Black Carbon Direct Radiative Forcing Research Articles

Source-specified atmospheric age distribution of black carbon and its impact on optical properties over the Yangtze River Delta

Black carbon (BC) exerts a profound and intricate impact on both air quality and climate due to its high light absorption. However, the uncertainty in representing the absorption enhancement of BC in climate models leads to an increased range in the modeled aerosol climate effects. Changes in BC optical properties could result either from atmospheric aging processes or from variations in its sources. In this study, a source-age model for identifying emission sources and aging states presented by University of California at Davis/California Institute of Technology (UCD/CIT) was used to simulate the atmospheric age distribution of BC from different sources and to quantify its impact on the optical properties of BC-containing particles. The results indicate that regions with greater aged BC concentrations do not correspond to regions with higher BC emissions due to atmospheric transport. High concentrations of aged BC are found in northern Yangtze River Delta (YRD) regions during summer. The chemical compositions of particles from different sources and with different atmospheric ages differ significantly. BC and primary organic aerosols (POA) are dominating in Traffic-dominated source while other components dominate in Industry-dominated source. As the atmospheric age increases, the mass fraction of secondary inorganic aerosols rises. Compared to the original model, the simulated mass absorption cross section of BC particles in the source-age model decreases while the single scattering albedo increases. This compensates for ~11 % of the overestimation of the simulated BC direct radiative forcing. Our study highlights that incorporating atmospheric age and source information into models can greatly improve the estimation of optical properties of BC-containing particles and deepen our understanding of their climate effects.

Responses of wildfire-induced global black carbon pollution and radiative forcing to climate change

The impacts of climate change on wildfires have been studied extensively. Along with declining emissions from fossil fuel combustion due to anthropogenic emission control, black carbon (BC) released from wildfires is expected to contribute a more significant portion to its atmospheric burden. However, from a global perspective, little is known about the BC burden and radiative forcing caused by wildfires. Here, we report the results from the long-term wildfire-induced BC concentration and direct radiative forcing (DRF) from 1981 to 2010 globally simulated by an Earth System Model using an updated wildfire BC emission inventory. We show that wildfire-induced BC concentration and DRF varied significantly spatially and temporarily, with the highest in sub-Saharan Africa, attributable to its highest level of wildfire BC emission worldwide. The temporal trends of near-surface air temperature, precipitation, and evapotranspiration and their association with wildfire-induced BC concentration are explored using the multidimensional ensemble empirical mode decomposition. A statistically significant relation between changes in climate parameters and wildfire-induced BC concentration was found for 53% of the land grid cells, explaining on average 33% of the concentration variations. The result suggests that the wildfire-induced BC DRF, with an increasing trend, would be an important contributor to climate change, especially in sub-Saharan Africa. Occurrences of wildfires in the Amazon Basin respond most strongly to climate change and play an increasingly important role in changing the global climate.

Numerical study of aerosol radiative forcing over East Asia and the impacts of cloud coverage and relative humidity

The spatiotemporal characteristics of aerosol direct radiative forcing (RF) and the relative contributions from aerosol species, as well as the impacts of cloud coverage and relative humidity on aerosol direct RF were quantified in East Asia using a regional climate model. Generally, the total aerosol produces net RFs of −12.78 W m−2 at surface, 1.72 W m−2 at TOA (top-of-atmosphere), and atmospheric heating of 14.50 W m−2. It was found that dust, black carbon, and sulfate made dominant contributions to the total RF at surface and TOA, and all aerosol species induced atmospheric heating, whereas more than 96% of which was induced by dust and black carbon. The remarkably seasonally decreasing tendency of the total and the absorbing aerosol RFs was found from spring to winter at surface. Moreover, dust contributes relatively larger to the positive TOA RF and to the atmospheric heating in spring and summer, which were weakened and smaller than black carbon in other seasons. Sensitivity studies further demonstrated cloud strengthens the dust and black carbon direct RF and weakens the other species direct RF at TOA, while induces weak direct RFs of all aerosol species at surface. Particularly, cloud induced larger reduction in dust longwave RF than shortwave leads to remarkable enhanced net surface direct RF of dust, especially in JJA. The aerosol swelling effect induced by relative humidity strengthens aerosol direct RF at both TOA and surface. The percentage changes in aerosol RF and its seasonal amplitude by cloud are considered larger at TOA than surface, however, the effects of relative humidity distribute relatively uniform vertically. Meteorological factors impact on scattering aerosols direct RF is assumed larger than absorbing aerosols. The impacts of cloud on aerosol direct RF are compared to the relative humidity and are supposed to be more important at TOA and surface.

