Vertical Distribution Of Black Carbon Research Articles

Vertical distribution of black carbon and its mixing state in the urban boundary layer in summer

Abstract. The vertical distribution of black carbon (BC), as well as its mixing state, is of great concern due to BC's strong regional climatic and environmental effects. In this study, vertical measurements were conducted through a moveable container based on a meteorological tower in the Beijing urban area during June and July. A total of 112 vertical profiles (0–240 m), including the concentrations of BC, O3, NOx and the optical properties of aerosols, were obtained. Based on BC concentration, the vertical profiles could be classified into four categories: uniform, gradual decrease, sharp decrease and sudden increase. The uniform type indicates strong vertical mixing with similar pollutant concentrations along the vertical direction. The gradual and sharp decrease types indicate stable vertical conditions with higher pollutant concentrations on the ground and lower concentrations at higher altitudes. Due to the strong radiation in summer, the vertical profiles exhibited a clear diurnal variation in which ∼ 80 % of profiles were uniform during the daytime and ∼ 40 %–90 % of profiles were of the gradual and sharp decrease types at night. O3 is an exception, and its concentration generally increases with height, even under strong vertical mixing conditions. The size distribution of the BC core varied slightly along the vertical direction, and the coating thickness, denoted by the diameter ratio between the BC-containing particle and BC core (Dp/Dc), of BC increased with height under stable conditions. Although the coating thickness could increase the absorption ability with an average absorption enhancement of 1.25 at 23:00 LT (local time: UTC+8), the vertical difference of Dp/Dc (2 %) was much lower than that of BC concentration (∼ 35 %). The vertical variation in absorption ability was mainly caused by the variation in BC concentration. In addition, O3 and Dp/Dc occasionally increased during 06:00–08:00 but remained stable during 08:00–10:00. Vertical mixing and transportation from upper heights, such as the residual layer, could significantly influence the pollutant properties on the surface during early mornings. This study exhibits a continuous vertical picture of BC and its mixing state in urban areas, which would be helpful for understanding BC's regional environmental effect.

Wintertime vertical distribution of black carbon and single scattering albedo in a semi-arid region derived from tethered balloon observations

The vertical distribution of atmospheric aerosols plays an essential role in aerosol–radiation and aerosol–cloud interactions. Because of strong light absorption, the radiative effects of black carbon (BC) are highly sensitive to its vertical distribution; the lack of high-resolution observations is the reason for their poor quantification. We used a tethered balloon platform to acquire high-resolution vertical profiles of BC, particle number concentration, and meteorological parameters in the semi-arid region of Northwest China in December 2018. A total of 112 BC profiles were classified into four vertical distribution categories, which were determined by local emissions, regional transport, vertical mixing due to the ABL evolution, and topography. BC profiles with peaks near or above the atmospheric boundary layer (ABL) accounted for 57% of the profiles. Vertical single scattering albedo (SSA) profiles were subsequently calculated using the profiles of BC and particle size distribution. The vertical SSA distribution is generally modulated by BC profiles. The diurnal variations of the BC and SSA profiles were summarized using a boundary-layer normalization method. In the ABL, BC decreased and SSA increased with increasing height at 02:00, 08:00, and 20:00, while both BC and SSA exhibited a uniform distribution at 14:00. The SSA decreased above the ABL at 14:00, which might have had a profound impact on ABL development. These results provide a better understanding of the vertical BC and SSA distributions, which can also be used to reduce uncertainties in estimating the BC radiative effects.

Estimation of radiative forcing and heating rate based on vertical observation of black carbon in Nanjing, China

