Advances in the Research on Brown Carbon Aerosols: Its Concentrations, Radiative Forcing, and Effects on Climate

Received: October 11, 2022
Revised: March 6, 2023
Accepted: May 15, 2023

 Copyright The Author(s). This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are cited.

Cite this article:

Li, S., Zhang, H., Wang, Z., Chen, Y. (2023). Advances in the Research on Brown Carbon Aerosols: Its Concentrations, Radiative Forcing, and Effects on Climate. Aerosol Air Qual. Res. 23, 220336. https://doi.org/10.4209/aaqr.220336

HIGHLIGHTS

• Global distribution and estimation of brown carbon (BrC) concentration are analyzed.
• Advance on radiative forcing of BrC and its effects on temperature and precipitation are discussed.
• Including key factors influencing the radiative forcing of BrC simulated by the model.

Keywords: Brown carbon, Climate effect, Radiative forcing, Temperature and precipitation, Snow/ice albedo effect

1 INTRODUCTION

The burning of biomass and fossil fuels has rapidly expanded in recent decades due to the rapid growth of the world economy and industry, and carbonaceous aerosols with light-absorbing capabilities have gradually grown in importance as a component of atmospheric aerosols (Bond, 2001; Yang et al., 2009; Castro et al., 1999). Carbonaceous aerosols include black carbon (BC) and organic carbon (OC). BC has long been regarded as the only light-absorbing substance in carbonaceous aerosols, with strong light absorption in the ultraviolet to infrared spectrum (Xu et al., 2009; Bhat et al., 2017), while OC has a good scattering effect on visible radiation (Kanakidou et al., 2005). Until the 1990s, it was found that there existed a large number of OCs with some light-absorbing ability between strongly light-absorbing BC and non-light-absorbing OC (Kriacsy et al., 2000).

Andreae et al. (2006) named such special OCs as brown carbon (BrC) for the first time.

BrC aerosols have strong light absorption in the near-ultraviolet and visible regions, and studies have shown that lab-generated BrC contains 41 compounds that are likely to be absorbed in the UV and visible regions (Budisulistiorini et al., 2017). The light absorption of BrC is strongly wavelength dependent (Laskin et al., 2015) and decreases with increasing wavelength (Jacobson, 1999). The contribution of BrC to light absorption in the shortwave band can reach 20%–50% (Kirchstetter and Thatcher, 2012; Shamjad et al., 2016; Wang et al., 2016), and the strong absorption of BrC in the shortwave band can lead to a change in the photolysis rate of some gases associated with photochemical reactions, thus changing concentration of oxides in the atmosphere and the radiation budget (Lu et al., 2012; Massabo et al., 2015). BrC produces a positive radiative forcing on climate, about 1/4 of that of BC (Feng et al., 2013). It can also enter the aquatic environment from the atmosphere through precipitation. There, it is broken down by microorganisms through a decomposition reaction and released into the atmosphere as CO₂, contributing to the global carbon cycle and causing global warming (Farjalla et al., 2009).

BrC can affect local or global climate change via a variety of mechanisms: first, it can directly absorb UV and visible radiation, disturbing the energy balance of the earth-atmosphere system, and directly affecting the climate (Barnard et al., 2008); second, it can act as cloud condensation nuclei or ice nuclei to modify the microphysical and radiative properties of clouds and cloud lifetimes, indirectly affecting the climate system (Gelencser et al., 2003); third, BrC in the atmosphere can also absorb solar radiation and heat the atmosphere, leading to evaporation of water vapor and reduction of cloud fraction (Lin et al., 2014). BrC also has snow/ice albedo effect (Wu et al., 2016; Tuccella et al., 2021). The main sources of BrC and its climate effects are shown in Fig. 1.

Fig. 1. The major sources of BrC and its effect ons climate.

BrC has therefore been the subject of much study and attention lately (Yan et al., 2014; Xie and Xu, 2017; Liu et al., 2020; Choudhary et al., 2021; Gao et al., 2022). Scholars have published many papers on BrC, including the sources of BrC (Zhi et al., 2015), measurement methods (Wang et al., 2020), model simulation (Xu et al., 2021), chemical composition (Laskin et al., 2015), optical properties and their influencing factors (Chen et al., 2022), mechanisms of generation and elimination (Guan et al., 2020) and pollution characteristics and sources (Zhao et al., 2021). However, the impacts of BrC on the climate have not been extensively studied. To this end, this paper reviews the research progress of BrC climate effects in terms of radiative forcing, its effects on temperature and precipitation, and snow/ice albedo effect. The existing problems are also pointed out. Finally, we propose suggestions and prospects for future research directions related to climate effects of BrC.