The Direct Radiative Forcing Impact of Agriculture‐Emitted Black Carbon Associated With India's Green Revolution

AbstractBiomass burning contributes considerably to black carbon (BC) emissions in South Asia, but such emissions have not been linked with the Green Revolution (GR) which has enabled substantial crop production growth in South Asian countries, India in particular. Here, we use an Earth system model to quantify climate change through the direct radiative forcing (DRF) by agriculture‐emitted BC associated with the GR in India. We show that the BC DRF in India has increased significantly since the GR, especially during the post‐GR period. The estimated BC DRF in India rose from +0.197 W/m2 in 1961 to +0.805 W/m2 in 2011; this represents a fourfold increase in DRF since the onset of the GR. The contribution of BC DRF by India's intensive agriculture to the global BC forcing also increased from 2.6% to 4.4% during the GR. Our results reveal that increasing BC emissions associated with the GR raises the importance of emission mitigation from agriculture source.

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).

Observations of black carbon and albedo over a Central Himalayan Glacier (Satopanth): Preliminary results

Simultaneous measurements of ambient atmospheric black carbon (BC) mass concentrations and radiative fluxes were carried out over Satopanth glacier in the central Himalayas from September 22 to October 2, 2016, as a part of a glacier campaign experiment. The daily mean atmospheric BC concentrations varied between 165 ± 20–263 ± 32 ng m−3 with a mean of 199 ± 54 ng m−3 during the observational period. The measured average surface albedo was found to be 0.24 ± 0.11 during the entire period of observation. Spectral albedo from Moderate Resolution Imaging Spectroradiometer - Bidirectional Reflectance Distribution Function (MODIS-BRDF) satellite observation and net radiometer derived glacier albedo was found to be in good agreement with a correlation of 0.64 over the region. Concentration weighted trajectory analysis (CWT) over the site indicates a 70% BC transport from the Indo-Gangetic plain, Pakistan, and the Middle East region. BC radiative forcing was estimated using an optical model along with a radiative transfer model. An average BC direct radiative forcing of −5.4 ± 0.25 W m−2 and 2.4 ± 0.19 W m−2 was found respectively in the surface and at the top of the atmosphere (TOA) during the experimental period. The estimated average BC induced heating rate was found to be 0.33 ± 0.04 K day−1over the region.

Black carbon over a central Himalayan Glacier (Satopanth): Pathways and direct radiative impacts

Continuous measurement of Black Carbon (BC) concentration was carried out during May–October 2018 periods over Satopanth Glacier in the central Himalayas. BC concentrations varied between 28 and 287 ngm−3 on different days during the observational period. High concentration of BC was observed in the month of May (monthly mean of 221 ± 79 ngm−3), and a lower concentration was observed in August (monthly mean of 92 ± 58 ngm−3). Biomass burning was found to contribute up to 58% of BC mass over the region, with lower contribution during June and higher during the month of May. Compensation parameter (K) values were found to vary between −0.005 and 0.005 in different months, asserting the presence of aged BC in June to October months and relatively fresh BC in the month of May. Concentration weighted trajectory (CWT) analysis showed that the air mass from Indo Gangetic Plains (IGP) was responsible for the majority of transported BC in July & August months (up to 65%) and partially in September (up to 40%). However, the transport from Middle East and far north-western regions was found to be the major contributor to BC concentrations in other months. The estimated BC direct radiative forcing was found to induce 4.5 to 7.6 Wm−2 reduction of radiation at the surface (SFC) and the forcing was +2.3 to +3.5 Wm−2 at the Top of the Atmosphere (TOA). The BC induced atmospheric heating rates were found to be up to 0.35 k day−1 over the region. The sensitivity of snow albedo to radiative forcing was studied, and it is found that BC albedo changes tend to decrease albedo with an increase in BC-snow deposition, leading to a decrease in atmospheric absorption.

