Aerosol Aging Processes Research Articles

Aerosol acidity and its impact on sulfate and nitrate of PM2.5 in southern city of Beijing-Tianjin-Hebei region

Aerosol acidity, similar to the pH of a solution, influences the water-phase reactions within aerosols, thereby impacting the formation and composition of particle components. This study investigated the chemical composition of PM2.5, pH of PM2.5 and its impact on formation of SO42− and NO3− in Handan city from March 2015 to February 2016. The results showed that NO3− was the dominant inorganic secondary ion in PM2.5, accounting for 16.4% of PM2.5, slightly higher than SO42− at 14.2%. The average pH of PM2.5 in Handan was 4.3, indicating moderate acidity, with higher acidity observed in spring and winter. Nitrogen oxidation ratio (NOR) was synergistically influenced by pH and aerosol water content (AWC), while sulfur oxidation ratio (SOR) was associated with high aerosol acidity or AWC. The activity coefficient of NO3− exhibited a negative correlation with activity coefficient of [H+], with an R2 value of −0.20 (“-” means negative correlation). Conversely, a positive correlation was observed between activity coefficient of SO42− and [H+], with an R2 value of 0.19. The partition coefficient (ε(NO3−)) of nitrate, representing its assignment to the solid phase, is given by the function y = −0.5x2+4.2x-7.6 (R2 = 0.60), where x corresponds to pH and y corresponds to ε(NO3−). It suggested that NO3− is most likely to enter the solid phase in significant quantities under moderately acidity condition (at around pH = 4.2). A significant correlation observed between pH, surface tension, and particle radius, indicating that pH influences the aging process of aerosols. However, the impact of pH varies depending on the pollution episode.

Long term observations of biomass burning aerosol over Warsaw by means of multiwavelength lidar.

High quality lidar measurements of PollyXT operating at the University of Warsaw in the years 2013-2022 were analyzed to present a comprehensive optical characterization of biomass burning aerosols over Warsaw. The directions of the aerosol inflows were analyzed by dividing advection cases into four types, according to the area of origin: Western Europe, Eastern Europe, the Iberian Peninsula, and North America. It was shown that optical properties of smoke vary in each of these types, and emphasized that aerosol aging processes are important. It was found that as aerosol's age increases, there is more backscattering and less extinction at 355 nm in relation to 532 nm. The analysis of the lidar ratio demonstrated that the main changes of the aging process were observed in the UV spectra.

Surface tension of single suspended aerosol microdroplets

The surface tension of troposphere aerosols can significantly influence their atmospheric processes and key properties, particularly on the morphology, the phase transition, the activation as cloud condensation nuclei, and the gas-particle partitioning. However, directly measuring the surface tension of single ambient aerosol is quite challenging, due to the limitations of their picolitre volumes and thermal motion. Here, we developed a dual laser tweezers Raman spectroscopy (DLT-RS) system to directly sense the surface tension of single airborne microdroplets (PM10 particles). A pair of aerosol droplets were trapped and driven to coalesce by the laser tweezers. Meanwhile, the backscattering light intensity and bright-field images during the coalescence process were recorded to characterize the aerosol surface tension. A remarkable advantage of directly sensing aerosol surface tension is that the solutes in aerosols are often supersaturated, which is common in atmospheric aerosols but almost unavailable in bulk solutions. We experimentally measured the surface tension of aerosols composed of nitrates or oxalic acid/nitrate mixture. Besides, the variation of surface tension during aerosol aging process was also explored, which brings possible implications on the surface evolution of actual ambient aerosol during their atmospheric lifetime.

Reinterpret the heterogeneous reaction of α-Fe2O3 and NO2 with 2D-COS: The role of SDS, UV and SO2

Heterogeneous reaction of mineral aerosols and atmospheric polluting gases play an important role in atmospheric chemistry. In this study, the reactions of NO2 with or without SO2 mixture gas on the surface of α-Fe2O3 particles under dry conditions were studied. The effects of sodium dodecyl sulfate (SDS) and the heterogeneous reaction under both dark and UV irradiation conditions were investigated. The infrared spectrum analyzed by the two-dimensional correlation spectroscopy (2D-COS) was used to obtain the products formation sequences. The results showed that UV irradiation can promote the production of nitrate. The 2D-COS analysis indicated SDS changed the sequence order of nitrate and nitrite species during reactions. In oxidation conditions, the final product of heterogeneous reaction of NO2 and α-Fe2O3 was monodentate nitrate. Only the heterogenous reaction of NO2 and α-Fe2O3 containing SDS (FOS) without UV light, the final product was bidentate nitrate. SDS was the catalysis agent supply and photoresist to the system. With surface active compounds, the environmental lifetime of heterogeneous reactions between trace gases and aerosols extends. Surfactants, ultraviolet light, and the types of gases involved in the reaction all have complex effects on the aerosol aging process. This study provided a reference for subsequent heterogeneous reaction studies and the formation of aerosols.