2 DISTRIBUTION OF BrC AEROSOLS

Sources of BrC come in a variety of forms, mostly primary source (POC) and secondary source (SOC). POC includes incomplete combustion of coal and biomass, motor vehicle emissions, etc.; and SOC includes reaction products of primary aerosols with nitrogen-containing substances and liquid-phase reaction products of carbon compounds, etc., The inclusions and mixing of BCs and inorganic ions with BrC further complicate its characteristics due to the unstable atmosphere (Chakrabarty et al., 2010; Cheng et al., 2011). The primary sources of global BrC are biofuel and biomass combustion (Cai et al., 2014; Lukács et al., 2007; Salehs et al., 2013). Jo et al. (2016) estimated the surface concentration of BrC (Fig. 2) and tropospheric emissions using a global 3D chemical transport model (GEOS-Chem). As can be seen from the figure, the global mean surface concentration of BrC is 0.129 µg Cm^–3, with the highest value in Southeast and South America (biomass burning), East Asia and Northeast India (biofuel) and Europe and North America (SOC). POC (3.9 ± 1.7 and 3.0 ± 1.3 TgC yr^–1, from biomass combustion and biofuel, respectively) accounts for 77% of BrC concentrations in surface air, while SOC (5.7 TgC yr^–1, from aromatic oxidation) accounts for 50% of total BrC concentrations (12.5 ± 3.0 TgC yr^–1) in the troposphere, it is lower in surface but increases with altitude. The mean global surface BrC/BC ratio is 1.24 (highest in the eastern North Pacific and the North Atlantic), and the mean BrC/OC ratio is 0.21 (Fig. 3). The mean BrC/BC ratio in the troposphere is 1.83 and the mean BrC/OC ratio is 0.27. The ratio of BrC to OC is relatively high in areas with high biofuel use (northern India and Central Asia); although China is one of the largest sources of BrC emissions, the proportion of BrC to BC and OC is relatively low due to high concentrations of BC and OC.

Fig. 2. Annual surface map of (a) total BrC and BrC concentrations from three source categories: (b) biomass burning, (c) biofuel, and (d) SOC (Unit is µg Cm⁻³). Mean values are presented in the upper right corner of each panel (Jo et al., 2016).
 

Fig. 3. Annual mean ratios of (a) BrC to BC and (b) OC in surface air. Global mean values are presented in the upper right corner of each panel (Jo et al., 2016).

It can also be seen from Fig. 4 that South America and South Africa, where biomass combustion or biofuel are dominant, have high BrC column concentrations; North America and Western Europe, where fossil fuel combustion emissions are dominant, have relatively low BrC column concentrations. There are large atmospheric BrC column concentrations all year round in Beijing, China due to the combined effects of biomass combustion, motor vehicle exhaust pollution, industrial emissions, and domestic combustion emissions.

 Fig. 4. Distribution of BrC aerosol concentration calculated by different methods in different area (where Region 1 is the USA and Europe (summer and autumn), Region 2 is South America and South Africa (summer and autumn), Region 3 is Beijing, China (winter), based on AERONET data (Arola et al., 2011); Region 4 is Beijing, China (summer), Region 5 is Beijing, China (autumn and winter), based on solar-sky radiometer measurements (Wang et al., 2013); Region 6 is Europe (year-round), based on spectral measurements of aerosol extracts (Lukács et al., 2007); Region 7 is the South African region (year-round), based on the Integrated Massively Parallel Atmospheric Chemistry Transport Model (IMPACT) (Barnard et al., 2008).
 

The differences in global BrC emission concentrations from different estimation methods may be mainly due to the large differences in the BrC emission source, inventories and their contributions from different studies (Table 1), as well as the differences in particle modes, composition, mixing state and aging characteristics of BrC (Wang et al., 2013). According to the IPCC Sixth Assessment Report, the knowledge of the complex chemical processes, source and sink budgets and atmospheric content of carbon aerosols is of low confidence due to the lack of global-scale observations of carbon aerosols. Therefore, future BrC estimation models need to consider the following aspects: (1) establish a more accurate inventory of BrC emissions from complex sources. In particular, SOC properties are complex and changeable. For example, BrCs produced by biological and man-made SOC have different optical properties (mass absorption efficiency, MAE, refractive index assumption, etc.) (Moiseg et al., 2015), which require different optical parameters for mode evaluation;

 
3 CLIMATE EFFECTS OF BrC AEROSOLS

 
3.1 The Optical Properties of BrC

The optical properties of BrC, including the absorption Ångström index (AAE), mass absorption efficiency (MAE), and the imaginary part of the refractive indices K_BrC and so on, are one of the key parameters for assessing its radiative forcing and climate effects (Wang et al., 2014; Brown et al., 2018), which are generally obtained by measuring in the laboratory and observing in the field (Park et al., 2010; Wang et al., 2018; Brown et al., 2018), model simulating (Feng et al., 2013; Zhang et al., 2020) and remote sensing modelling (Irie et al., 2019; Wang et al., 2015).

AAE is a measure characterizing the variation of aerosols light absorption capacity with wavelength. It has been shown that the AAE of BC is usually considered to be equal to approximately 1.0, and the AAE of BrC is larger than that of BC, with an AAE ≥ 1 (Moosmueller et al., 2011; Laskin et al., 2015), exhibiting a stronger wavelength dependence than BC. For light-absorbing aerosols mixed by BC and BrC, the AAE is influenced by both BC and BrC (Cheng et al., 2011). To some extent, the AAE of BrC could indicate the type of light-absorbing particles (Russell et al., 2010), and the size of the AAE is closely related to the particle size and composition of BrC (Moosmueller et al., 2011; Bahadur et al., 2012). Studies have shown that the differences in AAE of BrC may be due to different sources, with the AAE ranging from 1.0–2.0 in flue gas produced by low temperature combustion of coal (Cai et al., 2014). In the samples containing BrC produced by biomass combustion, the AAE is between 1.0 to 3.0 (Martinsson et al., 2015). In a few studies, AAE > 3 may be due to the oxidation of fresh BrC to secondary BrC after Long-range atmospheric Transport (Zhang et al., 2011; Cai et al., 2014). The light absorption of secondary BrC shows a strong wavelength dependence, with an AAE of about 4.7–7 (Bones et al., 2010).