Black Carbon Absorption Efficiency Under Preindustrial and Present‐Day Conditions Simulated by a Size‐ and Mixing‐State‐Resolved Global Aerosol Model

AbstractThe positive radiative forcing of black carbon (BC) aerosol depends not only on the spatial and temporal distribution of BC but also its absorption efficiency. The mass absorption cross section (MAC) of BC is enhanced by atmospheric aging processes that increase particle size and non‐BC coating amounts (mixing state) of BC‐containing particles. However, the representation of MAC (or absorption enhancement) in current global aerosol models has a large uncertainty. This study used a global aerosol model that resolves particle size and mixing state to show that the MAC of anthropogenic BC has increased by 50% from preindustrial to present‐day conditions (from 5.6 to 8.6 m2 g−1) because faster present‐day aging processes increase the fraction of thickly coated BC particles, which have high absorption efficiency. This effect is apparent only when the model considers BC mixing state with sufficient resolution. The impact of this MAC enhancement on BC direct radiative forcing is estimated to be 0.051–0.086 W m−2 globally (22–37% of anthropogenic BC direct radiative forcing, 0.23 W m−2) and exceeds 0.5 W m−2 near‐source regions in East Asia. Sensitivity simulations show that BC direct radiative forcing and MAC are highly sensitive to non‐BC emissions, secondary aerosol formation, and aerosol size distribution and mixing state in emissions. We must therefore improve our understanding of these factors by further observations and reduce their discrepancies between models to achieve better estimates of BC absorption efficiency and radiative forcing.

Direct Radiative Forcing Induced by Light‐Absorbing Aerosols in Different Climate Regions Over East Asia

AbstractLight‐absorbing aerosols (LAAs), mainly composed of black carbon (BC) and dust aerosols, are responsible for significant climate forcing through their strong absorption of solar radiation. A fully coupled meteorology‐chemistry model (WRF‐Chem) associated with satellite retrievals and in situ measurements is used to investigate the direct radiative forcing (DRF) induced by LAAs in different climate regions over East Asia. Results show that the annual all‐sky dust and BC DRF are −0.84 and 1.06 W m−2 at the top of atmosphere (TOA), −1.23 and −1.55 W m−2 at the surface (SUR), and 0.39 and 2.61 W m−2 within the atmosphere (ATM) over East Asia. Large LAAs DRF can be found in hyper‐arid, subhumid, and humid regions at the SUR and ATM where dust DRF dominates the surface cooling effect, while BC DRF is predominant in the eminent warming effect on ATM in most climate regions. The meteorological conditions in hyper‐arid region are associated with enhanced surface wind and weakened atmospheric wind, which is in favor of the emission and accumulation of dust supporting the positive LAAs DRF anomalies higher than 10 W m−2 in hyper‐arid region. The positive geopotential height anomalies over Northeast China weaken the westerly winds, which is beneficial to the accumulation of LAAs, and results in the positive LAAs DRF anomalies of 3 W m−2 in semiarid regions. The large LAAs mass loading, strong aerosol absorptive ability, and decreased cloudiness caused by northerly anomalies are responsible for the high LAAs DRF in humid region.

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.