Compositional evolution for mixed aerosols containing gluconic acid and typical nitrate and the effect of multiply factors on hygroscopicity

The aging process of atmospheric aerosols usually leads to a mixture of inorganic salts and organic compounds of anthropogenic origin. In organic compounds, polyhydroxy organic acids are important components, however, the study on composition and hygroscopic properties of the mixture containing inorganics and polyhydroxy organic acids is scanty. In this study, gluconic acid, the proxy of polyhydroxy organic acids, is mixed with the representative nitrate (Mg(NO3)2, Ca(NO3)2) to form aerosols. ATR–FTIR and optical microscopy are employed to study the component changes and hygroscopicity as a function of relative humidity. As relative humidity fluctuates, the FTIR–ATR spectra display that the internal mixed gluconic acid (CH2(CH)4(OH)5COOH) and nitrate can react to release acidic gases, forming relevant gluconate and further affecting the hygroscopicity. The specific presentation is particles cannot be recovered to their original size after the dehydration–hydration process and there will be some disparities in GF for mixed particles. For the gluconic acid–Ca(NO3)2/Mg(NO3)2 mixtures with molar ratios of 1:1, higher degree of reaction resulting in the production of large amounts of gluconate should be responsible to the lower hygroscopicity compared to ZSR model. For 1:2 gluconic acid–nitrate mixed systems (with higher nitrate content), the hygroscopicity of mixtures are higher than the ZSR prediction. A possible reason could be ‘salt-promoting effect’ on the organic fractions of the surplus inorganic salt in the mixture. These data can improve the chemical composition list evaluation, in turn hygroscopic properties and phase state of atmospheric aerosol, and then the climate effect.

Simultaneous retrievals of biomass burning aerosols and trace gases from the ultraviolet to near-infrared over northern Thailand during the 2019 pre-monsoon season

Abstract. With the advent of spaceborne spectroradiometers in a geostationary constellation, measuring high spectral resolution ultraviolet–visible (UV-VIS) and selected near-/shortwave-infrared (NIR/SWIR) radiances can enable the probing of the life cycle of key atmospheric trace gases and aerosols at higher temporal resolutions over the globe. The UV-VIS measurements are important for retrieving several key trace gases (e.g., O3, SO2, NO2, and HCHO) and particularly for deriving aerosol characteristics (e.g., aerosol absorption and vertical profile). This study examines the merit of simultaneous retrievals of trace gases and aerosols using a ground-based spectroradiometer covering the UV-NIR to monitor their physicochemical processes and to obtain reliable aerosol information for various applications. During the 2019 pre-monsoon season over northern Thailand, we deployed a ground-based SMART–s (Spectral Measurements for Atmospheric Radiative Transfer–spectroradiometer) instrument, which is an extended-range Pandora with reliable radiometric calibration in the 330–820 nm range, to retrieve remotely sensed chemical and aerosol properties for the first time near biomass burning sources. The high spectral resolution (∼ 1.0 nm full width half maximum with ∼ 3.7 × oversampling) of sun and sky measurements from SMART–s provides several key trace gases (e.g., O3, NO2, and H2O) and aerosol properties covering the UV where significant light absorption occurs by the carbonaceous particles. During the measurement period, highly correlated total column amounts of NO2 and aerosol optical thickness (τaer) retrieved from SMART–s (correlation coefficient, R=0.74) indicated their common emissions from biomass burning events. The SMART–s retrievals of the spectral single scattering albedo (ω0) of smoke aerosols showed an abrupt decrease in the UV, which is an important parameter dictating photochemical processes in the atmosphere. The values of ω0 and column precipitable water vapor (H2O) gradually increase with the mixing of biomass burning smoke particles and higher water vapor concentrations when approaching the monsoon season. The retrieved ω0 and weighted mean radius of fine-mode aerosols from SMART–s showed positive correlations with the H2O (R=0.81 for ω0 at 330 nm and 0.56 for the volume-weighted mean radius), whereas the real part of the refractive index of fine-mode aerosol (nf) showed negative correlations (R=-0.61 at 330 nm), which suggest that aerosol aging processes including hygroscopic growth (e.g., humidification and cloud processing) can be a major factor affecting the temporal trends of aerosol optical properties. Retrieved nf and ω0 were closer to those of the water droplet (i.e., nf of about 1.33 and ω0 of about 1.0) under lower amounts of NO2 during the measurement period; considering that the NO2 amounts in the smoke may indicate the aging of the plume after emission due to its short lifetime, the tendency is also consistent with active hygroscopic processes of the aerosols over this area. Retrieved UV aerosol properties from SMART–s generally support the assumed smoke aerosol models (i.e., the spectral shape of aerosol absorption) used in NASA's current satellite algorithms, and their spectral ω0 retrievals from ground and satellites showed good agreements (R = 0.73–0.79). However, temporal and spectral variabilities in the aerosol absorption properties in the UV emphasize the importance of a realistic optical model of aerosols for further improvements in satellite retrievals.

