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Activation of biomass burning aerosols (BBA) and fossil fuel combustion aerosols (FFA) in fogs and clouds significantly impact regional air quality through aqueous chemistry and climate by affecting cloud microphysics. However, we lack direct observations of how these aerosols behave in fogs and clouds. Using a newly developed aerosol‐cloud sampling system, we conducted observations during fog events and found that BBA, despite their high organic content, effectively contributed to super‐micron interstitial aerosols and fog droplets in low supersaturation fogs. In contrast, FFA, predominantly externally mixed organic, did not grow beyond the super‐micron size in fogs due to their near‐hydrophobic nature. Measurements conducted under supersaturations relevant for cloud formation revealed that portions of FFA could serve as cloud condensation nuclei, but only when supersaturation exceeded ∼0.14%. These findings have broad implications for future investigations into the influence of BBA and FFA on fog and cloud chemistry and their interactions with clouds. Plain Language Summary: Tiny particles, known as aerosols, emitted from combustion of biomass and fossil fuels, can impact air quality and global climate by interacting with fog and clouds. However, we lack direct observational evidence of how these aerosols behave in these conditions. In our study, we developed an advanced aerosol‐cloud sampling system to observe aerosol activation during fog events. Our findings highlight a crucial factor of aerosol activation: the mixing of these aerosols. Biomass burning aerosols with an internal mixture of organic and inorganic components activate more easily, even in conditions with low supersaturation like fog. In contrast, fossil fuel combustion aerosols are often externally mixed and almost water‐repellent, requiring higher supersaturation to become cloud condensation nuclei. Understanding these distinctions has significant implications for regional air quality and the intricate interactions between aerosols and clouds. By gaining insights into how various aerosols interact with fog and clouds, we can enhance our understanding of their impact on our environment and climate. Key Points: Advanced Aerosol‐Cloud sampling system was developed to characterize aerosol activation in fogsBiomass burning aerosols efficiently form fog droplets, while fossil fuel combustion aerosols are almost hydrophobic and don't contributeFossil fuel combustion organic aerosols can serve as CCN in high supersaturation conditions (>0.14%) [ABSTRACT FROM AUTHOR]

Both the size and mixing state of black carbon (BC)-containing aerosols are crucial in estimating the environmental, health and climate impacts of BC. Traffic emissions are a major global source of BC; however, parameterization of BC mass size distributions and mixing states associated with traffic remains lacking due to its dependence on vehicle types and driving conditions. To investigate BC mass size distributions and mixing states associated with traffic emissions, a field campaign was conducted in the Guangzhou urban area during winter, which used a system coupling a differential mobility analyzer (DMA) and a single-particle soot photometer (SP2) to measure BC mass size distributions in the range of 100 to 700 nm. The resolved primary organic aerosols were hydrocarbon-like organic aerosols (HOA) and cooking-like organic aerosols (COA), as well as refractory BC (rBC), which was detected by the DMA–SP2 and correlated highly with HOA (R2=0.88), confirming that traffic emissions are the dominant source of atmospheric BC during the observations. The BC mass size distribution was found to be best fitted by a lognormal distribution, with a geometric mean (Dg,BC) of 258±16 nm, varying between 200 and 300 nm. During daytime, active formation of secondary nitrate and organic aerosols was observed, but it had little effect on the variations of BC mass size distributions. Further analyses revealed that Dg,BC was moderately correlated with rBC / HOA (R2=0.41) in a linear form of Dg,BC=34×rBC/HOA+177 , demonstrating that the BC content of traffic emissions significantly impacts the BC mass size distributions. In addition, the size-dependent fractions of BC-containing aerosols in all types of aerosols (fBCc) and the fraction of identified externally mixed (bare/thinly coated) BC particles in all BC-containing aerosols (fext) were also characterized. It was found that the daytime secondary aerosol formation reduced both fBCc and fext , with the decrease in fext being more pronounced for larger particles, possibly due to the higher relative coating thickness. Variations in fext during nighttime were mainly controlled by the emission conditions. For example, fext for 600 nm particles decreased from 0.82 to 0.46 as rBC / HOA increased from 1 to 3.5, while the mass ratios of secondary aerosols to rBC varied little, demonstrating that the BC content also significantly affects the mixing states of freshly emitted BC from traffic emissions. This study suggests that BC content can be used as the key factor to parameterize both the BC mass size distribution and mixing states from traffic emissions, which warrants future comprehensive investigation. In addition, other sources such as biomass burning and coal combustion also contribute substantially to BC emissions, and it was important to investigate whether BC content of other major BC sources than traffic is also important in determining BC mass size distributions and mixing states. Overall, results of this study have significant implications for accurate representation of BC from different sources when modeling the impacts of BC. [ABSTRACT FROM AUTHOR]