In addition to the AAE, another key parameter of the BrC optical properties is the MAE, which determines the link between atmospheric abundance and radiative forcing (Conrad and Johnson, 2019). Studies have shown that the MAE of BrC emissions from diesel vehicle sources is stronger (up to 2 times) than that of biomass straw wood combustion emissions (Cheng et al., 2011; Xie and Xu, 2017), the MAE of BrC from fossil fuel combustion is higher than that from incomplete biomass combustion (Srinivas and Sarin, 2014; Yan et al., 2017). Secondary BrC can increase or decrease the amount of absorbed radiation, depending on the state of mixing of BrC with other aerosols (Lack et al., 2012; Zhong and Jang, 2014). Other studies have shown that the MAE of ambient atmospheric BrC in areas dominated by motor vehicle source contributions is higher than that than in areas dominated by natural source emissions (Zhang et al., 2011), that is, the absorption capacity of anthropogenic sources of BrC may be stronger than that of natural sources. A study by Chung et al. (2012) reported that strongly absorbing BrC typically exhibited a higher MAE but lower AAE values.

In addition, the refractive index (RI) represents the interaction between the aerosols particles and the optical radiation, which can be calculated by Eq. (1).

In Eq. (1), the real part n is related to the scattering of light by the particles; the imaginary part K is an important parameter that determines the absorption properties of aerosols particles, and is related to the strength of the absorption spectral correlation. the K_BrC of BrC is also one of the key parameters of its optical properties and can be calculated by the following equation Eq. (2) (Liu et al., 2013).

In Eq. (2), ρ is the particle density, it’s unit is g cm^–3; λ is the wavelength.

Studies have shown that the mixing state of the carbon particulate will directly affects the size of the RI, and the k value of organic aerosols mixed inside is 4–5 times that of the external mixed type. Changes in K_BrC may also be due to different types of biomass burning emissions (e.g., forest fires, agricultural residues, wood burning) (Lambe et al., 2013; Pani et al., 2021).The processes of BrC generation and mixing are highly variable in different regions and under different circumstances, leading to large differences in its K_BrC values (Yan et al., 2017; Zeng et al., 2020), such as K_BrC values in Chiang Mai (0.08–0.17, 370 nm) (Pani et al., 2021), which was significantly higher than that in the Ganges Plain of India (0.07 ± 0.03, 365 nm) (Choudhary et al., 2017), the Amazon Basin (0.0016–0.0019, 532 nm) (Hoffer et al., 2006), and Colorado (0.07 ± 0.005, 404 nm) (Lack et al., 2012), the differences can reach two orders of magnitude, and these differences ultimately affect regional and global climate effects of carbon aerosols.

Studies have shown that the light absorption characteristics of BrC directly discharged by different pollution sources and generated by different precursors under different reaction conditions are significantly different (Li et al., 2019; Choudhary et al., 2021). When BrC enters the atmosphere from source emissions, its optical properties are also changed by atmospheric chemical processes (Dasari et al., 2019). In most current climate models, the dynamic evolution of BrC optical properties in time and space is not fully considered, but the BrC optical parameters are assumed to be uniform and stable constants (Zhang et al., 2020), which may increase the uncertainty in the assessment of BrC radiative forcing.

 
3.2 Distribution and Model Simulation of Radiative Forcing

The mechanism of radiative forcing for aerosols particles is complicate, and has long been the subject of much scientific and policy interest (Forster et al., 2007; Shi et al., 2008). The effective radiative forcing for aerosols (ERF) includes aerosol-radiation interaction (ERFari) and aerosol-cloud interaction (ERFaci) effects, and allows shorter-timescale atmospheric elements to adjust to equilibrium. According to the IPCC Sixth Assessment Report, the global mean radiative forcing due to anthropogenic aerosols from 1750 to 2019 is –0.77 [–1.15 to –0.31] W m^–2 (with a 95% confidence level) (Forest, 2018). The estimated ERF of BC given in the IPCC Sixth Assessment Report is significantly reduced to 0.063 (–0.28–0.42) W m^–2 as the positive forcing of aerosol-radiation interactions is largely offset by negative atmospheric adjustments due to cloud cover changes, vertical temperature gradients, and changes in atmospheric water vapor. Zhang et al. (2020) used the Community Atmosphere Model (CAM5) in the Community Earth System Model (CESM) to estimate the direct radiation effect (DRE) of global BrC and BC (Fig. 5), which is the latest model-based DRE estimation of global BrC. It can be seen that BrC is an important absorber with DRE of 0.10 W m^–2, more than 25% (+0.39 W m^–2) of BC. Model results indicated that BrC atmospheric heating in the tropical mid and upper troposphere is larger than that of BC. BrC heating contributes as much to Hadley circulation and tropical expansion as BC heating.

Fig. 5. Annual averaged global distributions of (a) BC DRE, (b) BrC DRE, and (c) the ratio of BrC/BC DRE for 2010 (The unit is W m⁻², the global averaged DRE is shown in the upper right corner) (Zhang et al., 2020).