Owing to a lack of vertical observations, the impacts of black carbon (BC) on radiative forcing (RF) have typically been analyzed using ground observations and assumed profiles. In this study, a UAV platform was used to measure high-resolution in-situ vertical profiles of BC, fine particles (PM2.5), and relevant meteorological parameters in the boundary layer (BL). Further, a series of calculations using actual vertical profiles of BC were conducted to determine its impact on RF and heating rate (HR). The results show that the vertical distributions of BC were strongly affected by atmospheric thermodynamics and transport. Moreover. Three main types of profiles were revealed: Type I, Type II, Type III, which correspond to homogenous profiles (HO), negative gradient profiles (NG), and positive gradient profiles (PG), respectively. Types I and II were related to the diurnal evolution of the BL, and Type III was caused by surrounding emissions from high stacks and regional transport. There were no obvious differences in RF calculated for HO profiles and corresponding surface BC concentrations, unlike for NG and PG profiles. RF values calculated using surface BC concentrations led to an overestimate of 13.2 W m−2 (27.5%, surface) and 18.2 W m−2 (33.4%, atmosphere) compared to those calculated using actual NG profiles, and an underestimate of approximately 15.4 W m−2 (35.0%, surface) and 16.1 W m−2 (29.9%, atmosphere) compared to those calculated using actual PG profiles. In addition, the vertical distributions of BC HR exhibited clear sensitivity to BC profile types. Daytime PG profiles resulted in a positive vertical gradient of HR, which may strengthen temperature inversion at high altitudes. These findings indicate that calculations that use BC surface concentrations and ignore the vertical distribution of BC will lead to substantial uncertainties in the effects of BC on RF and HR.

The vertical variability of black carbon observed in the atmospheric boundary layer during DACCIWA

Abstract. This study underlines the important role of the transported black carbon (BC) mass concentration in the West African monsoon (WAM) area. BC was measured with a micro-aethalometer integrated in the payload bay of the unmanned research aircraft ALADINA (Application of Light-weight Aircraft for Detecting IN situ Aerosol). As part of the DACCIWA (Dynamics–Aerosol–Chemistry–Cloud Interactions in West Africa) project, 53 measurement flights were carried out at Savè, Benin, on 2–16 July 2016. A high variability of BC (1.79 to 2.42±0.31 µg m−3) was calculated along 155 vertical profiles that were performed below cloud base in the atmospheric boundary layer (ABL). In contrast to initial expectations of primary emissions, the vertical distribution of BC was mainly influenced by the stratification of the ABL during the WAM season. The article focuses on an event (14 and 15 July 2016) which showed distinct layers of BC in the lowermost 900 m above ground level (a.g.l.). Low concentrations of NOx and CO were sampled at the Savè supersite near the aircraft measurements and suggested a marginal impact of local sources during the case study. The lack of primary BC emissions was verified by a comparison of the measured BC with the model COSMO-ART (Consortium for Small-scale Modelling–Aerosols and Reactive Trace gases) that was applied for the field campaign period. The modelled vertical profiles of BC led to the assumption that the measured BC was already altered, as the size was mainly dominated by the accumulation mode. Further, calculated vertical transects of wind speed and BC presume that the observed BC layer was transported from the south with maritime inflow but was mixed vertically after the onset of a nocturnal low-level jet at the measurement site. This report contributes to the scope of DACCIWA by linking airborne BC data with ground observations and a model, and it illustrates the importance of a more profound understanding of the interaction between BC and the ABL in the WAM region.