Insights into the triplet photochemistry of atmospheric aerosol and subfractions isolated with different polarity

Atmospheric aerosols contain organic compounds that can form triplet organics (3C*) excited by sunlight, which plays a significant role in the aging process of aerosols. The formation characteristics of 3C* of aerosol components with different polarities were investigated, including highly polar water-soluble matter (HP-WSM), humic-like substances (HULIS), and methanol-soluble matter (MSM). Carbon analysis, probe method and electron paramagnetic resonance were used to investigate the photochemical properties of ambient samples. The coupling effect of generation of 3C* and reactive oxygen species (ROS) between different aerosol components was also examined. The results showed that the 3C* generation characteristics are strongly dependent on the polarity of these components. HULIS has the strongest generation ability of 3C*, and the MSM contributes the most to the total generation of 3C*. It was found that the high-energy triplet states (ET ≥ 250 kJ mol−1) of HULIS and HP-WSM components account for up to 80% of the water soluble components. These 3C* have an important contribution to the photochemically generation of ROS, and the generated ROS of different components are also different, which is determined by the chromophore composition of complex organic compounds. A tyrosine-like chromophore is the main substance leading to the formation of water-soluble 3C*, while the highly oxidized HULIS chromophore plays a leading role in the water-insoluble component. This study also found that there is a coupling effect between HP-WSM and HULIS on 3C* and ROS generation. The generation rate of 3C* increases by about 40% after mixing, but the generation of 1O2 is severely reduced. In general, this study provided deep insights into the generation characteristics and aging impact of the triplet state on atmospheric aerosols.

Planetary Boundary Layer Height Modulates Aerosol—Water Vapor Interactions During Winter in the Megacity of Delhi

AbstractThe Indo‐Gangetic Plain (IGP) is one of the dominant sources of air pollution worldwide. During winter, the variations in planetary boundary layer (PBL) height, driven by a strong radiative thermal inversion, affect the regional air pollution dispersion. To date, measurements of aerosol‐water vapor interactions, especially cloud condensation nuclei (CCN) activity, are limited in the Indian subcontinent, causing large uncertainties in radiative forcing estimates of aerosol‐cloud interactions. We present the results of a one‐month field campaign (February‐March 2018) in the megacity, Delhi, a significant polluter in the IGP. We measured the composition of fine particulate matter (PM1) and size‐resolved CCN properties over a wide range of water vapor supersaturations. The analysis includes PBL modeling, backward trajectories, receptor models and fire spots to elucidate the influence of PBL and air mass origins on aerosols. The aerosol properties depended strongly on PBL height and a simple power‐law fit could parameterize the observed correlations of PM1 mass, aerosol particle number and CCN number with PBL height, indicating PBL induced changes in aerosol accumulation. The low inorganic mass fractions, low aerosol hygroscopicity and high externally mixed weakly CCN‐active particles under low PBL height (100 m) indicated the influence of PBL on aerosol aging processes. In contrast, aerosol properties did not depend strongly on air mass origins or wind direction, implying that the observed aerosol and CCN are from local emissions. An error function could parameterize the relationship between CCN number and supersaturation throughout the campaign.