Amplification of light absorption of black carbon associated with air pollution

Abstract. The impacts of black carbon (BC) aerosols on air quality, boundary layer dynamics and climate depend not only on the BC mass concentration but also on the light absorption capability of BC. It is well known that the light absorption capability of BC depends on the amount of coating materials (namely other species that condense and coagulate on BC). However, the difference of light absorption capability of ambient BC-containing particles under different air pollution conditions (e.g., clean and polluted conditions) remains unclear due to the complex aging process of BC in the atmosphere. In this work, we investigated the evolution of light absorption capability for BC-containing particles with changing pollution levels in urban Beijing, China. During the campaign period (17 to 30 November 2014), with an increase in PM1 concentration from ∼ 10 to ∼ 230 µg m−3, we found that the mass-weighted averages of the aging degree and theoretical light absorption capability of BC-containing particles increased by ∼ 33 % and ∼ 18 %, respectively, indicating stronger light absorption capability of BC-containing particles under more polluted conditions due to more coating materials on the BC surface. By using an effective emission intensity (EEI) model, we further found that aging during regional transport plays an important role in the difference in the light absorption capability of BC-containing particles under different air pollution levels. During the pollution episode, ∼ 63 % of the BC over Beijing originated from regional sources outside of Beijing. These regionally sourced BC-containing particles were characterized by more coating materials on BC surfaces due to more coating precursors within more polluted air, which contributed ∼ 75 % of the increase in theoretical light absorption capability of BC observed in Beijing during the polluted period (PM1 of ∼ 230 µg m−3) compared to that in the clean period (PM1 of ∼ 10 µg m−3). Due to the increase in theoretical light absorption capability of BC associated with air pollution, the direct radiative forcing of BC was estimated to be increased by ∼ 18 % based on a simple radiation transfer model. Our work identified an amplification of theoretical light absorption and direct radiative forcing under a more polluted air environment due to more coating materials on BC. The air pollution control measures may, however, break the amplification effect by reducing emissions of both BC and the coating precursors and achieve co-benefits of both air quality and climate.

Assessment of emission scenarios for 2030 and impacts of black carbon emission reduction measures on air quality and radiative forcing in Southeast Asia

Abstract. Our previously published paper (Permadi et al. 2018) focused on the preparation of emission input data and evaluation of WRF–CHIMERE performance in 2007. This paper details the impact assessment of the future (2030) black carbon (BC) emission reduction measures for Southeast Asia (SEA) countries on air quality, health and BC direct radiative forcing (DRF). The business as usual (BAU2030) projected emissions from the base year of 2007 (BY2007), assuming “no intervention” with the linear projection of the emissions based on the past activity data for Indonesia and Thailand and the sectoral GDP growth for other countries. The RED2030 featured measures to cut down emissions in major four source sectors in Indonesia and Thailand (road transport, residential cooking, industry, biomass open burning) while for other countries the Representative Concentration Pathway 8.5 (RCP8.5) emissions were assumed. WRF–CHIMERE simulated levels of aerosol species under BAU2030 and RED2030 for the modeling domain using the base year meteorology and 2030 boundary conditions from LMDZ-INCA. The extended aerosol optical depth module (AODEM) calculated the total columnar AOD and BC AOD for all scenarios with an assumption on the internal mixing state. Under RED2030, the health benefits were analyzed in terms of the number of avoided premature deaths associated with ambient PM2.5 reduction along with BC DRF reduction. Under BAU2030, the average number of the premature deaths per 100 000 people in the SEA domain would increase by 30 from BY2007 while under RED2030 the premature deaths would be cut down (avoided) by 63 from RED2030. In 2007, the maximum annual average BC DRF in the SEA countries was 0.98 W m−2, which would increase to 2.0 W m−2 under BAU2030 and 1.4 W m−2 under RED2030. Substantial impacts on human health and BC DRF reduction in SEA could result from the emission measures incorporated in RED2030. Future works should consider other impacts, such as for agricultural crop production, and the cost–benefit analysis of the measures' implementation to provide relevant information for policy making.

Spatial Representativeness Error in the Ground-Level Observation Networks for Black Carbon Radiation Absorption.

There is high uncertainty in the direct radiative forcing of black carbon (BC), an aerosol that strongly absorbs solar radiation. The observation‐constrained estimate, which is several times larger than the bottom‐up estimate, is influenced by the spatial representativeness error due to the mesoscale inhomogeneity of the aerosol fields and the relatively low resolution of global chemistry‐transport models. Here we evaluated the spatial representativeness error for two widely used observational networks (AErosol RObotic NETwork and Global Atmosphere Watch) by downscaling the geospatial grid in a global model of BC aerosol absorption optical depth to 0.1° × 0.1°. Comparing the models at a spatial resolution of 2° × 2° with BC aerosol absorption at AErosol RObotic NETwork sites (which are commonly located near emission hot spots) tends to cause a global spatial representativeness error of 30%, as a positive bias for the current top‐down estimate of global BC direct radiative forcing. By contrast, the global spatial representativeness error will be 7% for the Global Atmosphere Watch network, because the sites are located in such a way that there are almost an equal number of sites with positive or negative representativeness error.

