. Dust was the most influential forcing agent in July among five aerosol types due to the largest light-absorption capacity. Such unilateral tendencies of total aerosols and a part of the linearity of individual aerosols were exerted on the weather systems. The verification of medium-range forecasts showed that aerosols alleviated the overestimation of surface shortwave (SW) downward fluxes, the negative biases of temperature and geopotential heights at TOA and surface, and the underestimation in light and moderate precipitation. In contrast, they enhanced warm biases at the mid-atmosphere and underestimation in heavy precipitations, particularly negative biases in the intertropical convergence zone (ITCZ). Weather forecast scores including current aerosol information were improved in geopotential height (GPH) of the northern hemisphere (NH); however, they got worse in the temperature and the upper atmosphere GPH of the southern hemisphere (SH), which was mostly due to black carbon (BC) aerosols in the tropical regions. The missing mechanisms such as aerosol–cloud interactions, better aerosol spectral optical properties including mixing states and aging, and the near-real-time (NRT) based aerosol loading data are worthwhile to be tried in the near future for fixing the intrinsic underestimation of precipitation in ITCZ and surface radiative fluxes in the desert and biomass burning area.

Long-term (2008–2017) analysis of atmospheric composite aerosol and black carbon radiative forcing over a semi-arid region in southern India: Model results and ground measurement

To achieve an in-depth understanding of radiative forcing due to aerosols is a crucial challenge for climate change studies. The first-ever long-term measurement of direct shortwave composite and black carbon aerosol radiative properties over a semi-arid region, Anantapur, in southern India is presented. Long-term variations in Aerosol Optical Depth (AOD) and Black Carbon (BC) mass concentration from December 2007 to November 2017 are discussed with specific emphasis on intra-seasonal variation in aerosol optical properties, meteorology, transport pathways, and their implications for direct short wave radiative forcing over Anantapur. The intra-seasonal mean AOD showed strong seasonal dependence with the highest (0.47 ± 0.03) during summer and lowest (0.28 ± 0.03) during the monsoon. Meanwhile, the intra-seasonal mean (±σ) BC mass concentration was about 3.57 ± 0.45, 2.60 ± 0.58, 1.22 ± 0.18 and 2.24 ± 0.28 μg m−3 during winter, summer, monsoon and post-monsoon respectively. Furthermore, there is an obvious temporal variation in intra-seasonal BC mass concentration during the dry season (winter and summer). To be more specific, the intra-seasonal mean (±σ) BC mass concentration before 2012 (after 2012) during the dry season was about 3.37 ± 0.7 μg m−3 (2.80 ± 0.58 μg m−3), respectively. Concentration weighted trajectory analyses (CWT) revealed that the air masses originated from the continental and polluted environments located in the central and northern parts of India (except monsoon), in regulating BC mass concentration over measurement location. Further, Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) derived aerosol vertical extinction profiles (532 nm) showed that majority aerosols (>250 Mm−1) are confined within 2 km from the surface during winter while in summer particles are distributed throughout the profile (~6 km) with extinction coefficient varying between 200 and 250 Mm−1. The Santa Barbara Discrete Ordinate Radiative Transfer (SBDART) model estimated intra-seasonal mean direct shortwave composite aerosol radiative forcing (DARF) in the atmosphere (ATM) was about 31.13 ± 3.36, 34.82 ± 3.89, 17.10 ± 1.15, and 17.44 ± 1.81 Wm-2 during winter, summer, monsoon and post-monsoon, respectively. The positive signs of ATM forcing in all seasons indicate a warming of the atmosphere, and the corresponding heating rate was around a factor of two higher during the dry season (0.92 ± 0.12 Kday−1) than the wet season (monsoon and post-monsoon) (0.49 ± 0.04 Kday−1). The intra-seasonal mean BC forcing in ATM before 2012 (After 2012) during the dry season was about 24.14 ± 2.85Wm-2(20.09 ± 2.59Wm-2), respectively. The contribution of BC alone to the composite forcing during the study period over the station was ~68%. These findings would be helpful for regional climate studies and making air pollution control policy over the region.