Organic aerosol formation and aging processes in Beijing constrained by size-resolved measurements of radiocarbon and stable isotopic 13C

This study investigates the sources and atmospheric processes of size-resolved carbonaceous aerosols in winter 2018 in urban Beijing, based on analysis of dual-carbon isotopes (i.e., radiocarbon and the stable isotope 13C). We found a size dependence of fossil source contributions to elemental carbon (EC), but no clear size dependence for organic carbon (OC). Comparable fossil source contributions to water-insoluble OC (WIOC; 55 ± 3%) and to water-soluble OC (WSOC; 54 ± 4%) highlight the importance of secondary aerosol formation, considering that fossil sources emit only small amounts of primary WSOC. OC concentrations increased during high PM2.5 pollution events, with increased fossil and non-fossil WSOC concentrating at larger particles (0.44–2.5 µm) than WIOC (0.25–2.5 µm), highlighting the aqueous-phase chemistry as an important pathway for OC production. The ratio of 13C/12C (expressed as δ13C) of total carbon (−27.0‰ to −23.3‰) fell in the range of anthropogenic aerosol, reflecting small biogenic influence. δ13C of OC increased with desorption temperature steps (200 °C, 350 °C and 650 °C). The strongly enriched δ13COC,650 (−26.9‰ to −20.3‰) and large mass fraction of OC650°C in total desorbed OC, both increasing with the increase of particle sizes, were caused by photochemical aging, especially during low and moderate PM2.5 pollution events, when regional, aged aerosol played an important role. During low pollution events, higher δ13COC,650 and WSOC/OC ratios reflect a larger contribution and more extensive chemical processing of aged aerosol. In contrast, relatively low δ13COC,200 (−27.2‰ to −25.7‰) suggests the influence of secondary OC formation on the more volatile OC desorbed at 200 °C. δ13COC,200 was similar for all particle sizes and for different pollution events, pointing to an internal mixture of local and aged regional OC. Our results show that the organic aerosol in Beijing arises from a mixture of various sources and complex formation processes, spanning local to regional scales. Particle sizes < 250 nm show strong contribution from local secondary OC formation, whereas refractory OC in particles around 1 µm shows strong evidence for regional aging processes. In summary, primary emission, secondary and aqueous-phase formation, and (photo-)chemical aging all need to be considered to understand organic aerosol in this region and their importance varies with particle size.

Real‐Time Characterization of Aerosol Compositions, Sources, and Aging Processes in Guangzhou During PRIDE‐GBA 2018 Campaign

AbstractTo investigate the chemical compositions, sources, and aging processes of submicron particles (PM1), a comprehensive field campaign was conducted in Guangzhou urban area of China during the autumn (October–November) of 2018. The average mass concentration of PM1 was 35.6 ± 20.8 µg m−3, which was mainly contributed by organic aerosols (OA, 42%), then followed by sulfate (25%) and inorganic nitrate (11%). The inorganic nitrate was found to be the main driving component (up to ∼50%) to account for the fast increase of PM1 during the polluted periods of Guangzhou autumn. The promotion effects of sulfate, aerosol liquid water content, and particles acidity on nitrate formation were systematically discussed. Source apportionment results showed 72% of OA in Guangzhou autumn was contributed by secondary OA (SOA), and 28% of primary OA (POA), including vehicle emission related hydrocarbon‐like OA (HOA, 16%), nitrogen‐containing OA (NOA, 3%) and cooking OA (COA, 8%). To explore the aging processes of OA, the dynamic variations of OA and its oxidation level as a function of ambient photochemical age are shown. Using an in situ field‐deployed oxidation flow reactor, the heterogeneous reaction rate coefficients of ambient POA with OH radicals () were estimated to be 4.0–5.4 × 10−13 cm3 molecules−1 s−1, which is equivalent to a lifetime of POA &gt;2 weeks. The long heterogeneous lifetime of POA supports gas phase oxidation was the major pathway for ambient OA aging. The OH uptake coefficient () was estimated to be 0.76–0.84, underlining that OH radicals can be taken up efficiently on ambient aerosols.

Unraveling interfacial properties of organic-coated marine aerosol with lipase incorporation

