External Mixing State Research Articles

Backscattering Linear Depolarization Ratio of Smoke Aerosols From Biomass Burning

AbstractThe linear depolarization ratio (LDR) of backscattering is a key parameter for distinguishing particle types. The measured LDRs of smoke in previous studies are highly variable and different from each other. Existing research on smoke aerosols considers only the internal mixing state of the black carbon (BC) particle population either by ignoring the externally mixed organic carbon (OC) particle population or by evaluating the LDR of smoke aerosols using the results of individual particles rather than the particle population. The field‐measured LDR of smoke then becomes difficult to interpret properly. The recent prescribed forest burning experiment in China showed that the LDR of freshly emitted smoke varies between 0.0% and 20.1% in wavelength (λ) of 532 nm. Electron microscopy images also showed that coated BC and pure OC exist simultaneously in biomass burning aerosols. Therefore, this study evaluates the influence of various parameters on the LDR of smoke by considering the internal and external mixing states. The calculated LDRs of the smoke aerosols were found to vary between 0.0% and 28.2% when λ is equal to 532 nm. The calculation results indicate that the LDR of smoke is slightly influenced by BC, considerably affected by the externally mixed OC, and is essentially dominated by the latter's morphology and particle size distribution. High levels and rapid changes of LDR of smoke can be well explained by nonsphericity and particle size distribution of externally mixed OC. This study advances the research on the measurement and evaluation of the LDR of smoke aerosols.

Properties and Processing of Aviation Exhaust Aerosol at Cruise Altitude Observed from the IAGOS-CARIBIC Flying Laboratory.

The characteristics of aviation-induced aerosol, its processing, and effects on cirrus clouds and climate are still associated with large uncertainties. Properties of aviation-induced aerosol, however, are crucially needed for the assessment of aviation's climate impacts today and in the future. We identified more than 1100 aircraft plume encounters during passenger aircraft flights of the IAGOS-CARIBIC Flying Laboratory from July 2018 to March 2020. The aerosol properties inside aircraft plumes were similar, independent of the altitude (i.e., upper troposphere, tropopause region, and lowermost stratosphere). The exhaust aerosol was found to be mostly externally mixed compared to the internally mixed background aerosol, even at a plume age of 1 to 3 h. No enhancement of accumulation mode particles (diameter >250 nm) could be detected inside the aircraft plumes. Particle number emission indices (EIs) deduced from the observations in aged plumes are in the same range as values reported from engine certifications. This finding, together with the observed external mixing state inside the plumes, indicates that the aviation exhaust aerosol almost remains in its emission state during plume expansion. It also reveals that the particle number EIs used in global models are within the range of the EIs measured in aged plumes.

Impact of aerosol optics on vertical distribution of ozone in autumn over Yangtze River Delta

Abstract. Tropospheric ozone, an important secondary pollutant, is greatly impacted by aerosols within boundary layer (BL). Previous studies have mainly attributed ozone variation to either aerosol–BL or aerosol–photolysis interactions at the near-surface level. In this study, we analyze the sensitivities of ozone response to aerosol mixing states (e.g., mixing behavior hypothesis of scattering and absorbing components) in the vertical direction and address the effects of aerosol–BL and aerosol–photolysis interactions on ozone profiles in autumn by Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) simulations. The aerosol internal mixing state experiment reasonably reproduces the vertical distribution and time variation in meteorological elements and ozone. Sensitivity experiments show that aerosols lead to turbulent suppression, precursor accumulation, lower-level photolysis reduction, and upper-level photolysis enhancement. Consequently, ozone basically decreases within entire the BL during daytime (08:00–17:00 LT), and the decrease is the least in the external mixing state (2.0 %) when compared with internal (10.5 %) and core shell mixing states (8.6 %). The photolysis enhancement is the most significant in the external mixing state due to its strong scattering ability. By process analysis, lower-level ozone chemical loss is enhanced due to photolysis reduction and NOx accumulation under a volatile organic compound (VOC)-limited regime. Upper-level ozone chemical production is accelerated due to a higher photolysis rate resulting from aerosol backscattering. Therefore, the increased ozone entrainment from BL aloft to the surface induced by the boosted ozone vertical gradient outweighs the decreased ozone entrainment induced by turbulent suppression after 11:00 LT. Additional simulations support the finding that the aerosol effect on precursors, photolysis, and ozone is consistent under different underlying surface and pollution conditions.