Parametric analysis for global single scattering albedo calculations

Abstract Single scatter albedo (SSA) is a key parameter in radiative transport models for estimating aerosol direct radiative forcing (DRF) and is also a major contributor to DRF uncertainty. We investigate the sensitivity of SSA calculations to physical input parameters (e.g., mixing state, size distribution, density, and refractive index of aerosols) associated with absorbing aerosols (e.g., black carbon [BC], brown carbon [BrC], and soil dust). We attempted to estimate global aerosol SSAs using the 3-D global chemical transport model (GEOS-Chem) and a post-processing tool of aerosol optical properties (FlexAOD) and evaluated the model by comparing it with observed values. The model reproduces the observed variability of both the surface aerosol concentrations and aerosol optical depth (AOD) obtained from the Surface Particulate Matter Network (SPARTAN), the global Aerosol Mass Spectrometer (AMS), and the Aerosol Robotic Network (AERONET). Our sensitivity tests show that the physical input parameters, which are not as well understood as aerosol mass concentrations, can lead to large uncertainties in global SSA values. We find that BC mixing state, BrC, and a dust size distribution have significant impacts on the global SSA calculation. Their combined use can reduce aerosol SSA bias in the model by 43% at 440 nm, compared to observations. We also find that the direct radiative effect (DRE) of global aerosols increases by 10% (from −2.62 W m−2 to −2.36 W m−2) when the SSA bias is corrected.

Effects of black carbon mitigation on Arctic climate

Abstract. We use the ECHAM-HAMMOZ aerosol-climate model to assess the effects of black carbon (BC) mitigation measures on Arctic climate. To this end we constructed several mitigation scenarios that implement all currently existing legislation and then implement further reductions of BC in a successively increasing global area, starting from the eight member states of the Arctic Council, expanding to its active observer states, then to all observer states, and finally to the entire globe. These scenarios also account for the reduction of the co-emitted organic carbon (OC) and sulfate (SU). We find that, even though the additional BC emission reductions in the member states of the Arctic Council are small, the resulting reductions in Arctic BC mass burdens can be substantial, especially in the lower troposphere close to the surface. This in turn means that reducing BC emissions only in the Arctic Council member states can reduce BC deposition in the Arctic by about 30 % compared to the current legislation, which is about 60 % of what could be achieved if emissions were reduced globally. Emission reductions further south affect Arctic BC concentrations at higher altitudes and thus only have small additional effects on BC deposition in the Arctic. The direct radiative forcing scales fairly well with the total amount of BC emission reduction, independent of the location of the emission source, with a maximum direct radiative forcing in the Arctic of about −0.4 W m−2 for a global BC emission reduction. On the other hand, the Arctic effective radiative forcing due to the BC emission reductions, which accounts for aerosol–cloud interactions, is small compared to the direct aerosol radiative forcing. This happens because BC- and OC-containing particles can act as cloud condensation nuclei, which affects cloud reflectivity and lifetime and counteracts the direct radiative forcing of BC. Additionally, the effective radiative forcing is accompanied by very large uncertainties that originate from the strong natural variability of meteorology, cloud cover, and surface albedo in the Arctic. We further used the TM5-FASST model to assess the benefits of the aerosol emission reductions for human health. We found that a full implementation in all Arctic Council member and observer states could reduce the annual global number of premature deaths by 329 000 by the year 2030, which amounts to 9 % of the total global premature deaths due to particulate matter.

Multi-year analysis of aerosol optical properties and implications to radiative forcing over urban Pretoria, South Africa