Marine aerosols are believed to have an organic surface coating on which fatty acids act as an important component due to their high surface activity. In addition, various kinds of enzyme species are abundantly found in seawater, some of which have been identified to exist in marine aerosols. Herein, from the perspective of marine aerosol interface simulation, we investigate the effect of Burkholderia cepacia lipase on the surface properties of stearic acid (SA) monolayer at the air-water interface by using surface-sensitive techniques of Langmuir trough and Infrared reflection-absorption spectroscopy (IRRAS). Our findings indicate that the stearic acid film undergoes a significant expansion, especially when the lipase concentration is 500 nM, because of the incorporation of lipase as observed from the surface pressure-area (π-A) isotherms. IRRAS spectra also show reduced intensities and ordering in the methylene stretching vibration region of stearic acid as a result of low surface density and disordered packing as the enzyme concentration increases. In particular, when the concentration of lipase is 500 nM, the lowest Ias/Is values are shown on both pure water subphase and artificial seawater subphase, indicating more gauche conformations for SA. Furthermore, SA films with lipase incorporation were also studied at three different pH of subphase environment, considering the decrease of pH caused by the reaction with acidic gases during the aerosol aging process. The results reflect a more pronounced expansion of SA monolayer in acidic environment at pH 2.5, suggesting that hydrophobic interaction plays an important role in the disorder of the SA monolayer. In view of the coexistence of fatty acids and enzymes in the marine environment, this study provides a further understanding of the surface organization and behavior of organic-coated marine aerosols and deepen the knowledge of lipid-enzyme interfacial interactions occurring in the atmosphere.

Sulfur isotope composition during heterogeneous oxidation of SO2 on mineral dust: The effect of temperature, relative humidity, and light intensity

Sulfate as the major constituent of atmospheric aerosol plays a key role in aerosol formation and aging processes. However, little attention has been paid to the impacts of mineral dust on the oxidation of SO2, along with the effects of temperature, moisture, and light intensity. In this study, the influences of these parameters on the formation of sulfate and sulfur isotope composition were investigated in detail. Temperature promoted the heterogeneous oxidation of SO2 on the surface of hematite (α-Fe2O3), while excessive temperature was unfavorable for sulfate formation. Besides, the effect of temperature on sulfur isotope fractionation was not apparent. In addition, relative humidity (RH, < 40%) boosted the oxidation of SO2 with the concurrent enrichment of heavy isotope. Meanwhile, light with high intensity inhibited the formation of sulfate due to the photo-induced reductive dissolution of hematite, and had little effects on the sulfur isotope compositions. Furthermore, compared with other minerals, α-Fe2O3 exhibited greater uptake capacity of SO2, favoring the enrichment of light sulfur isotopes, whereas the enrichment of heavy sulfur isotopes existed on the surface of MgO. This study contributes to the investigation of the heterogeneous oxidation of SO2 and provides useful parameters for atmospheric modeling studies.

Insights into the aging of biomass burning aerosol from satellite observations and 3D atmospheric modeling: evolution of the aerosol optical properties in Siberian wildfire plumes

Abstract. Long-range transport of biomass burning (BB) aerosol from regions affected by wildfires is known to have a significant impact on the radiative balance and air quality in receptor regions. However, the changes that occur in the optical properties of BB aerosol during long-range transport events are insufficiently understood, limiting the adequacy of representations of the aerosol processes in chemistry transport and climate models. Here we introduce a framework to infer and interpret changes in the optical properties of BB aerosol from satellite observations of multiple BB plumes. Our framework includes (1) a procedure for analysis of available satellite retrievals of the absorption and extinction aerosol optical depths (AAOD and AOD) and single-scattering albedo (SSA) as a function of the BB aerosol photochemical age and (2) a representation of the AAOD and AOD evolution with a chemistry transport model (CTM) involving a simplified volatility basis set (VBS) scheme with a few adjustable parameters. We apply this framework to analyze a large-scale outflow of BB smoke plumes from Siberia toward Europe that occurred in July 2016. We use AAOD and SSA data derived from OMI (Ozone Monitoring Instrument) satellite measurements in the near-UV range along with 550 nm AOD and carbon monoxide (CO) columns retrieved from MODIS (Moderate Resolution Imaging Spectroradiometer) and IASI (Infrared Atmospheric Sounding Interferometer) satellite observations, respectively, to infer changes in the optical properties of Siberian BB aerosol due to its atmospheric aging and to get insights into the processes underlying these changes. Using the satellite data in combination with simulated data from the CHIMERE CTM, we evaluate the enhancement ratios (EnRs) that allow isolating AAOD and AOD changes due to oxidation and gas–particle partitioning processes from those due to other processes, including transport, deposition, and wet scavenging. The behavior of EnRs for AAOD and AOD is then characterized using nonlinear trend analysis. It is found that the EnR for AOD strongly increases (by about a factor of 2) during the first 20–30 h of the analyzed evolution period, whereas the EnR for AAOD does not exhibit a statistically significant increase during this period. The increase in AOD is accompanied by a statistically significant enhancement of SSA. Further BB aerosol aging (up to several days) is associated with a strong decrease in EnRs for both AAOD and AOD. Our VBS simulations constrained by the observations are found to be more consistent with satellite observations of strongly aged BB plumes than “tracer” simulations in which atmospheric transformations of BB organic aerosol were disregarded. The simulation results indicate that the upward trends in EnR for AOD and in SSA are mainly due to atmospheric processing of secondary organic aerosol (SOA), leading to an increase in the mass scattering efficiency of BB aerosol. Evaporation and chemical fragmentation of the SOA species, part of which is assumed to be absorptive (to contain brown carbon), are identified as likely reasons for the subsequent decrease in the EnR for both AAOD and AOD. Hence, our analysis reveals that the long-range transport of smoke plumes from Siberian fires is associated with major changes in BB aerosol optical properties and chemical composition. Overall, this study demonstrates the feasibility of using available satellite observations for evaluating and improving representations in atmospheric models of the BB aerosol aging processes in different regions of the world at much larger temporal scales than those typically addressed in aerosol chamber experiments.