Emission controls have substantially brought down aerosol pollution in China; however, aerosol mass reductions have slowed down in recent years in the Pearl River Delta (PRD) region, where secondary organic aerosol (SOA) formation poses a major challenge for air quality improvement. In this study, we characterized the roles of SOA in haze formation in urban Guangzhou in the PRD using year-long aerosol mass spectrometer measurements for the first time and discussed possible pathways of SOA formations. On average, organic aerosols (OA) contribute dominantly (50 %) to nonrefractory submicron aerosol mass (NR-PM 1). The average mass concentration of SOA (including by less- and more-oxidized OA, LOOA and MOOA) contributed most to NR-PM 1 , reaching about 1.7 times that of primary organic aerosols (POA, including hydrocarbon-like and cooking-related OA) and accounting for 32 % of NR-PM 1 , even more than sulfate (22 %) and nitrate (16 %). Seasonal variations of NR-PM 1 revealed that haze-formation mechanisms differed much among distinct seasons. Sulfate mattered more than nitrate in fall, while nitrate was more important than sulfate in spring and winter, with SOA contributing significantly to haze formations in all the seasons. Daytime SOA formation was weak in winter under low oxidant levels and air relative humidity, whereas prominent daytime SOA formation was observed in fall, spring and summer almost on a daily basis, suggesting important roles of photochemistry in SOA formations. Further analysis showed that the coordination of gas-phase photochemistry and subsequent aqueous-phase reactions likely played significant roles in quick daytime SOA formations. Obvious nighttime SOA formations were also frequently observed in spring, fall and winter, and it was found that daytime and nighttime SOA formations together had resulted in the highest SOA concentrations in these seasons and contributed substantially to severe haze formations. Simultaneous increases in nitrate with SOA after sunset suggested the important roles of NO 3 radical chemistry in nighttime SOA formations, and this was further confirmed by a continuous increase in the NO + / NO 2+ fragment ratio that related to measured particulate nitrate after sunset. The findings of this study have promoted our understanding of haze pollution characteristics of the PRD and laid down future directions for investigations of SOA-formation mechanisms in urban areas of southern China that share similar emission sources and meteorological conditions. [ABSTRACT FROM AUTHOR]

Biomass burning organic aerosol (BBOA) impacts significantly on climate directly through scattering and absorbing solar radiation and indirectly through acting as cloud condensation nuclei. However, fundamental parameters in the simulation of BBOA radiative effects and cloud activities such as size distribution and refractive index remain poorly parameterized in models. In this study, biomass burning events with high combustion efficiency characterized by a high black carbon (BC) to BBOA ratio (0.22 on average) were frequently observed during autumn in the Pearl River Delta region, China. An improved absorption Ångström exponent (AAE) ratio method considering both variations and spectral dependence of black carbon AAE was proposed to differentiate brown carbon (BrC) absorptions from total aerosol absorptions. BBOA size distributions, mass scattering and absorption efficiency were retrieved based on the changes in aerosol number size distribution, scattering coefficients and derived BrC absorptions that occurred with BBOA spikes. Geometric mean diameter of BBOA volume size distribution Dgv depended largely on combustion conditions, ranging from 245 to 505 nm, and a linear relationship between Dgv and ΔBC/ΔBBOA ​​​​​​​ was achieved. The retrieved real part of the BBOA refractive index ranges from 1.47 to 1.64, with evidence showing that its variations might depend largely on combustion efficiency, which is rarely investigated in existing literature but which however requires further comprehensive investigations. Retrieved imaginary parts of BBOA refractive index (mi,BBOA) correlated highly with ΔBC/ΔBBOA (R>0.88) but differ a lot from previous parameterization schemes. The reason behind the inconsistency might be that single formula parameterizations of mi,BBOA over the whole BC/BBOA range were used in previous studies which might deviate substantially for specific BC/BBOA ranges. Thus, a new scheme that parameterizes wavelength-dependent mi,BBOA was presented, which filled the gap for field-based BBOA absorptivity parameterizations of ​​​​​​​BC/BBOA >0.1. These findings have significant implications for simulating BBOA climate effects and suggest that linking both BBOA refractive index and BBOA volume size distributions to BC content might be a feasible and a good choice for climate models. [ABSTRACT FROM AUTHOR]