Impacts of East Asian summer and winter monsoons on interannual variations of mass concentrations and direct radiative forcing of black carbon over eastern China

Abstract. We applied a global three-dimensional chemical transport model (GEOS-Chem) to examine the impacts of the East Asian monsoon on the interannual variations of mass concentrations and direct radiative forcing (DRF) of black carbon (BC) over eastern China (110–125° E, 20–45° N). With emissions fixed at the year 2010 levels, model simulations were driven by the Goddard Earth Observing System (GEOS-4) meteorological fields for 1986–2006 and the Modern Era Retrospective-analysis for Research and Applications (MERRA) meteorological fields for 1980–2010. During the period of 1986–2006, simulated June–July–August (JJA) and December–January–February (DJF) surface BC concentrations were higher in MERRA than in GEOS-4 by 0.30 µg m−3 (44 %) and 0.77 µg m−3 (54 %), respectively, because of the generally weaker precipitation in MERRA. We found that the strength of the East Asian summer monsoon (EASM; East Asian winter monsoon, EAWM) negatively correlated with simulated JJA (DJF) surface BC concentrations (r = −0. 7 (−0.7) in GEOS-4 and −0.4 (−0.7) in MERRA), mainly by the changes in atmospheric circulation. Relative to the 5 strongest EASM years, simulated JJA surface BC concentrations in the 5 weakest monsoon years were higher over northern China (110–125° E, 28–45° N) by 0.04–0.09 µg m−3 (3–11 %), but lower over southern China (110–125° E, 20–27° N) by 0.03–0.04 µg m−3 (10–11 %). Compared to the 5 strongest EAWM years, simulated DJF surface BC concentrations in the 5 weakest monsoon years were higher by 0.13–0.15 µg m−3 (5–8 %) in northern China and by 0.04–0.10 µg m−3 (3–12 %) in southern China. The resulting JJA (DJF) mean all-sky DRF of BC at the top of the atmosphere was 0.04 W m−2 (3 %; 0.03 W m−2, 2 %) higher in northern China but 0.06 W m−2 (14 %; 0.03 W m−2, 3 %) lower in southern China. In the weakest monsoon years, the weaker vertical convection at the elevated altitudes led to the lower BC concentrations above 1–2 km in southern China, and therefore the lower BC DRF in the region. The differences in vertical profiles of BC between the weakest and strongest EASM years (1998–1997) and EAWM years (1990–1996) reached up to −0.09 µg m−3 (−46 %) and −0.08 µg m−3 (−11 %) at 1–2 km in eastern China.

Source attribution of black carbon and its direct radiative forcing in China

Abstract. The source attributions for mass concentration, haze formation, transport and direct radiative forcing of black carbon (BC) in various regions of China are quantified in this study using the Community Earth System Model (CESM) with a source-tagging technique. Anthropogenic emissions are from the Community Emissions Data System that is newly developed for the Coupled Model Intercomparison Project Phase 6 (CMIP6). Over north China where the air quality is often poor, about 90 % of near-surface BC concentration is contributed by local emissions. Overall, 35 % of BC concentration over south China in winter can be attributed to emissions from north China, and 19 % comes from sources outside China in spring. For other regions in China, BC is largely contributed from nonlocal sources. We further investigated potential factors that contribute to the poor air quality in China. During polluted days, a net inflow of BC transported from nonlocal source regions associated with anomalous winds plays an important role in increasing local BC concentrations. BC-containing particles emitted from East Asia can also be transported across the Pacific. Our model results show that emissions from inside and outside China are equally important for the BC outflow from East Asia, while emissions from China account for 8 % of BC concentration and 29 % in column burden in the western United States in spring. Radiative forcing estimates show that 65 % of the annual mean BC direct radiative forcing (2.2 W m−2) in China results from local emissions, and the remaining 35 % is contributed by emissions outside of China. Efficiency analysis shows that a reduction in BC emissions over eastern China could have a greater benefit for the regional air quality in China, especially in the winter haze season.