Overview paper: New insights into aerosol and climate in the Arctic

Abstract. Motivated by the need to predict how the Arctic atmosphere will change in a warming world, this article summarizes recent advances made by the research consortium NETCARE (Network on Climate and Aerosols: Addressing Key Uncertainties in Remote Canadian Environments) that contribute to our fundamental understanding of Arctic aerosol particles as they relate to climate forcing. The overall goal of NETCARE research has been to use an interdisciplinary approach encompassing extensive field observations and a range of chemical transport, earth system, and biogeochemical models. Several major findings and advances have emerged from NETCARE since its formation in 2013. (1) Unexpectedly high summertime dimethyl sulfide (DMS) levels were identified in ocean water (up to 75 nM) and the overlying atmosphere (up to 1 ppbv) in the Canadian Arctic Archipelago (CAA). Furthermore, melt ponds, which are widely prevalent, were identified as an important DMS source (with DMS concentrations of up to 6 nM and a potential contribution to atmospheric DMS of 20 % in the study area). (2) Evidence of widespread particle nucleation and growth in the marine boundary layer was found in the CAA in the summertime, with these events observed on 41 % of days in a 2016 cruise. As well, at Alert, Nunavut, particles that are newly formed and grown under conditions of minimal anthropogenic influence during the months of July and August are estimated to contribute 20 % to 80 % of the 30–50 nm particle number density. DMS-oxidation-driven nucleation is facilitated by the presence of atmospheric ammonia arising from seabird-colony emissions, and potentially also from coastal regions, tundra, and biomass burning. Via accumulation of secondary organic aerosol (SOA), a significant fraction of the new particles grow to sizes that are active in cloud droplet formation. Although the gaseous precursors to Arctic marine SOA remain poorly defined, the measured levels of common continental SOA precursors (isoprene and monoterpenes) were low, whereas elevated mixing ratios of oxygenated volatile organic compounds (OVOCs) were inferred to arise via processes involving the sea surface microlayer. (3) The variability in the vertical distribution of black carbon (BC) under both springtime Arctic haze and more pristine summertime aerosol conditions was observed. Measured particle size distributions and mixing states were used to constrain, for the first time, calculations of aerosol–climate interactions under Arctic conditions. Aircraft- and ground-based measurements were used to better establish the BC source regions that supply the Arctic via long-range transport mechanisms, with evidence for a dominant springtime contribution from eastern and southern Asia to the middle troposphere, and a major contribution from northern Asia to the surface. (4) Measurements of ice nucleating particles (INPs) in the Arctic indicate that a major source of these particles is mineral dust, likely derived from local sources in the summer and long-range transport in the spring. In addition, INPs are abundant in the sea surface microlayer in the Arctic, and possibly play a role in ice nucleation in the atmosphere when mineral dust concentrations are low. (5) Amongst multiple aerosol components, BC was observed to have the smallest effective deposition velocities to high Arctic snow (0.03 cm s−1).

High Arctic aircraft measurements characterising black carbon vertical variability in spring and summer

Abstract. The vertical distribution of black carbon (BC) particles in the Arctic atmosphere is one of the key parameters controlling their radiative forcing and thus role in Arctic climate change. This work investigates the presence and properties of these light-absorbing aerosols over the High Canadian Arctic (>70∘ N). Airborne campaigns were performed as part of the NETCARE project (Network on Climate and Aerosols: Addressing Key Uncertainties in Remote Canadian Environments) and provided insights into the variability of the vertical distributions of BC particles in summer 2014 and spring 2015. The observation periods covered evolutions of cyclonic disturbances at the polar front, which favoured the transport of air pollution into the High Canadian Arctic, as otherwise this boundary between the air masses largely impedes entrainment of pollution from lower latitudes. A total of 48 vertical profiles of refractory BC (rBC) mass concentration and particle size, extending from 0.1 to 5.5 km altitude were obtained with a Single-Particle Soot Photometer (SP2). Generally, the rBC mass concentration decreased from spring to summer by a factor of 10. Such depletion was associated with a decrease in the mean rBC particle diameter, from approximately 200 to 130 nm at low altitude. Due to the very low number fraction, rBC particles did not substantially contribute to the total aerosol population in summer. The analysis of profiles with potential temperature as vertical coordinate revealed characteristic variability patterns within specific levels of the cold and stably stratified, dome-like, atmosphere over the polar region. The associated history of transport trajectories into each of these levels showed that the variability was induced by changing rates and efficiencies of rBC import. Generally, the source areas affecting the polar dome extended southward with increasing potential temperature (i.e. altitude) level in the dome. While the lower dome was mostly only influenced by low-level transport from sources within the cold central and marginal Arctic, for the mid-dome and upper dome during spring it was found that a cold air outbreak over eastern Europe caused intensified northward transport of air from a corridor over western Russia to central Asia. This sector was affected by emissions from gas flaring, industrial activity and wildfires. The development of transport caused rBC concentrations in the second lowest level to gradually increase from 32 to 49 ng m−3. In the third level this caused the initially low rBC concentration to increase from <15 to 150 ng m−3. A shift in rBC mass-mean diameter, from above 200 nm in the lower polar dome dominated by low-level transport to <190 nm at higher levels, may indicate that rBC was affected by wet removal mechanisms preferential to larger particle diameters when lifting processes were involved during transport. The summer polar dome had limited exchange with the mid-latitudes. Air pollution was supplied from sources within the marginal Arctic as well as by long-range transport, but in both cases rBC was largely depleted in absolute and relative concentrations. Near the surface, rBC concentrations were <2 ng m−3, while concentrations increased to <10 ng m−3 towards the upper boundary of the polar dome. The mass mean particle diameter of 132 nm was smaller than in spring; nonetheless the summer mean mass size distribution resembled the spring distribution from higher levels, with depletion of particles >300 nm. Our work provides vertical, spatial and seasonal information of rBC characteristics in the polar dome over the High Canadian Arctic, offering a more extensive dataset for evaluation of chemical transport models and for radiative forcing assessments than those obtained before by other Arctic aircraft campaigns.