There are two general methods to calculate aerosols radiative forcing. The first method is based on model-based estimates, relying on emission inventory, chemical transport models (Tuccella et al., 2020) or climate model (Zhang et al., 2020) to determine global aerosols concentration distribution, Optical module to calculate aerosols optical properties (aerosols optical depth, AOD, aerosols absorption optical depth, AAOD, and asymmetry parameters etc.), and radiative transport models to determine radiative forcing (Tang et al., 2021). The second is the observation constraint method, which uses the cylindrical aerosols optical properties obtained from satellite and ground observations as constrain functions and estimates the radiative forcing of aerosols by radiative transport models. The results show that, when using global model to retrieve aerosols radiative forcing, the simulated value of aerosols light absorption will deviate greatly from the observed value, and the light absorption of aerosols will be underestimated by nearly 20%–30%, provided BC is simplified as the only light-absorbing substance and BrC is ignored (Park et al., 2010). Climate effects of aerosols can be represented more accurately when including BrC absorption in climate models (Chung et al., 2012; Ramanathan and Carmichael, 2008). Compared with the pure scattering effect of OC, BrC absorption increases the warming effect (0.11 W m⁻²) at the top of the atmosphere (Feng et al., 2013), and enhances (decreases) the surface (TOA) cooling effect of total OC by 9 ± 5% (9 ± 3%) (Zhang et al., 2021b).

There have been numerous attempts to quantify the global BrC radiative forcing in recent years (Table 2), as well as their impacts on global and regional climate. but the corresponding estimates are subject to substantial uncertainty due to unknowns regarding its sources, composition, evolution, and optical characteristics (Sun et al., 2021; Zhu et al., 2021; Kimet al., 2021), and spans over an order of magnitude from 0.03 to 0.57 W m^–2 (Lin et al., 2014; Hammer et al., 2016). Feng et al. (2013) initially estimated the radiative forcing of BrC to be about 0.25 W m^–2, accounting for about 19% of the global contribution to anthropogenic aerosols light absorption, using the integrated massively parallel atmospheric chemistry transport (IMPACT) model and the Monte Carlo aerosol, cloud and radiation model (MACR). They pointed out that the radiation effect of organic aerosols at tropopause in some areas even changed from cooling (–0.08 W m^–2) to warming (0.025 W m^–2) after BrC was included in the radiation simulation. Lin et al. (2014) estimated the global radiative forcing of BrC to be about 0.22–0.57 W m^–2, equivalent to 27%–70% of BC, using the IMPACT model. Anthropogenic emissions have a great impact on the production rate of secondary organic aerosols (SOA). Since the Industrial Revolution, the direct radiative forcing due to the increase of SOA is 0.12–0.31 W m^–2, and the direct radiative forcing due to the increase of primary organic aerosols (POA) is 0.06–0.11 W m^–2. Hammer et al. (2016), based on the GEOS-Chem model and the radiative transfer model RRTMG (GC-RT), considered the effect of atmospheric photochemical reactions on DRE estimation, and their estimated DRE for global BrC was the lowest of existing studies at 0.03 W m^–2.

It was found that the optical parameters of the BrC have a great influence on the estimation of radiative forcing, with lens effect and photobleaching being important factors affecting radiative forcing of BrC. Saleh et al. (2015) estimated the DRE of BrC to be 0.22 W m^–2 and concluded that the model DRE estimation for BrC was dependent on the accuracy of optical parameters, which were largely influenced by photochemical reactions of BrC. Jo et al. (2016) estimated that the DRE of global BrC was 0.11 W m^–2, which reduced the direct radiative cooling effect of OC by 16%. In addition, BrC absorption led to a general decline in NO₂ photolysis rate, with the largest decline occurring in Asia, averaging –8% (–17%) per year (spring). In Asia, where the average annual (spring) surface ozone concentration decreases by as much as –6% (–13%), the impact of BrC on photochemical pollution in the region cannot be ignored. Wang et al. (2014) estimated the DRF of global BrC and BC to be 0.11 and 0.21 W m^–2, respectively. The DRF of BC was at the low end of 0.2–1.0 W m^–2 in previous studies, and was significantly lower than the 0.6 W m^–2 estimated in the IPCC Fifth Assessment Report. They argued that the DRF of BC had been overestimated in previous studies, mainly due to overestimation of BC longevity and misattribution of BrC absorption to BC. Wang et al. (2018) compared simulated BrC absorption with direct measurements from an aircraft for the first time on the basis of previous studies. They believed that the DRF of BrC was overestimated due to the lack of observation constraints of direct measurements and the neglect of photobleaching. The lens effect and photobleaching are significant elements affecting radiative forcing of BrC, and the inclusion of BrC photochemical reactions can enhance the model estimation of radiative forcing. Considering these effects in this estimate, the DRF of global BrC and BC was only 0.048 and 0.17 W m^–2, respectively.

Brown et al. (2018) included the BrC module in CESM-CAM5 for the first time and used a parameterization for BrC absorptivity of Saleh et al. (2014). Considering the effect of BrC photobleaching, they estimated that the radiative effects from aerosol-cloud interactions (REaci) of global BrC were decreased from 0.13 ± 0.01 W m^–2 to 0.06 ± 0.008 W m^–2. They argued that global low clouds decrease due to BrC semi-direct effects and suggested that lense effects and OA absorption should be included in radiative transfer calculations. Zhang et al. (2020) was the first attempt to comprehensively analyze how convective transport and photobleaching affect global atmospheric heating by BrC absorption relative to BC based on CESM-CAM5 Model.