Photochemical degradation of iron(III) citrate/citric acid aerosol quantified with the combination of three complementary experimental techniques and a kinetic process model

Abstract. Iron(III) carboxylate photochemistry plays an important role in aerosol aging, especially in the lower troposphere. These complexes can absorb light over a broad wavelength range, inducing the reduction of iron(III) and the oxidation of carboxylate ligands. In the presence of O2, the ensuing radical chemistry leads to further decarboxylation, and the production of .OH, HO2., peroxides, and oxygenated volatile organic compounds, contributing to particle mass loss. The .OH, HO2., and peroxides in turn reoxidize iron(II) back to iron(III), closing a photocatalytic cycle. This cycle is repeated, resulting in continual mass loss due to the release of CO2 and other volatile compounds. In a cold and/or dry atmosphere, organic aerosol particles tend to attain highly viscous states. While the impact of reduced mobility of aerosol constituents on dark chemical reactions has received substantial attention, studies on the effect of high viscosity on photochemical processes are scarce. Here, we choose iron(III) citrate (FeIII(Cit)) as a model light-absorbing iron carboxylate complex that induces citric acid (CA) degradation to investigate how transport limitations influence photochemical processes. Three complementary experimental approaches were used to investigate kinetic transport limitations. The mass loss of single, levitated particles was measured with an electrodynamic balance, the oxidation state of deposited particles was measured with X-ray spectromicroscopy, and HO2. radical production and release into the gas phase was observed in coated-wall flow-tube experiments. We observed significant photochemical degradation with up to 80 % mass loss within 24 h of light exposure. Interestingly, we also observed that mass loss always accelerated during irradiation, resulting in an increase of the mass loss rate by about a factor of 10. When we increased relative humidity (RH), the observed particle mass loss rate also increased. This is consistent with strong kinetic transport limitations for highly viscous particles. To quantitatively compare these experiments and determine important physical and chemical parameters, a numerical multilayered photochemical reaction and diffusion (PRAD) model was developed that treats chemical reactions and the transport of various species. The PRAD model was tuned to simultaneously reproduce all experimental results as closely as possible and captured the essential chemistry and transport during irradiation. In particular, the photolysis rate of FeIII, the reoxidation rate of FeII, HO2. production, and the diffusivity of O2 in aqueous FeIII(Cit) ∕ CA system as function of RH and FeIII(Cit) ∕ CA molar ratio could be constrained. This led to satisfactory agreement within model uncertainty for most but not all experiments performed. Photochemical degradation under atmospheric conditions predicted by the PRAD model shows that release of CO2 and repartitioning of organic compounds to the gas phase may be very important when attempting to accurately predict organic aerosol aging processes.

Measurement of Density and Shape for Single Black Carbon Aerosols in a Heavily Polluted Urban Area