In-Situ Aircraft Measurements of the Vertical Distribution of Black Carbon in the Lower Troposphere of Beijing, China, in the Spring and Summer Time

Due to rapid economic development in recent years, China has become a major global source of refractory black carbon (rBC) particles. However, surface rBC measurements have been limited, and the lower troposphere suffers from a complete lack of measurements, especially in heavily rBC-polluted regions such as China’s capital, Beijing (BJ). In this study, we present the first concentration measurements using an airborne Single Particle Soot Photometer (SP2) instrument, including vertical distributions, size distributions, and the mixing state of rBC particles in the lower troposphere in BJ and its surrounding areas. The measurements were conducted from April to June 2012 during 11 flights. The results show that the vertical rBC distributions had noticeable differences between different air masses. When an air mass originated from the south of BJ (polluted region), the rBC particles were strongly compressed in the planetary boundary layer (PBL), and showed a large vertical gradient at the top of the PBL. In contrast, when an air mass originated from the north of BJ (clean region), there was a small vertical gradient. This analysis suggests that there was significant regional transport of rBC particles that enhanced the air pollution in BJ, and the transport not only occurred near the surface but also in the middle levels of the PBL (around 0.5 to 1 km). The measured size distributions show that about 80% of the rBC particles were between the diameters of 70 and 400 nm, and the mean diameter of the peak rBC concentrations was about 180–210 nm. This suggests that the rBC particles were relatively small particles. The mixing state of the rBC particles was analyzed to study the coating processes that occurred on the surface of these particles. The results indicate that the air mass strongly affected the number fraction (NF) of the coated particles. As for a southern air mass, the local air pollution was high, which was coupled with a lower PBL height and higher humidity. Consequently, hygroscopic growth occurred rapidly, producing a high NF value (~65%) of coated rBC particles. The correlation coefficient between the NF and the local relative humidity (RH) was 0.88, suggesting that the rBC particles were quickly converted from hydrophobic to hydrophilic particles. This rapid conversion is very important because it suggests a shorter lifetime of rBC particles under heavily polluted conditions. In contrast, under a northern air mass, there was no clear correlation between the NF and the local humidity. This suggests that the coating process occurred during the regional transport in the upwind region. In this case, the lifetime was longer than the southern air mass condition.

Increase in elemental carbon values between 1970 and 2004 observed in a 300-year ice core from Holtedahlfonna (Svalbard)

Abstract. Black carbon (BC) is a light-absorbing particle that warms the atmosphere–Earth system. The climate effects of BC are amplified in the Arctic, where its deposition on light surfaces decreases the albedo and causes earlier melt of snow and ice. Despite its suggested significant role in Arctic climate warming, there is little information on BC concentrations and deposition in the past. Here we present results on BC (here operationally defined as elemental carbon (EC)) concentrations and deposition on a Svalbard glacier between 1700 and 2004. The inner part of a 125 m deep ice core from Holtedahlfonna glacier (79°8' N, 13°16' E, 1150 m a.s.l.) was melted, filtered through a quartz fibre filter and analysed for EC using a thermal–optical method. The EC values started to increase after 1850 and peaked around 1910, similar to ice core records from Greenland. Strikingly, the EC values again increase rapidly between 1970 and 2004 after a temporary low point around 1970, reaching unprecedented values in the 1990s. This rise is not seen in Greenland ice cores, and it seems to contradict atmospheric BC measurements indicating generally decreasing atmospheric BC concentrations since 1989 in the Arctic. For example, changes in scavenging efficiencies, post-depositional processes and differences in the vertical distribution of BC in the atmosphere are discussed for the differences between the Svalbard and Greenland ice core records, as well as the ice core and atmospheric measurements in Svalbard. In addition, the divergent BC trends between Greenland and Svalbard ice cores may be caused by differences in the analytical methods used, including the operational definitions of quantified particles, and detection efficiencies of different-sized BC particles. Regardless of the cause of the increasing EC values between 1970 and 2004, the results have significant implications for the past radiative energy balance at the coring site.