BrC is an important contributor to aerosols direct radiative forcing, which is prevalent in the measurement range from 1 to 12 km altitude and can increase short-wave solar absorption in the atmosphere by about 20% (Liu et al., 2015). BrC is increasingly sensitive to climate with increasing altitude and absorbs more short-wavelength radiation than BC at altitudes of 5–12 km (Zhang et al., 2017). Xu et al. (2021) found that the effect of the carbon aerosols aging process on optical properties and differences in aerosols mixing methods could bias the simulation results by up to 40%. Shamjad et al. (2018) also concluded that radiative forcing of the lense effects caused by BrC and absorption coating at the top of the atmosphere was –0.93 ± 0.27 and 0.13 ± 0.06 W m⁻², respectively. As compared to scattering OC, externally mixed absorbing OC reduced radiative forcing by 48%. The presence of internally mixed absorbing OC as a BC shell caused 31% of the warming compared to a similar shell composed of scattering OC. Therefore, BrC photobleaching and lense effects need to be included into global climate models when calculating radiative forcing. In addition, the following factors need to be considered when estimating the DRE of BrC:

(1) With few direct observations (satellite or ground measurements) of global BrC, field and laboratory observations of BrC parameters should be expanded, and measured optical parameters such as BrC concentration, photobleaching, absorption efficiency, and the mixed state of carbon aerosols should be included in the atmospheric radiation model.

(2) The current emission inventories do not provide enough accurate information, leading to deviation in the model-estimated BrC concentrations. Emission measurements for different combustion conditions and fuel types, as well as cross-checks between field and laboratory combustion experiments should be conducted to refine the emission inventories (Yan et al., 2018).

(3) Methods for distinguishing between BrC and BC absorption contributions are important and need further study.

(4) The lack of a comprehensive understanding of BrC light-absorbing characteristics, including its sources, chromophores, atmospheric processes, pH conditions and concomitant metals, is one of the key factors leading to uncertainty in the assessment of radiative forcing (Guan et al., 2020).

 
3.3 Effect of BrC Aerosols on Temperature and Precipitation

3.3.1 Effect on temperature

Carbonaceous aerosols affect the global radiation balance through absorption and scattering, leading to atmospheric warming or cooling. BC and BrC are the main absorbing aerosols. Studies show that BC causes changes in radiation flux at the top of the atmosphere and surface through instantaneous absorption of short-wave radiation in the atmosphere, thus affecting 850 hPa clouds and air temperature, and further affecting meteorological elements of boundary layer and ground (Li et al., 2007; Wang et al., 2010; Jiang et al., 2017; Lin et al., 2022), as well as ITCZ location (Zhang et al., 2021a). In addition, BC may have an important influence on the formation of altostratus clouds (Wu et al., 2021). BC tends to heat the lower atmosphere in the northern mid-latitude, resulting in elevated near-surface shortwave heating rates (Jacobson, 2002). BrC tends to heat the tropical troposphere (Zhang et al., 2020). It is transported to the troposphere through deep convection, and in-cloud heterogeneous processing may also produce BrC. Results based on aircraft-borne instrument observations of the vertical distribution of aerosols over the United States indicate that BrC accounts for about 24% of the combined warming effect of BC and BrC at the top of the troposphere. Most BrCs are below 5 km, but about 2/3 of their radiative forcing occurs above 5 km. Upper-air BrCs generated by biomass combustion are an underappreciated component of climate forcing (Zhang et al., 2017). Zhang et al. (2020) found that BrC and BC heating caused tropical expansion of 1.0 ± 0.9° and 1.2 ± 2.9°, respectively. BrC heating reduced the deep convective mass flux in the upper troposphere (Feingold et al., 2005; Yoshimori and Broccoli, 2008) by 4.41 × 10^–5 kg m^–2 s^–1 or 4.1% in the tropics, which is about 1/3 of the BC heating effect (1.52 × 10^–4 kg m^–2 s^–1 or 12.9%).

Ferrero et al. (2021) found that the heating rate (HR) of light-absorbing aerosols decreased with the increase of cloud fraction, and HR could be reduced by 20%–30% and 80% under low cloud and complete haze conditions, respectively. This suggests that the simplified assumption of using clear sky conditions in radiative transfer calculations may overestimate HR by more than 400%. For different cloud types, cirrus had little effect on HR-BC and HR-BRC, which decreased by 5% at most; cumulus, cirrus-cirrostratus, altocumulus, stratocumulus and altostratus reduced HR-BC and HR-BRC by –31 ± 12% and –26 ± 7%, –60 ± 8% and –54 ± 4%, –60 ± 6% and –46 ± 4%, –63 ± 6% and –58 ± 4%, and –78 ± 5%, –73 ± 4%, respectively; stratus had the greatest effect, inhibiting HR-BC and HR-BrC to –85 ± 5% and –83 ± 3%. This study also showed that the role of cloud fraction and different cloud types need to be taken into account when estimating the HR due to BC and BrC, thus reducing and quantifying the uncertainties in their impact on climate.