Black carbon (BC) aerosol imposes adverse effects on atmospheric visibility, climate, and health. The particle density and morphology are often needed to investigate the mixing state and aging process of BC particles. A method, combining an aerodynamic aerosol classifier (AAC), a differential mobility analyzer (DMA), a single-particle soot photometer (SP2) and a single particle aerosol mass spectrometer (SPAMS), was developed to determine the density and dynamic shape factor (χ) of ambient BC particles with three different aerodynamic diameters (Da, 200 nm, 350 nm, and 500 nm) in Shanghai, China, a typical urban area. The BC particles were either classified as “BC-dominated particle” which is mainly made of black carbon or “BC-mixed particle” which is a mixture of both BC and non-BC substances. The results show that BC-dominated particles whose BC core mass (~2.2 fg) was almost equal to particle mass (~2.3 fg) were observed in particles with 200 nm Da. The morphology of these BC-dominated particles was near-spherical (χ ≈ 1.02), indicating that they had undergone rapid morphology modification from the initial highly irregular morphology to near-spherical shape. Most BC particles with 350 nm or 500 nm Da were BC-mixed particles. Combining the effective densities (1.62~1.77 g cm-3) and average single particle mass spectra of particle, the ammonium sulfate and ammonium nitrate were found to be the main secondary substances of these BC-mixed particles, indicating that condensation of inorganic species such as nitrates and sulfates could play a significant role in the aging process of fresh BC in Shanghai. Generally, the morphology and density information of single BC particle is crucial to identify the mixing state and aging process of BC aerosols.

Chemical composition, water content and size distribution of aerosols during different development stages of regional haze episodes over the North China Plain

Aerosol size distribution and chemical composition are found to have significant effects on its hygroscopicity, acidity/alkalinity and light extinction. Heavy haze pollution occurred frequently in the wintertime of NCP, with long duration time and large impact area, which had important influences on air quality and human health. However, the study concerning the formation mechanism and physicochemical characteristics of aerosols in different haze stages have been rarely carried out. In this study, aerosol size distribution in the range of 10 nm - 10 μm, water soluble inorganic ions (WSIIs), PM (PM2.5, PM10), trace gases, organic carbon (OC), elemental carbon (EC) and meteorological elements were derived during a regional haze pollution episode from Nov. 9 to 16, 2018. The aerosol water content and pH were further calculated using the ISORROPIA model. We divided the whole observation period into segments of clean days, fog processes, and haze processes, including three stages of I: accumulation, II: growth, and III: explosion, and a dry haze (D-haze) process based on the PM2.5 concentration, visibility and RH. Given the shift of particle size to larger segment ascribed to the ageing process during fog/haze process, the size distribution peak in aerosol number concentration was located at 100 nm in the fog/haze episode and was 70 nm larger than that on clean days. The concentration descended significantly in stage III, peaking at 160 nm, suggesting a strong aging process of aerosols. The PM2.5 increased sharply under high RH, static weather conditions and strengthening oxidation that favored liquid and heterogeneous reactions. The nitrate concentration was found to account for 20.5% (D-haze) - 29.0% (fog) of the total water soluble inorganic ions (WSIIs) during the fog/haze process, while sulfate constituted only 13.8% (stage III) - 21.3% (stage I), revealing dominant nitrate pollution. Hence, nitrate was considered to originate mainly from photochemical and heterogeneous reactions in haze episodes and was generated only by heterogeneous reactions. Higher (lower) aerosol water content made aerosols more acidic (alkaline) under similar chemical compositions. Therefore, the chemical reactions may differ under dry and wet haze pollution. Sources of carbonaceous aerosols changed in different haze stages with descending (enhancing) contributions from coal combustion (vehicle exhaust).

Seasonal variation of dicarboxylic acids in PM2.5 in Beijing: Implications for the formation and aging processes of secondary organic aerosols

Dicarboxylic acids are a group of highly oxidized components, which can provide insights into the formation mechanism and aging process of secondary organic aerosols (SOA). Based on the 12-h day and night PM2.5 samples collected in downtown Beijing in January, April, July and October of 2017, dicarboxylic acids and relevant components were measured to investigate their seasonal variation pattern and sources. High concentrations of the identified organic acids were observed, following the decreasing order of July > January > October > April. The high fractions of phthalic acid and maleic acid in January indicated severe aromatic SOA pollution during the sampling period in winter, and the high malonic acid to succinic acid and malic acid to succinic acid ratios in July suggested strong photochemical formation over the sampling period in summer. Based on the calculation of principle component analysis and multiple linear regression, water-soluble organic acids were mainly formed from the aerosol aging process during the sampling periods except for January, while water-soluble organic carbon (WSOC) mostly originated from combustion sources. Correlation analysis was conducted between the CO-normalized concentrations of organic acids and PM2.5, O3, as well as the meteorological parameters. The results suggested that gas-phase photooxidation contributed significantly to the formation of these organic acids during the entire sampling period, and the aqueous-phase process played an important role over the severe haze event in January. Our results also suggested that the intensity of photooxidation and the aging degree of SOA were enhanced along with the reduction of PM2.5 in Beijing in recent years.