The vertical distribution of black carbon in CMIP5 models: Comparison to observations and the importance of convective transport

Large uncertainty in the direct radiative forcing of black carbon (BC) exists, with published estimates ranging from 0.25 to 0.9 W m−2. A significant source of this uncertainty relates to the vertical distribution of BC, particularly relative to cloud layers. We first compare the vertical distribution of BC in Coupled Model Intercomparison Project Phase 5 (CMIP5) models to aircraft measurements and find that models tend to overestimate upper tropospheric/lower stratospheric (UT/LS) BC, particularly over the central Pacific from Hiaper Pole‐to‐Pole Observations Flight 1 (HIPPO1). However, CMIP5 generally underestimates Arctic BC from the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites campaign, implying a geographically dependent bias. Factors controlling the vertical distribution of BC in CMIP5 models, such as wet and dry deposition, precipitation, and convective mass flux (MC), are subsequently investigated. We also perform a series of sensitivity experiments with the Community Atmosphere Model version 5, including prescribed meteorology, enhanced vertical resolution, and altered convective wet scavenging efficiency and deep convection. We find that convective mass flux has opposing effects on the amount of black carbon in the atmosphere. More MC is associated with more convective precipitation, enhanced wet removal, and less BC below 500 hPa. However, more MC, particularly above 500 hPa, yield more BC aloft due to enhanced convective lofting. These relationships—particularly MC versus BC below 500 hPa—are generally stronger in the tropics. Compared to the Modern‐Era Retrospective Analysis for Research and Applications, most CMIP5 models overestimate MC, with all models overestimating MC above 500 hPa. Our results suggest that excessive convective transport is one of the reasons for CMIP5 overestimation of UT/LS BC.

Constraints on aerosol processes in climate models from vertically-resolved aircraft observations of black carbon

Abstract. Evaluation of the aerosol schemes in current climate models is dependent upon the available observational data. In-situ observations from flight campaigns can provide valuable data about the vertical distribution of aerosol that is difficult to obtain from satellite or ground-based platforms, although they are localised in space and time. Using single-particle soot-photometer (SP2) measurements from the HIAPER Pole-to-Pole Observations (HIPPO) campaign, which consists of many vertical profiles over a large region of the Pacific, we evaluate the meridional and vertical distribution of black carbon (BC) aerosol simulated by the HadGEM3–UKCA and ECHAM5–HAM2 models. Both models show a similar pattern of overestimating the BC column burden compared to that derived from the observations, in many areas by an order of magnitude. However, by sampling the simulated BC mass mixing ratio along the flight track and comparing to the observations, we show that this discrepancy has a rather different vertical structure in the two models: in HadGEM3–UKCA the discrepancy is dominated by excess aerosol in the tropical upper troposphere, while in ECHAM5–HAM2 areas of discrepancy are spread across many different latitudes and altitudes. Using this methodology, we conduct sensitivity tests on two specific elements of the models: biomass-burning emissions and scavenging by convective precipitation. We show that, by coupling the convective scavenging more tightly with convective transport, both the column burden and vertical distribution of BC in HadGEM3–UKCA are much improved with respect to the observations, with a substantial and statistically significant increase in correlation – this demonstrates the importance of a realistic representation of this process. In contrast, updating from GFED2 to GFED3.1 biomass-burning emissions makes a more modest improvement in both models, which is not statistically significant. By comparing our results with a more traditional approach using regional- and monthly-mean vertical profile curves, we show that the point-by-point analysis allows the model improvements to be demonstrated more clearly. We also demonstrate the important role that nudged simulations (where the large-scale model dynamics are continuously relaxed towards a reanalysis) can play in this type of evaluation, allowing statistically significant differences between configurations of the aerosol scheme to be seen where the differences between the corresponding free-running simulations would not be significant.