Secondary organic aerosols (SOAs) account for a large fraction of atmospheric aerosol mass and play significant roles in visibility impairment by scattering solar radiation. However, comprehensive evaluations of SOA scattering abilities under ambient relative humidity (RH) conditions on the basis of field measurements are still lacking due to the difficulty of simultaneously direct quantifications of SOA scattering efficiency in dry state and SOA water uptake abilities. In this study, field measurements of aerosol chemical and physical properties were conducted in winter in Guangzhou (lasting about 3 months) using a humidified nephelometer system and aerosol chemical speciation monitor. A modified multilinear regression model was proposed to retrieve dry-state mass scattering efficiencies (MSEs, defined as scattering coefficient per unit aerosol mass) of aerosol components. The more oxidized oxygenated organic aerosol (MOOA) with an O/C ratio of 1.17 was identified as the most efficient light scattering aerosol component. On average, 34 % mass contribution of MOOA to total submicron organic aerosol mass contributed 51 % of dry-state organic aerosol scattering. The overall organic aerosol hygroscopicity parameter κOA was quantified directly through hygroscopicity closure, and hygroscopicity parameters of SOA components were further retrieved using a multilinear regression model by assuming hydrophobic properties of primary organic aerosols. The highest water uptake ability of MOOA among organic aerosol factors was revealed with κMOOA reaching 0.23, thus further enhancing the fractional contribution of MOOA in ambient organic aerosol scattering. In particular, the scattering abilities of MOOA were found to be even higher than those of ammonium nitrate under RH of <70% , which was identified as the most efficient inorganic scattering aerosol component, demonstrating that MOOA had the strongest scattering abilities in ambient air (average RH of 57 %) during winter in Guangzhou. During the observation period, secondary aerosols contributed dominantly to visibility degradation (∼70%), with substantial contributions from MOOA (16 % on average), demonstrating significant impacts of MOOA on visibility degradation. The findings of this study demonstrate that more attention needs to be paid to SOA property changes in future visibility improvement investigations. Also, more comprehensive studies on MOOA physical properties and chemical formation are needed to better parameterize its radiative effects in models and implement targeted control strategies on MOOA precursors for visibility improvement. [ABSTRACT FROM AUTHOR]

Water uptake abilities of organic aerosol under sub-saturated conditions play critical roles in direct aerosol radiative effects and atmospheric chemistry; however, field characterizations of the organic aerosol hygroscopicity parameter κOA under sub-saturated conditions remain limited. In this study, a field campaign was conducted to characterize κOA at a relative humidity of 80 % with hourly time resolution for the first time in the Pearl River Delta region of China. Observation results show that, during this campaign, secondary organic aerosol (SOA) dominated total organic aerosol mass (mass fraction > 70 % on average), which provides a unique opportunity to investigate influences of SOA formation on κOA. Results demonstrate that the commonly used organic aerosol oxidation level parameter O/C was weakly correlated with κOA and failed to describe the variations in κOA. However, the variations in κOA were well reproduced by mass fractions of organic aerosol factor resolved based on aerosol mass spectrometer measurements. The more oxygenated organic aerosol (MOOA) factor, exhibiting the highest average O/C (∼ 1) among all organic aerosol factors, was the most important factor driving the increase in κOA and was commonly associated with regional air masses. The less oxygenated organic aerosol (LOOA; average O/C of 0.72) factor revealed strong daytime production, exerting negative effects on κOA. Surprisingly, the aged biomass burning organic aerosol (aBBOA) factor also formed quickly during daytime and shared a similar diurnal pattern with LOOA but had much lower O/C (0.39) and had positive effects on κOA. The correlation coefficient between κOA and mass fractions of aBBOA and MOOA in total organic aerosol mass reached above 0.8. The contrasting effects of LOOA and aBBOA formation on κOA demonstrate that volatile organic compound (VOC) precursors from diverse sources and different SOA formation processes may result in SOA with different chemical composition, functional properties and microphysical structure, consequently exerting distinct influences on κOA and rendering single oxidation level parameters (such as O/C) unable to capture those differences. Aside from that, distinct effects of aBBOA on κOA were observed during different episodes, suggesting that the hygroscopicity of SOA associated with similar sources might also differ much under different emission and atmospheric conditions. Overall, these results highlight that it is imperative to conduct more research on κOA characterization under different meteorological and source conditions and examine its relationship with VOC precursor profiles and formation pathways to formulate a better characterization and develop more appropriate parameterization approaches in chemical and climate models. [ABSTRACT FROM AUTHOR]