Source apportionment of absorption enhancement of black carbon in different environments of China

Black carbon (BC) is an important pollutant for both air quality and earth's radiation balance because of its strong absorption enhancement. The enhanced light absorption of BC caused by other pollutants is one of the most important sources of uncertainty in global radiative forcing. The light absorption of BC is highly dependent on the emission source and very few studies have been carried out for the source apportionment of BC absorption enhancement. Thus, with this objective, continuous measurements of particulate matter (PM2.5) were performed at three different sites: a traffic site in Nanjing, an urban site in Jinan, and a rural site in Yucheng; the BC absorption enhancement and its source contributions were determined. The mass absorption cross-section (MAC) of BC aerosols was reduced after the removal of the coating material. The maximum MAC enhancement (EMAC) was found to be 2.25 ± 0.5 at the rural site, followed by 2.07 ± 0.7 at the urban site and 1.7 ± 0.6 at the traffic site, suggesting an approximately double enhancement in BC absorption due to different coating materials. The source apportionment of absorption enhancement of BC analysis using the positive matrix factorization model suggests five major emission sources. Among them, secondary sources were the main source of EMAC at all the three sites with a percentage contribution of 43.4% (rural site), 34.6% (traffic site), and 31% (urban site). However, other emission sources, such as biomass burning (21.1% at rural site) and vehicular emissions (33.8% at traffic site) also had a significant contribution to EMAC, suggesting that there could be large variations in BC absorption enhancement due to differences in emission sources together with aerosol aging processes.

Technical note: Mismeasurement of the core-shell structure of black carbon-containing ambient aerosols by SP2 measurements

The core diameter and shell thickness of black carbon (BC)-containing ambient aerosols directly determine their corresponding optical properties and aging processing, influencing significantly on the environment and climate change. The information of ambient particle core-shell structure is usually obtained using the single particle soot photometer (SP2) measurements. In this study, we found that the particle sizes measured by SP2 were limited within the core diameter larger than 84 nm and particle scattering diameter larger than 178 nm. There were about 34.9% of the ambient BC-containing aerosols that are mistaken for pure BC aerosols without coating. The lower limits of SP2 measurement lead to overestimation of the pure BC aerosol number concentrations and missing information of the shell thickness of BC-containing aerosol. Thus the statistical shell thickness of ambient BC-containing particles was underestimated and the aging process of ambient BC-containing aerosols derived from the SP2 measurements is misleading.

Variations in &amp;lt;i&amp;gt;N&amp;lt;/i&amp;gt;&amp;lt;sub&amp;gt;cn&amp;lt;/sub&amp;gt; and &amp;lt;i&amp;gt;N&amp;lt;/i&amp;gt;&amp;lt;sub&amp;gt;ccn&amp;lt;/sub&amp;gt; over marginal seas in China related to marine traffic emissions, new particle formation and aerosol aging

Abstract. In this study, a cruise campaign was conducted over marginal seas in China to measure the concentrations of condensation nuclei (Ncn), cloud condensation nuclei (Nccn) and other pollutants from day of year (DOY) 110 to DOY 135 of 2018. The ship self-emission signals were exhaustively excluded, and the mean values of Nccn during the cruise campaign were found to slightly increase from 3.2±1.1×103 cm−3 (mean ± standard deviation) at supersaturation (SS) of 0.2 % to 3.9±1.4×103 cm−3 at SS of 1.0 %, and the mean value for Ncn was 8.1±4.4×103 cm−3. Data analysis showed that marine traffic emissions apparently largely contributed to the increase in Ncn in the daytime, especially in the marine atmospheres over heavily traveled sea zones; however, the fresh sources made no clear contribution to the increase in Nccn. This finding was supported by the quantitative relations between Ncn and Nccn at SS = 0.2 %–1.0 % against the mixing ratios of SO2 in the ship self-emission plumes – i.e., a 1 ppb increase in SO2 corresponded to a 1.4×104 cm−3 increase in Ncn but only a 30–170 cm−3 increase in Nccn, possibly because of abundant organics in the aerosols. Smooth growth can be observed in the marine-traffic-derived particles, reflecting aerosol aging. The estimated hygroscopicity parameter (κ) values were generally as high as 0.46–0.55 under the dominant onshore winds, suggesting that inorganic ammonium aerosols likely acted as the major contributor to Nccn largely through aerosol aging processes of decomposing organics. Moreover, the influences of the new transported particles from the continent on the Ncn and Nccn in the marine atmosphere were investigated.