How much can the vertical distribution of black carbon affect its global direct radiative forcing?

Black carbon (BC) has an increased forcing per unit mass when it is located above reflective clouds. To explore sensitivity of forcing to aerosol vertical location, we used a column radiative transfer model to produce globally‐averaged values of normalized direct radiative forcing (NDRF) for BC over and under different types of clouds. We developed a simple column‐weighting scheme based on the mass fractions of BC that are over and under clouds in measured vertical profiles. The resulting NDRF is in good agreement with global 3‐D model estimates, supporting the column‐weighted model as a tool for exploring uncertainties due to diversity in vertical distribution. BC above low clouds accounts for about 20% of the global burden but 50% of the forcing. We estimate maximum‐minimum spread in NDRF due to modeled profiles as about 40% and uncertainty as about 25%. Because models overestimate BC in the upper troposphere compared with measurements, modeled NDRF might need to be reduced by about 15%. Redistributing BC within the lowest 4 km of the atmosphere affects modeled NDRF by only about 5% and cannot account for very high forcing estimates.

Historical record of black carbon in urban soils and its environmental implications

Energy use in urbanization has fundamentally changed the pattern and fluxes of carbon cycling, which has global and local environmental impacts. Here we have investigated organic carbon (OC) and black carbon (BC) in six soil profiles from two contrast zones in an ancient city (Nanjing) in China. BC in soils was widely variable, from 0.22 to 32.19 g kg −1. Its average concentration in an ancient residential area (Zone 1) was, 0.91 g kg −1, whereas in Zone 2, an industrial and commercial area, the figure was 8.62 g kg −1. The ratio of BC/OC ranged from 0.06 to 1.29 in soil profiles, with an average of 0.29. The vertical distribution of BC in soil is suggested to reflect the history of BC formation from burning of biomass and/or fossil fuel. BC in the surface layer of soils was mainly from traffic emission (especially from diesel vehicles). In contrast, in cultural layers BC was formed from historical coal use. The contents of BC and the ratio of BC/OC may reflect different human activities and pollution sources in the contrasting urban zones. In addition, the significant correlation of heavy metals (Cu, Pb, and Zn) with BC contents in some culture layers suggests the sorption of the metals by BC or their coexistence resulted from the coal-involved smelting.

Equilibrium Response of an Atmosphere–Mixed Layer Ocean Model to Different Radiative Forcing Agents: Global and Zonal Mean Response

Abstract The equilibrium response to various forcing agents, including CO2, solar irradiance, tropospheric ozone, black carbon, organic carbon, sulfate, and volcanic aerosols, is investigated using an atmospheric general circulation model coupled to a mixed layer ocean model. The experiments are carried out by altering each forcing agent separately. Realistic spatial patterns of forcing constituents are applied but the magnitude of the forcing is adjusted so that each forcing constituent yields approximately the same strength of radiative forcing. It is demonstrated that the global mean temperature response depends on the types of forcing agents and the efficacy with respect to CO2 forcing ranges from 58% to 100%. The smallest efficacy is seen in one of the black carbon experiments and is associated with negative cloud feedback. The sign of the cloud feedback is shown to be sensitive to the vertical distribution of black carbon. The feedback analysis suggests that the small efficacy in tropospheric ozone is due to a large negative lapse rate feedback. Global mean precipitation increases when the earth warms except for the case of black carbon in which precipitation decreases. In all experiments, the global mean convective mass flux decreases when the earth’s surface warms. When the applied radiative forcing resulting from a particular forcing agent is stronger in one hemisphere, anomalous heat exchange between the hemispheres results in conjunction with changes in the Hadley circulation. The magnitude of interhemispheric heat transport is little sensitive to the details of the forcing, but is determined primarily by the interhemispheric contrast in forcing. The change in the Hadley circulation strongly impacts the precipitation changes in low latitudes.