Secondary aerosols (SAs, including secondary organic and inorganic aerosols, SOAs and SIAs) are predominant components of aerosol particles in the North China Plain (NCP), and their formation has significant impacts on the evolution of particle size distribution (PNSD) and hygroscopicity. Previous studies have shown that distinct SA formation mechanisms can dominate under different relative humidity (RH). This would lead to different influences of SA formation on the aerosol hygroscopicity and PNSD under different RH conditions. Based on the measurements of size-resolved particle activation ratio (SPAR), hygroscopicity distribution (GF-PDF), PM 2.5 chemical composition, PNSD, meteorology and gaseous pollutants in a recent field campaign, McFAN (Multiphase chemistry experiment in Fogs and Aerosols in the North China Plain), conducted during the autumn–winter transition period in 2018 at a polluted rural site in the NCP, the influences of SA formation on cloud condensation nuclei (CCN) activity and CCN number concentration (NCCN) calculation under different RH conditions were studied. Results suggest that during daytime, SA formation could lead to a significant increase in NCCN and a strong diurnal variation in SPAR at supersaturations lower than 0.07 %. During periods with daytime minimum RH exceeding 50 % (high RH conditions), SA formation significantly contributed to the particle mass and size changes in a broad size range of 150 to 1000 nm, leading to NCCN (0.05 %) increases within the size range of 200 to 500 nm and mass concentration growth mainly for particles larger than 300 nm. During periods with daytime minimum RH below 30 % (low RH conditions), SA formation mainly contributed to the particle mass and size and NCCN changes for particles smaller than 300 nm. As a result, under the same amount of mass increase induced by SA formation, the increase of NCCN (0.05 %) was stronger under low RH conditions and weaker under high RH conditions. Moreover, the diurnal variations of the SPAR parameter (inferred from CCN measurements) due to SA formation varied with RH conditions, which was one of the largest uncertainties within NCCN predictions. After considering the SPAR parameter (estimated through the number fraction of hygroscopic particles or mass fraction of SA), the relative deviation of NCCN (0.05 %) predictions was reduced to within 30 %. This study highlights the impact of SA formation on CCN activity and NCCN calculation and provides guidance for future improvements of CCN predictions in chemical-transport models and climate models. [ABSTRACT FROM AUTHOR]

Submicron aerosol (PM1) species measured by aerosol mass spectrometers have been widely used to validate chemical transport models; however, the uncertainties due to chemical differences between PM1 and PM2.5 are poorly constrained. Here we characterized such differences in a highly polluted environment in north China in winter. Our results showed that the changes in PM1/PM2.5 ratios as a function of relative humidity (RH) were largely different for primary and secondary species. Secondary organic and inorganic aerosol (SOA and SIA) presented clear decreases in PM1/PM2.5 ratios at RH > 60% during periods with high SIA contributions (>50%), likely driven by the changes in aerosol hygroscopicity and phase states, while the traffic and coal combustion OA had limited dependence on RH. Thermodynamic modeling showed negligible impacts of PM differences on predictions of particle acidity, yet these impacts can cause a difference in aerosol water content by up to 50–70%. Plain Language Summary: Current air pollution studies rely largely upon aerosol mass spectrometers that provide real‐time measurements of submicron aerosol (PM1) species, and in many studies, PM1 aerosol species are used to validate those of PM2.5 in chemical transport models and estimate particle acidity and aerosol water content which are key parameters in studying heterogeneous reactions. However, the uncertainties in air pollution studies due to the chemical differences of PM1 and PM2.5 are poorly constrained, particularly in highly polluted environment, for example, China and India. We found that the changes in PM1/PM2.5 ratios as a function of relative humidity were largely different for primary and secondary aerosol species in highly polluted environment, which was likely driven by the changes in aerosol hygroscopicity and phase states. The chemical differences of PM1/PM2.5 ratios were also found to have negligible impacts on predictions of particle acidity, yet these impacts can cause a difference in aerosol water content by up to 50–70%. Considering the rapid increases in the deployments of aerosol mass spectrometers worldwide, the results in this study are of great importance for a better understanding of the uncertainties in both modelling and observations. Key Points: We observed large chemical differences between PM1 and PM2.5 under high relative humidity periods in highly polluted environmentThe RH dependence of PM1/PM2.5 ratios of primary and secondary species was largely different due to different hygroscopicityThe chemical differences between PM1 and PM2.5 have negligible influences on particle acidity prediction, yet have a large impact on AWC [ABSTRACT FROM AUTHOR]