Impact of water uptake and mixing state on submicron particle deposition in the human respiratory tract (HRT) based on explicit hygroscopicity measurements at HRT-like conditions

Abstract. Particle hygroscopicity plays a key role in determining the particle deposition in the human respiratory tract (HRT). In this study, the effects of hygroscopicity and mixing state on regional and total deposition doses on the basis of the particle number concentration for children, adults, and the elderly were quantified using the Multiple-Path Particle Dosimetry model, based on the size-resolved particle hygroscopicity measurements at HRT-like conditions (relative humidity = 98 %) performed in the North China Plain. The measured particle population with an external mixing state was dominated by hygroscopic particles (number fraction = (91.5 ± 5.7) %, mean ± standard deviation (SD); the same below). Particle hygroscopic growth in the HRT led to a reduction by around 24 % in the total doses of submicron particles for all age groups. Such a reduction was mainly caused by the growth of hygroscopic particles and was more pronounced in the pulmonary and tracheobronchial regions. Regardless of hygroscopicity, the elderly group of people had the highest total dose among three age groups, while children received the maximum total deposition rate. With 270 nm in diameter as the boundary, the total deposition doses of particles smaller than this diameter were overestimated, and those of larger particles were underestimated, assuming no particle hygroscopic growth in the HRT. From the perspective of the daily variation, the deposition rates of hygroscopic particles with an average of (2.88 ± 0.81) × 109 particles h−1 during the daytime were larger than those at night ((2.32 ± 0.24) × 109 particles h−1). On the contrary, hydrophobic particles interpreted as freshly emitted soot and primary organic aerosols exhibited higher deposition rates at nighttime ((3.39 ± 1.34) × 108 particles h−1) than those in the day ((2.58 ± 0.76) × 108 particles h−1). The traffic emissions during the rush hours enhanced the deposition rate of hydrophobic particles. This work provides a more explicit assessment of the impact of hygroscopicity and mixing state on the deposition pattern of submicron particles in the HRT.

Mixing state of printer generated ultrafine particles: Implications for the complexity of indoor aerosols

The operation of laser printers can lead to the emission of high numbers of ultrafine particles. Evidence on the toxicology and adverse health effects of these particles has been mounting, however few studies have investigated the complexity of these particles in terms of their volatility, hygroscopicity and mixing state. This study utilized a Volatility Hygroscopic Tandem Differential Mobility Analyzer (VH-TDMA) to explore the internal and external mixing states of printer-generated particles. Up to 6.0 × 105 particles. cm−3 were observed during the operation of the laser printer, and these ultrafine particles could be classified into three groups, each with its own particular volatility (i.e., gradually shrinkable, suddenly shrinkable and expandable), owing to the different internal mixing states. In particular, we propose shell-core structures to explain the special volatility of these particles. In addition, whilst the majority of the generated particles were initially hydrophobic at 90% relative humidity (RH), it was observed that there were a very small number of volatilized particles (less than 5%) that shrank (i.e., decreased in size) after humidification. This study can be used as a model for investigating the complex particle formation processes in relation to secondary organic aerosols from other sources. Furthermore, our results shed light on the complexity of indoor aerosols, which should be investigated further in future indoor air quality studies.