The present study focused on investigating the aerosol optical properties and direct aerosol radiative forcing (DARF) using the AERONET Sunphotometer measurements conducted during 2011–2017 over an urban-industrial city, Pretoria (25.75° S, 28.28° E) located in the Northwest of South Africa (SA). Results revealed high aerosol optical depth (AOD440) and Angstrom exponent (AE440–870) during SON (0.28 ± 0.09, 1.46 ± 0.16) and DJF (0.24 ± 0.07, 1.48 ± 0.18) indicating the dominance of anthropogenic fine-mode aerosols from biomass burning. The single scattering albedo (SSA440) reached a maximum of 0.90 ± 0.05 in DJF and minimum (0.85 ± 0.04) during JJA. The source analysis from the concentration-weighted trajectory (CWT) model exhibited large heterogeneity in all seasons. However, the region is influenced by distinct aerosol types with the abundance of anthropogenic fine absorbing aerosols (86%) and the invisible ratio of dust particles. In addition, the aerosol volume size distribution (VSD) increased with an increasing AOD, exhibiting a bimodal lognormal structure, with a more pronounced peak in fine- relative to coarse-mode particles. Further, the inversion products showed a strong spectral dependence in SSA with substantial heterogeneity in all seasons. At last, the Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART) model showed that the DARF within the atmosphere was more pronounced during SON (46.71 W m−2) and JJA (46.39 W m−2) due to significant differences in AOD and SSA, with an annual mean value of 33.16 W m−2, and the corresponding atmospheric heating rate of 0.96 K day−1. The study provides information on the existing aerosol distribution and their potential impact on climatic change over an urban city in the Northwest of SA and could form a basis for policymaking over the region.

Retrieval of gridded aerosol direct radiative forcing based on multiplatform datasets

Abstract. Atmospheric aerosols play a crucial role in regional radiative budgets. Previous studies on clear-sky aerosol direct radiative forcing (ADRF) have mainly been limited to site-scale observations or model simulations for short-term cases, and long-term distributions of ADRF in China have not been portrayed yet. In this study, an accurate fine-resolution ADRF estimate at the surface was proposed. Multiplatform datasets, including satellite (MODIS aboard Terra and Aqua) and reanalysis datasets, served as inputs to the Santa Barbara Discrete Atmospheric Radiative Transfer (SBDART) model for ADRF simulation with consideration of the aerosol vertical profile over eastern China during 2000–2016. Specifically, single-scattering albedo (SSA) from the Modern-Era Retrospective Analysis for Research and Application, Version 2 (MERRA-2) was validated with sun photometers over eastern China. The gridded asymmetry parameter (ASY) was then simulated by matching the calculated top-of-atmosphere (TOA) radiative fluxes from the radiative transfer model with satellite observations (Clouds and the Earth's Radiant Energy System, CERES). The high correlation and small discrepancy (6–8 W m−2) between simulated and observed radiative fluxes at three sites (Baoshan, Fuzhou, and Yong'an) indicated that ADRF retrieval is feasible and has high accuracy over eastern China. Then this method was applied in each grid of eastern China, and the overall picture of ADRF distributions over eastern China during 2000–2016 was displayed. ADRF ranges from −220 to −20 W m−2, and annual mean ADRF is −100.21 W m−2, implying that aerosols have a strong cooling effect at the surface in eastern China. With the economic development and rapid urbanization, the spatiotemporal changes of ADRF during the past 17 years are mainly attributed to the changes of anthropogenic emissions in eastern China. Our method provides the long-term ADRF distribution over eastern China for the first time, highlighting the importance of aerosol radiative impact under climate change.

Air quality and climate change, Topic 3 of the Model Inter-Comparison Study for Asia Phase III (MICS-Asia III) – Part 2: aerosol radiative effects and aerosol feedbacks

Abstract. Topic 3 of the Model Inter-Comparison Study for Asia (MICS-Asia) Phase III examines how online coupled air quality models perform in simulating wintertime haze events in the North China Plain region and evaluates the importance of aerosol radiative feedbacks. This paper discusses the estimates of aerosol radiative forcing, aerosol feedbacks, and possible causes for the differences among the participating models. Over the Beijing–Tianjin–Hebei (BTH) region, the ensemble mean of estimated aerosol direct radiative forcing (ADRF) at the top of atmosphere, inside the atmosphere, and at the surface are −1.1, 7.7, and −8.8 W m−2 during January 2010, respectively. Subdivisions of direct and indirect aerosol radiative forcing confirm the dominant role of direct forcing. During severe haze days (17–19 January 2010), the averaged reduction in near-surface temperature for the BTH region can reach 0.3–1.6 ∘C. The responses of wind speeds at 10 m (WS10) inferred from different models show consistent declines in eastern China. For the BTH region, aerosol–radiation feedback-induced daytime changes in PM2.5 concentrations during severe haze days range from 6.0 to 12.9 µg m−3 (&lt;6 %). Sensitivity simulations indicate the important effect of aerosol mixing states on the estimates of ADRF and aerosol feedbacks. Besides, black carbon (BC) exhibits a large contribution to atmospheric heating and feedbacks although it accounts for a small share of mass concentration of PM2.5.