A potential large and persistent black carbon forcing over Northern Pacific inferred from satellite observations

Black carbon (BC) aerosol strongly absorbs solar radiation, which warms climate. However, accurate estimation of BC’s climate effect is limited by the uncertainties of its spatiotemporal distribution, especially over remote oceanic areas. The HIAPER Pole-to-Pole Observation (HIPPO) program from 2009 to 2011 intercepted multiple snapshots of BC profiles over Pacific in various seasons, and revealed a 2 to 5 times overestimate of BC by current global models. In this study, we compared the measurements from aircraft campaigns and satellites, and found a robust association between BC concentrations and satellite-retrieved CO, tropospheric NO2, and aerosol optical depth (AOD) (R2 > 0.8). This establishes a basis to construct a satellite-based column BC approximation (sBC*) over remote oceans. The inferred sBC* shows that Asian outflows in spring bring much more BC aerosols to the mid-Pacific than those occurring in other seasons. In addition, inter-annual variability of sBC* is seen over the Northern Pacific, with abundances varying consistently with the springtime Pacific/North American (PNA) index. Our sBC* dataset infers a widespread overestimation of BC loadings and BC Direct Radiative Forcing by current models over North Pacific, which further suggests that large uncertainties exist on aerosol-climate interactions over other remote oceanic areas beyond Pacific.

Estimation of global black carbon direct radiative forcing and its uncertainty constrained by observations

AbstractBlack carbon (BC) contributes to global warming by absorbing sunlight. However, the size of this contribution, namely, the direct radiative forcing (RF), ranges from +0.1 to +1.0 W m−2, largely due to differences between bottom‐up and observation‐based estimates. Current global models systematically underestimate BC radiation absorption relative to observations, which is often attributed to the underestimation of BC emissions. Several studies that adjusted emissions to correct biases of global aerosol models resulted in a revised upward estimate of the BC RF. However, the BC RF was never optimized against observations in a rigorous mathematical manner. Here we simulated the absorption of solar radiation by BC from all sources at the 10 km resolution by combining a highly disaggregated emission inventory with a nested aerosol climate model and a downscaling method. As a result, the normalized mean bias in BC radiation absorption was reduced from −51% to −24% in Asia and from −57% to −50% elsewhere. We applied a Bayesian method that makes the best account of all model, representativeness and observational uncertainties to estimate the BC RF and its uncertainty. Using the new emission inventory and high‐resolution model reduces uncertainty in BC RF from −101%/+152% to −70%/+71% over Asia and from −83%/+108% to −64%/+68% over other continental regions. Finally we derived an observationally constrained BC RF of 0.61 Wm−2 (0.16 to 1.40 as 90% confidence) as our best estimate. Our estimate implies that reduction in BC emissions would contribute to slow down global warming, but the contribution could be less than previously thought.

Black carbon aerosol in winter northeastern Qinghai–Tibetan Plateau, China: the source, mixing state and optical property

Abstract. Black carbon (BC) aerosol at high altitudes of the Qinghai–Tibetan Plateau has potential effects on the regional climate and hydrological cycle. An intensive measurement campaign was conducted at Qinghai Lake (~ 3200 m above sea level) at the edge of the northeastern Qinghai–Tibetan Plateau during winter using a ground-based single particle soot photometer (SP2) and a photoacoustic extinctiometer (PAX). The average concentration of refractory BC (rBC) and number fraction of coated rBC were found to be 160 ± 190 ng m−3 and 59 % for the entire campaign, respectively. Significant enhancements of rBC loadings and number fraction of coated rBC were observed during a pollution episode, with an average value of 390 ng m−3 and 65 %, respectively. The mass size distribution of rBC particles showed log-normal distribution, with a peak diameter of ~ 187 nm regardless of the pollution level. Five-day backward trajectory analysis suggests that the air masses from north India contributed to the increased rBC loadings during the campaign. The potential source contribution function (PSCF) model combined with the fire counts map further proves that biomass burning from north India is an important potential source influencing the northeastern Qinghai–Tibetan Plateau during the pollution episode. The rBC mass absorption cross section (MACrBC) at λ = 532 nm was slightly larger in clean days (14.9 m2 g−1) than during the pollution episode (9.3 m2 g−1), likely due to the effects of brown carbon and the uncertainty of the MACrBC calculation. The MACrBC was positively correlated with number fraction of coated rBC during the pollution episode with an increasing rate of 0.18 (m2 g−1) %−1. The number fraction of coated rBC particles showed positive correlation with light absorption, suggesting that the increase of coated rBC particles will enhance the light absorption. Compared to rBC mass concentration, rBC mixing sate is more important in determining absorption during the pollution episode, estimated from the same percentage-wise increment of either rBC mass concentration or the number fraction of coated rBC. The estimated BC direct radiative forcing was +0.93 W m−2 for the pollution episode, which is 2 times larger than that in clean days. Our study provides insight into the potential climatic impacts of rBC aerosol transported to the Qinghai–Tibetan Plateau from south Asian regions, and is also useful for future modeling studies.