The hygroscopicity of organic aerosol (OA) is important for investigation of its climatic and environmental impacts. However, the hygroscopicity parameter κOA remains poorly characterized, especially in the relatively polluted environment on the North China Plain (NCP). Here we conducted simultaneous wintertime measurements of bulk aerosol chemical compositions of PM 2.5 and PM 1 and bulk aerosol hygroscopicity of PM 10 and PM 1 on the NCP using a capture-vaporizer time-of-flight aerosol chemical speciation monitor (CV-ToF-ACSM) and a humidified nephelometer system which measures the aerosol light-scattering enhancement factor fRH. A method for calculating κOA based on fRH and bulk aerosol chemical-composition measurements was developed. We found that κOA varied in a wide range with significant diurnal variations. The derived κOA ranged from almost 0.0 to 0.25, with an average (±1σ) of 0.08 (±0.06) for the entire study. The derived κOA was highly correlated with f44 (fraction of m/z 44 in OA measured by CV-ToF-ACSM), an indicator of the oxidation degree of OA (R=0.79), and the relationship can be parameterized as κOA=1.04×f44-0.02 (κOA=0.3×O:C-0.02 , based on the relationship between the f44 and O/C ratio for CV-ToF-ACSM). On average, κOA reached the minimum (0.02) in the morning near 07:30 local time (LT) and then increased rapidly, reaching the peak value of 0.16 near 14:30 LT. The diurnal variations in κOA were highly and positively correlated with those of mass fractions of oxygenated OA (R=0.95), indicating that photochemical processing played a dominant role in the increase in κOA in winter on the NCP. Results in this study demonstrate the potential wide applications of a humidified nephelometer system together with aerosol composition measurements for investigating the hygroscopicity of OA in various environments and highlight that the parameterization of κOA as a function of OA aging processes needs to be considered in chemical transport models for better evaluating the impacts of OA on cloud formation, atmospheric chemistry, and radiative forcing. [ABSTRACT FROM AUTHOR]

Large uncertainties exist when estimating radiative effects of ambient black carbon (BC) aerosol. Previous studies about the BC aerosol radiative forcing mainly focus on the BC aerosols' mass concentrations and mixing states, while the effects of BC mass size distribution (BCMSD) were not well considered. In this paper, we developed a method of measuring the BCMSD by using a differential mobility analyzer in tandem with an Aethalometer. A comprehensive method of multiple charging corrections was proposed and implemented in measuring the BCMSD. Good agreement was obtained between the BC mass concentration integrated from this system and that measured in the bulk phase, demonstrating the reliability of our proposed method. Characteristics of the BCMSD and corresponding radiative effects were studied based on a field measurement campaign conducted in the North China Plain by using our own measurement system. Results showed that the BCMSD had two modes and the mean peak diameters of the modes were 150 and 503 nm. The BCMSD of the coarser mode varied significantly under different pollution conditions with peak diameter varying between 430 and 580 nm, which gave rise to significant variation in aerosol bulk optical properties. The direct aerosol radiative forcing was estimated to vary by 8.45 % for different measured BCMSDs of the coarser mode, which shared the same magnitude with the variation associated with assuming different aerosol mixing states (10.5 %). Our study reveals that the BCMSD as well as its mixing state in estimating the direct aerosol radiative forcing matters. Knowledge of the BCMSD should be fully considered in climate models. [ABSTRACT FROM AUTHOR]

The study of atmospheric nitrous acid (HONO), which is the primary source of OH radicals, is crucial with respect to understanding atmospheric photochemistry and heterogeneous chemical processes. Heterogeneous NO2 chemistry under haze conditions has been identified as one of the missing sources of HONO on the North China Plain, and also produces sulfate and nitrate. However, controversy exists regarding the various proposed HONO production mechanisms, mainly regarding whether SO2 directly takes part in the HONO production process and what roles NH3 and the pH value play. In this paper, never before seen explosive HONO production was reported and evidence was found – for the first time in field measurements during fog (usually with 4< pH <6) and haze episodes under high relative humidity (pH ≈4) – that NH3 was the key factor that promoted the hydrolysis of NO2 , leading to the explosive growth of HONO and nitrate under both high and relatively lower pH conditions. The results also suggest that SO2 plays a minor or insignificant role in HONO formation during fog and haze events, but was indirectly oxidized upon the photolysis of HONO via subsequent radical mechanisms. Aerosol hygroscopicity significantly increased with rapid inorganic secondary aerosol formation, further promoting HONO production as a positive feedback. For future photochemical and aerosol pollution abatement, it is crucial to introduce effective NH3 emission control measures, as NH3 -promoted NO2 hydrolysis is a large daytime HONO source, releasing large amounts of OH radicals upon photolysis, which will contribute largely to both atmospheric photochemistry and secondary aerosol formation. [ABSTRACT FROM AUTHOR]