 
3.3.2 Effect on Precipitation

Zhang et al. (2009a) also considered the influence of BC and OC aerosols, and the simulation results showed that carbon aerosols heated the atmosphere through their direct and semi-direct effects, leading to the decrease of cloud fraction and precipitation in southern China, the increase of solar radiation reaching the ground, and the rise of surface temperature. In contrast, the above phenomena in northern China were exactly the opposite, leading to the exact opposite conclusion as Menon et al. (2002). Lau et al. (2005) and Lau and Kim (2006), based on observations and climate model studies, pointed out that the absorbing aerosols on the north and south sides of the Qinghai-Tibet Plateau may have contributed to the strengthening of southwesterly flow and increased precipitation in the Bay of Bengal from late May to early June, leading to the outbreak of the South Asian summer monsoon. Hodnebrog et al. (2016) proposed that aerosols generated by biomass burning inhibited precipitation in South Africa due to the interaction between aerosols and clouds, which was the main reason for the decrease of precipitation in the dry season. Zhang et al. (2020) also showed that, on a global scale, BrC heating reduced precipitation by 0.9% ± 7.0%, about 60% of the precipitation reduced by BC. However, in tropical areas with high convective and precipitation intensity, BrC heating reduced precipitation by 3.9% ± 17.8%, close to BC (4.0% ± 17.1%). BrC heating had a greater effect on tropical precipitation than on convective mass flux (about 1/3) as BrC heating in the upper troposphere was more potent than BC heating in the lower troposphere. BrC heating reduced precipitation by 0.3% ± 10.7% in northern middle and high latitudes, much less than BC heating (–4.8% ± 13.5%). Brown et al. (2018) showed (Fig. 6) that changes in cloud fraction, liquid water path, precipitation and surface flux were obvious in CAM5 due to the addition of BrC. Globally, With the increase of BrC, the large-scale precipitation (PRECL) changed little, but at regional scale, the PRECL changed significantly, the Arctic Ocean, near Kazakhstan, off the west coast of the United States declined slightly; Iran, off the east coast of the United States, parts of South America increased slightly, these changes corresponded to changes of low cloud fraction and in some cases were negatively correlated with REaci. Globally, precipitation from convective clouds (PRECC) declined slightly, particularly over the tropical Gulf Coast, western Australia and parts of South America, but increased significantly over Bangladesh, China and parts of Africa.

Fig. 6. The effect of BrC addition in the CAM5 model run on (a) atmospheric absorption (W m⁻²), (b) surface solar flux (W m⁻²), (c) low level cloud fraction (%; clouds below 700 mb), (d) mid-level cloud fraction (%; clouds between 700 and 400 mb), (e) large-scale precipitation (mm day⁻¹), (f) convective precipitation (mm day⁻¹), (g) liquid water path (g m⁻²), and (h) surface air temperature (°K). Hatching indicates significance in ensemble year change to the 0.1 level. Global mean values are presented in the upper right corner of each panel (Brown et al., 2018).

Precipitation responses depend on the climate forcing mechanism, which also differ from timescale and have strong regional scale. The precipitation responses can be split into two parts, a fast atmospheric response that strongly correlates with the atmospheric component of radiative forcing, and a slower response to global surface temperature change that is independent of the climate change mechanism (Andrews et al., 2010; Kvalevåg et al., 2013). BrC affects precipitation mainly through its direct and semi-direct effects, and very little work has been carried out to investigate the mechanisms of BrC effects on precipitation. The distribution of BrC in Section 2 of this paper, suggest that BrC from biomass burning accounts for about 45% of all sources (Jo et al., 2016), so the mechanism of aerosols from biomass burning effect on precipitation will be described below. Some studies have shown that aerosols from biomass burning could lead to increased precipitation, and Jo et al. (2016) estimated that Southeast Asia and South America were regions with high BrC values generated from biomass burning, which is coincides with the increasing regions of PRECL in the research results of Brown et al. (2018). The reason for this may be mainly due to strong atmospheric absorption from these particles which can cool the surface and enhance low-level convergence and vertical upward motion, these changes will increase sea level pressure over land in areas of biomass burning and accelerate the hydrological cycle by increasing clouds, atmospheric water vapour and a lesser extent precipitation. Meanwhile, an increase in cloudiness will also reinforce the surface radiative cooling tendency of the aerosols (Randles and Ramaswamy, 2008, 2010). However, other studies have shown that it inhibits regional precipitation, mainly due to aerosol-cloud interactions, with aerosols from higher biomass burning limiting upward vertical motion, raising surface pressure, and increasing low-level divergence meteorological indicators of convective suppression (Ming et al., 2010; Tosca et al., 2015). Some researches shows that impacts of absorbing aerosols on clouds and precipitation highly depend on the altitude of added absorbing aerosols (Samset and Myhre, 2015), and this could be one reason for the differing results. Absorbing aerosols in the lowest atmospheric layer increases precipitation, while those at higher altitudes decreases precipitation (Ban-Weiss et al., 2012). That is, when the main atmospheric heating from biomass burning aerosols occurs at an altitude high enough to inhibit the instability of vertical motion under some certain conditions, it may suppress convective activity (Hodnebrog et al., 2016).