Source sector and region contributions to concentration and direct radiative forcing of black carbon in China

Abstract We quantify the contributions from five domestic emission sectors (residential, industry, transportation, energy, and biomass burning) and emissions outside of China (non-China) to concentration and direct radiative forcing (DRF) of black carbon (BC) in China for year 2010 using a nested-grid version of the global chemical transport model (GEOS-Chem) coupled with a radiative transfer model. The Hemispheric Transport of Air Pollution (HTAP) anthropogenic emissions of BC for year 2010 are used in this study. Simulated surface-layer BC concentrations in China have strong seasonal variations, which exceed 9 μg m−3 in winter and are about 1–5 μg m−3 in summer in the North China Plain and the Sichuan Basin. Residential sector is simulated to have the largest contribution to surface BC concentrations, by 5–7 μg m−3 in winter and by 1–3 μg m−3 in summer, reflecting the large emissions from winter heating and the enhanced wet deposition during summer monsoon. The contribution from industry sector is the second largest and shows relatively small seasonal variations; the emissions from industry sector contribute 1–3 μg m−3 to BC concentrations in the North China Plain and the Sichuan Basin. The contribution from transportation sector is the third largest, followed by that from biomass burning and energy sectors. The non-China emissions mainly influence the surface-layer concentrations of BC in western China; about 70% of surface-layer BC concentration in the Tibet Plateau is attributed to transboundary transport. Averaged over all of China, the all-sky DRF of BC at the top of the atmosphere (TOA) is simulated to be 1.22 W m−2. Sensitivity simulations show that the TOA BC direct radiative forcings from the five domestic emission sectors of residential, industry, energy, transportation, biomass burning, and non-China emissions are 0.44, 0.27, 0.01, 0.12, 0.04, and 0.30 W m−2, respectively. The domestic and non-China emissions contribute 75% and 25% to BC DRF in China, respectively. These results have important implications for taking measures to reduce BC emissions to mitigate near-term climate warming and to improve air quality in China.

Using an explicit emission tagging method in global modeling of source‐receptor relationships for black carbon in the Arctic: Variations, sources, and transport pathways

AbstractWe introduce an explicit emission tagging technique in the Community Atmosphere Model to quantify source‐region‐resolved characteristics of black carbon (BC), focusing on the Arctic. Explicit tagging of BC source regions without perturbing the emissions provides a physically consistent and computationally efficient approach to establish source‐receptor relationships and transport pathways. Our analysis shows that the contributions of major source regions to the global BC burden are not proportional to the respective emissions due to strong region‐dependent removal rates and lifetimes, while the contributions to BC direct radiative forcing show a near‐linear dependence on their respective contributions to the burden. Arctic BC concentrations, deposition, and source contributions all have strong seasonal variations. Eastern Asia contributes the most to the wintertime Arctic BC burden, but has much less impact on lower‐level concentrations and deposition. Northern Europe emissions are more important to both surface concentration and deposition in winter than in summer. The largest contribution to Arctic BC in the summer is from Northern Asia. Although local emissions contribute less than 10% to the annual mean BC burden and deposition within the Arctic, the per‐emission efficiency is much higher than for non‐Arctic sources. The interannual variability (1996–2005) due to meteorology is small in annual mean BC burden and radiative forcing but is significant in yearly seasonal means over the Arctic. When a slow aging treatment of BC is introduced, the increase of BC lifetime and burden is source dependent. Global BC forcing‐per‐burden efficiency also increases primarily due to changes in BC vertical distributions.