Black carbon (BC) aerosols play an important role in modifying the PBL meteorology and severity of haze pollution through its radiative effect (Ding et al. 2016). The radiative effect of BC strongly depends on its mixing state or coating thickness, which is subject to the aging process in the atmosphere (Cheng et al. 2006, 2008). North China Plain (NCP) is one of the areas where both severe haze and high abundance of BC have been reported. By using a single particle soot photometer (SP2) and complimentary measurements, we investigated the aging of BC during several severe haze events in the North China Plain during our winter campaign in 2018. We calculated the size-resolved mixing state and aging rate of BC in the ambient air under different pollution levels. Our results show a fast aging of BC especially during the haze events with high concentrations of air pollutants and condensable vapors. The occurrence of fog episodes in the campaign enables us to analyze the BC-droplet interactions, especially the effect of aerosol size and coating thickness on its activation and removal, which is key for understanding the aerosol-cloud and aerosol fog interactions. Moreover, we find that most coated BC particles can be classified to a "core-shell" type while only a small fraction of BC-containing particles is in an "attached" type. It suggests that the traditional Core-Shell model should still be applicable for modeling the radiative properties of BC in the North China Plain. Based on the Mie model calculation, we determined their absorption enhancement factor of BC due to the aging process and discussed its implication to the formation of severe haze.References:Ding, A. J., Huang, X., Nie, W., Sun, J. N., Kerminen, V. M., Petaja, T., Su, H., Cheng, Y. F., Yang, X. Q., Wang, M. H., Chi, X. G., Wang, J. P., Virkkula, A., Guo, W. D., Yuan, J., Wang, S. Y., Zhang, R. J., Wu, Y. F., Song, Y., Zhu, T., Zilitinkevich, S., Kulmala, M., and Fu, C. B.: Enhanced haze pollution by black carbon in megacities in China, Geophys. Res. Lett., 43, 2873-2879, 10.1002/2016gl067745, 2016.Cheng, Y. F., Eichler, H., Wiedensohler, A., Heintzenberg, J., Zhang, Y. H., Hu, M., Herrmann, H., Zeng, L. M., Liu, S., Gnauk, T., Bruggemann, E., and He, L. Y.: Mixing state of elemental carbon and non-light-absorbing aerosol components derived from in situ particle optical properties at Xinken in Pearl River Delta of China, J. Geophys. Res.-Atmos., 111, 18, 10.1029/2005jd006929, 2006.Cheng, Y. F., Wiedensohler, A., Eichler, H., Su, H., Gnauk, T., Brueggemann, E., Herrmann, H., Heintzenberg, J., Slanina, J., Tuch, T., Hu, M., and Zhang, Y. H.: Aerosol optical properties and related chemical apportionment at Xinken in Pearl River Delta of China, Atmos. Environ., 42, 6351-6372, 10.1016/j.atmosenv.2008.02.034, 2008. [ABSTRACT FROM AUTHOR]

Black carbon (BC) is the most important light-absorbing species in the atmosphere and has a strong positive direct radiative forcing. Recent studies found that BC-containing particles may also modify the structure of planetary boundary layer and thus enhance haze pollution, especially in densely populated areas, for instance, in China. It is however difficult to quantify these effects due to their complex mixing state, hygroscopicity and the high spatial variability of concentration. During an intensive field campaign conducted in the North China Plain in a cold season, the mass size distribution and mixing state of BC was measured with a single particle soot photometer (SP2). During a severe haze event, the BC mass concentration exceeded 15 ug m-3. Stemming from the strong aging process under high relative humidity condition, freshly emitted BC particle was observed to turn into internally mixing within a short time scale. To understand the variation of the hygroscopicity of BC during its aging process, the hygroscopic growth factor of BC-containing particles was measured with a combination of a hygroscopic tandem differential mobility analyzer and a SP2. The particle hygroscopic growth factor was found to be correlated with the volume fraction of BC. During haze events, a fraction of the more-hygroscopic mode particles was observed to contain BC cores; while a minor fraction of the hydrophobic mode particles contained no BC. [ABSTRACT FROM AUTHOR]