 
3.4 Snow/ice Albedo Effect of BrC Aerosols

The cryosphere, composed of snow, river and lake ice, sea ice, glaciers, ice shelves and ice caps, and permafrost, is a vital component of the Earth's climate system and is extremely vulnerable to both climate changes and human activities. BC, BrC and dust are the main light-absorbing substances in atmospheric aerosols. They can be transported over long distances through atmospheric circulation, and deposited on snow and glacier surfaces through dry/wet deposition, reducing the snow/ice albedo, enhancing the absorption of solar radiation by snow and ice, thus contributing to the increase in snow and ice temperature, reducing upwelling radiation and accelerating snow and ice melting (Doherty et al., 2010; Kaspari et al., 2015; Chernenkov and Kostrykin, 2021; Chen et al., 2021; Usha et al., 2021). The results of Usha et al. (2021) suggested that the aerosol-induced decrease in snow albedo led to an early reversal of the sign of the direct radiative forcing of aerosols at the top of the atmosphere. In recent years, the effect of BrC on snow and ice has received increasing attention. Fig. 1 also briefly illustrates the source, transport and deposition of BrC in snow and ice and its effect on snow and ice. At present, the effects of BrC on snow and ice are mainly targeted at total light-absorbing particles (LAPs) in snow and ice (Pu et al., 2018; Svensson et al., 2021), mainly due to the difficulty of dividing the effects of BrC and non-BrC on the radiative forcing of snow and ice. Studies have shown that LAPs accelerate melt through perturbations to snow albedo and their associated radiative forcing. LAPs in snow enhance the light absorption due to surface darkening (Fig. 7), and when at or near the snow surface, LAPs immediately lower albedo across the visible wave-lengths at which snow is most reflective, produce a positive radiative forcing (Skiles et al., 2018). Lowered albedo means increased absorption of solar radiation by snow, which contributes to higher snow temperatures and faster melting, and the direct effect of LAPs enhances the growth of snow grains, effectively accelerates snow aging (Skiles and Painter, 2016). The larger size the snow grain grow, the more solar radiation penetrates into the underlying snow, which further promotes the accumulation of LAPs in the surface snow. The increased LAPs would absorb more heat and cause more snow to melt and continuous recrystallization, result in accelerating the growth of snow grains, and change in the structure of the snow surface layer, faster aging of snow further reduces snow albedo (Doherty et al., 2013; Jiang et al., 2006). The combination of these two processes accelerates the retreat of snow cover, resulting in a large-scale decrease in albedo, a decrease in snow cover time, and a change in spring runoff time. The highest radiative forcing value for LAPs occurs at mid-latitudes and all the maximum instantaneous radiative forcing values based on observations are controlled by dust (Skiles et al., 2018).

 Fig. 7. A representation of how LAPs impact snow albedo and net solar radiation (The data in the graph comes from article of Skiles et al. (2018)).

Cui et al. (2021) quantified the reduction in albedo caused by LAPs in snow and ice using MODIS data and the snow and ice and aerosols radiation model (SNICAR). The results showed that 84% and 70% of the albedo reduction and spatial changes in radiative forcing were caused by LAPs in snow and ice, respectively. Niwano et al. (2021) developed a new model chain (NHM-Chem-SMAP) consisting of regional meteorological chemical models and multi-layer physical snow cover models to estimate the seasonal variation of mass concentrations of BC and dust in surface snow cover in Japan. The study found that LAPs from domestic and foreign sources reduced snow cover duration by 5 and 10 days, respectively, compared to pure snow conditions.

Fewer studies have been conducted only for the effect of BrC on snow and ice, and some studies have used small-scale burning of biomass/peat, etc., to deposit BrC on the snow surface and analyze the effect of BrC on ice albedo and radiative forcing. Beres et al. (2017) deposited BrC generated by small biomass combustion on natural snow surfaces in the Carson Range, Sierra Nevada, USA, and monitored the spectral reflectance of these disturbed snow surfaces and adjacent undisturbed snow surfaces. It was found that in the blue and ultraviolet spectral regions, the reflectivity of snow surface decreased obviously, and BrC exerted a radiative forcing on snow surface, which had a certain influence on snow melting. Beres et al. (2020) also conducted similar experiments in Alaska using small-scale peat combustion to deposit BrC on the surface of snow. The results showed that BrC had an effect on snow albedo and radiative forcing. The mass weighted value of BrC (or combustion OC) per unit mass ppm deposited on natural snow was (+0.14/–0.11) W m^–2. The instantaneous radiative forcing was 2.68 (+0.27/–0.22) W m^–2 when deposited in clean snow (without other light-absorbing impurities). At the same time, BrC deposition could greatly reduce the UV photochemical flux and thus reduce the photochemical reaction in snow.

Other researchers used climate models to estimate radiative forcing from BrC deposited in snow. Lin et al. (2014) assessed radiative forcing of organic aerosols in both atmospheric and terrestrial snow/sea ice based on IMPACT models. The results showed that the radiative forcing of organic aerosols deposited in land snow and sea ice was +0.0011 to +0.0031 W m^–2, equivalent to 24% that of BC deposited in snow and ice. They have not been able to separate the radiative forcing of BrC from the total organic matter. Tuccella et al. (2021) calculated the radiative forcing of radiation-absorbing aerosols (RAA) in snow (taking into account the presence of BC, BrC, and dust in the snowpack) using the GEOS-Chem model, and concluded that the average radiative forcing of RAA, BC, BrC, and dust in the global snowpack was 0.068, 0.033, 0.0066 and 0.012 W m^–2, respectively (Fig. 8). Globally, non-BC compounds in snow account for 40% of the radiative forcing of RAA, while the contribution of anthropogenic RAA is 56%. BrC has its greatest impact in summer in the Arctic, with radiative forcing of +0.13 W m^–2; in the mid-latitudes of Asia, the radiative forcing of spring dust is +0.24 W m^–2, accounting for 50% of RAA. Uncertainties in optical properties, RAA mixing ratio in snow, snow grain size, and snow cover result in overall uncertainties of –50%/+61%, –57%/+183%, –63%/+112%, and –49%/+77% in the radiative forcing of BC, BrC, dust, and RAA in snow, respectively. With a variety of absorbing impurities in the snow, the upper bound of uncertainty for BrC and dust is two and three times larger than that with BC.