Preliminary comparison of the direct aerosol radiative forcing over Ukraine and Antarctic AERONET sites

Objectives. To analyze data on aerosol optical thickness (AOT) in the atmosphere over some Ukraine and Antarctic AERONET (AErosol RObotic NETwork) sites. To determine and compare direct aerosol radiative forcing (DRF) typical values using the data from midlatitude and Antarctic AERONET sites. Methods. Retrieval and visualization of the AERONET aerosol optical thickness and radiative forcing, data analysis and interpretation of the data. Radiative forcing evaluation using Global Atmospheric ModEl (GAME) and the AERONET operational product. Results. Aerosol optical thickness measurements are considered using observations by sun/sky photometers that are part of the AERONET sites in Ukraine (Kyiv) and at two additional sites in Antarctica (Vechernaya Hill and ARM_McMurdo sites). According to the 2015–2018 measurements at the Vechernaya Hill and ARM_McMurdo sites, the AOT values are small and are in the range of 0.05–0.1 at the 340 nm wavelength. In contrast, the corresponding AOT values from Kyiv observational site are reached 0.3–0.5 and sometimes higher. Using these AOT values from Kyiv site and the urban aerosol types, the aerosol direct radiative forcing has been evaluated by the GAME code. The top of atmosphere (TOA) DRF assessment using GAME suggests the instantaneous values of –5.7 W m–2 over vegetation surface for AOT equal 0.1. Conclusions. The AERONET derived aerosol optical thicknesses over Kyiv site show the mean value of 0.3 at 340 nm; the values over two Antarctic sites are in range of 0.03 to 0.06. Calculations using a numerical code (GAME) suggested the associated TOA instantaneous DRF of –6 W m–2 to –14 W m–2 over the Kyiv site. The values calculated as part of the AERONET Kyiv site operational product are about –20 W m–2 (BOA) and about –10 W m–2 (TOA) during 2018.

Estimation of marine dimethyl sulfide emissions from East Asian seas and their impact on natural direct radiative forcing

This study investigated the impacts of marine dimethyl sulfide (DMS) emitted from East Asian seas on natural direct radiative forcing (DRF) based on a numerical modeling approach. The high-resolution (4 km) DMS emissions (i.e., DMS flux) were estimated from sea surface DMS concentrations and transfer velocity in the air-sea interface using satellite observations and model-predicted data during the period 2014–2016. We inputted these emission data into an aerosol optical model and then estimated the natural DRF of sulfate aerosols derived from the DMS flux (i.e., DMS-derived sulfate) using a radiative transfer model. The total mean DMS flux in the study area during the study period was estimated to be approximately 0.73 μmol/m2/day, with a maximum of 3.4 μmol/m2/day (December 2014) and a minimum of 0.12 μmol/m2/day (March 2014). The total mean negative forcing at the top of the atmosphere (DRFTOA) due to the DMS-derived sulfate was approximately −1.3 W m−2 in the target area during the study period. The monthly maximum (−2.8 W m−2) and minimum DRFTOA (−0.82 W m−2) occurred in December and March 2014, respectively, with the highest and lowest forcing at the surface and in the atmosphere during the same months. In terms of spatial distributions over the entire study area, negative DRFTOA exhibited a maximum of up to −8.8 W m−2 in the coastal sea near Shanghai, China, and a minimum of −0.6 W m−2 in the offshore sea of the East Sea, Korea. These increases and decreases in the natural DRFs were strongly attributed to the DMS fluxes, which were affected mainly by sea surface wind and/or partially by DMS concentrations, and relative humidity conditions in the study region.