Fig. 8. All-sky annual mean (2010–2014) (a) radiation-absorbing aerosols, (b) black carbon, (c) brown carbon, and (d) soil dust in snow radiative forcing calculated from the CTRL experiment (Unit is W m^–2). Global mean values are presented in the upper right corner of each panel (Tuccella et al., 2021).

The model-based simulation of BrC concentrations in snow and ice and the resulting radiative forcing in different regions has not been investigated in depth. BrC in the ice layer can absorb solar radiation directly, and generate additional radiative forcing through interaction with BC. Current results on BrC radiative forcing may be greatly modified if lense effects are included in modeling (Skiles and Painter, 2016). In its Special Report on Oceans and the Cryosphere in Climate Change (SROCC), the IPCC6 noted that radiative forcing from BrC and dust in snow was of low and medium confidence, respectively. To reduce this uncertainty, the following may be needed in the future: (1) enhance the application of satellite data; (2) methods to distinguish BrC and non-BrC in snow and their absorption contributions are important and need further study; (3) through controlled experiments and snow melt rate model calculations, simulation tests are conducted to optimize the model and analyze the climate effects of BrC in snow and ice in detail.

 
4 CONCLUSIONS AND PROSPECT

Given that light-absorbing properties of BrC has been taken into account, the IPCC Sixth Assessment Report demonstrates that the uncertainty range for the total effective radiative forcing of OC now includes positive values for the first time, and the BrC discovery adds to the uncertainty surrounding the effects of carbonaceous aerosols on radiation and climate. BrC research is crucial to lessen the ambiguity regarding how aerosols may affect local or global climate change. This paper reviews recent advances in research on the radiative forcing of BrC (global distribution and model simulation), its effects on temperature and precipitation, and snow/ice albedo effect and the main conclusions are as follows:

(1) BrC concentration distributions can be obtained via observations (single site, foundation networking, and satellite retrieval) and model estimations. The regions with the highest global mean surface BrC concentrations predicted by models are mostly Southeast Asia and South America (biomass burning), East Asia and northeast India (biofuel burning), and Europe and North America (secondary sources); the global BrC radiative forcing estimated by different models are quite erratic, with a range of around 0.03–0.57 W m^–2. However, assessment of climate effects can be made more accurate by adding BrC absorption into climate models.

(2) BC and BrC heating lead to tropical expansion and a reduction in deep convective mass fluxes in the upper troposphere; BC tends to heat the lower atmosphere in the northern mid-latitude, while BrC tends to heat the troposphere in the tropics. Cloud fraction and cloud type have a substantial impact on the HR estimates of BC and BrC. The inclusion of BrC in the model results in a clear shift in the cloud fraction, liquid water path, precipitation, and surface flux, which is in some cases correlated with regional changes of REaci. BrC heating decreases precipitation on a global scale, particularly in tropical regions with high convective and precipitation intensity, but different in some regions; PRECC is on a slight downward trend globally, but increases in some regions; on a global level, PRECL changes little, while it varies greatly on a regional level.

(3) The climate effects of BrC in snow and ice are just beginning to be studied. Uncertain optical properties of BrC, mixing ratio of RAA in snow, snow grain size and snow cover lead to larger uncertainties and lower confidence in the model simulated distribution and radiative forcing of BrC in snow than BC due to the presence of multiple absorbing substances.

A review from the above three aspects generally shows that some studies on BrC radiative forcing and its climate effects have been carried out in recent years, and the development of atmospheric radiation models has played an important role in resolving the uncertainties involved. The following areas require further attention in future study:

(1) The BrC emission inventory is essential for estimation of its concentration distribution and radiative forcing. It is suggested to further study the calculation method of emission factors and establish a more accurate emission inventory of BrC from complex sources. In addition, most models cannot clearly distinguish between BrC and OC emissions and physical processes. Therefore, it is important to determine the exact mass ratio of BrC to total OC and BC, and to distinguish the contribution of BrC and BC to absorption for the calculation of BrC concentration, which needs further study.

(2) The lack of global observations of BrC, coupled with its complex optical and chemical atmospheric characteristics, results in low confidence in the quantification of BrC in atmospheric distribution. The accurate simulation of BrC concentration distribution and radiative forcing depends on a comprehensive understanding of its source, chemical composition and optical properties. Future research should focus on improving model research, taking into account the photobleaching and lense effects of BrC, and reducing the uncertainty of its climate effects by combining the observation data analysis with the numerical simulation.

(3) The changes in snow/ice albedo brought on by BrC deposition and its impact on the climate are high uncertainty. The BrC concentration in snow and ice can be further observed and estimated, and its influence on snow depth, snow cover, snow/ice albedo, surface temperature and radiative forcing can be analyzed. In addition, satellite observation data are included into the models to distinguish BrC and non-BrC in snow and their contribution to light absorption. Simulation tests are carried out to optimize the model and analyze the climate effect of BrC in snow and ice in detail through controlled experiments and calculation of snow melt rate model.

 
ACKNOWLEDGEMENT

This study was financially supported by the National Key R&D Program of China (2022YFC3701202), the National Natural Science Foundation of China (42275039), the National Key R&D Program of China (2017YFA0603502), and S&T Development Fund of CAMS (2021KJ004&2022KJ019).