Influence of aerosol hygroscopicity and mixing state on aerosol optical properties in the Pearl River Delta region, China

Both the effects of aerosol hygroscopicity and mixing state on aerosol optical properties were analyzed using ground-based measurements and a Mie model in this study. The sized-resolved particle hygroscopic growth factor at RH = 90% (Gf(90%)) and the enhancement factor for the scattering coefficients (f(RH)sp) were measured by a self-constructed Hygroscopic Tandem Differential Mobility Analyzer (H-TDMA) and two nephelometers in parallel (PNEPs) respectively from 22nd February to 18th March 2014 in the Pearl River Delta, China. In addition, the particle number size distribution (PNSD) and BC mass concentration (MBC) were measured simultaneously.During the observation period, the f(RH)sp increased sharply along with increasing RH (40%–85%) and the value of f(80%)sp was 1.77 ± 0.18. The mean Gf(90%) for all particles are 1.44 (80 nm), 1.48 (110 nm), 1.52 (150 nm) and 1.55 (200 nm), and the mean Gf(90%) for more-hygroscopic particles are 1.58 (80 nm), 1.63 (110 nm), 1.66 (150 nm) and 1.67 (200 nm) respectively. Based on Gf, PNSD and MBC, the enhancement factor of the aerosol optical properties (extinction (f(RH)ep), scattering (f(RH)sp), backscattering (f(RH)hbsp), absorption (f(RH)absp), and hemispheric backscatter fraction (f(RH)hbsp)) were calculated under three aerosol mixing state assumptions. The results show that the calculated f(80%)sp values agreed well with the ones measured by PNEPs, illustrating that the Gf size distribution fittings are reasonable. The f(RH)ep, f(RH)sp and f(RH)hbsp increased along with increasing RH for three mixtures, while f(RH)HBF decreased. The f(RH)absp increased for the homogenously internal mixture, but remained stable for the external mixture. For the core-shell mixture, the f(RH)absp increased from RH = 0 to 75% and then decreased, due to a decrease of light entering the BC core. The enhancement factor of aerosol direct radiative forcing (f(RH)Fr) increased sharply as the RH elevated for the external mixing state. However, f(RH)Fr increased or decreased along with the elevated RH for the homogenously internal mixture and the core-shell mixture depending on initial value of the aerosol direct radiative forcing (∆Fr) in a dry condition.

Size-resolved chemical composition, effective density, and optical properties of biomass burning particles

Abstract. Biomass burning aerosol has an important impact on the global radiative budget. A better understanding of the correlations between the mixing states of biomass burning particles and their optical properties is the goal of a number of current studies. In this work, the effective density, chemical composition, and optical properties of rice straw burning particles in the size range of 50–400 nm were measured using a suite of online methods. We found that the major components of particles produced by burning rice straw included black carbon (BC), organic carbon (OC), and potassium salts, but the mixing states of particles were strongly size dependent. Particles of 50 nm had the smallest effective density (1.16 g cm−3) due to a relatively large proportion of aggregate BC. The average effective densities of 100–400 nm particles ranged from 1.35 to 1.51 g cm−3 with OC and inorganic salts as dominant components. Both density distribution and single-particle mass spectrometry showed more complex mixing states in larger particles. Upon heating, the separation of the effective density distribution modes confirmed the external mixing state of less-volatile BC or soot and potassium salts. The size-resolved optical properties of biomass burning particles were investigated at two wavelengths (λ = 450 and 530 nm). The single-scattering albedo (SSA) showed the lowest value for 50 nm particles (0.741 ± 0.007 and 0.889 ± 0.006) because of the larger proportion of BC content. Brown carbon played an important role for the SSA of 100–400 nm particles. The Ångström absorption exponent (AAE) values for all particles were above 1.6, indicating the significant presence of brown carbon in all sizes. Concurrent measurements in our work provide a basis for discussing the physicochemical properties of biomass burning aerosol and its effects on the global climate and atmospheric environment.

Deriving aerosol hygroscopic mixing state from size-resolved CCN activity and HR-ToF-AMS measurements

The ability of a particle to uptake water and form a cloud droplet depends on its hygroscopicity. To understand its impact on cloud properties and ultimately radiative forcing, knowledge of chemically-resolved mixing state information or the one based on hygroscopic growth is crucial. Typically, global models assume either pure internal or external mixing state which might not be true for all conditions and sampling locations. To investigate into this, the current study employed an indirect approach to infer the probable mixing state. The hygroscopic parameters derived from κ-Kohler theory using size-resolved CCN measurements (κCCN) and bulk/size-resolved aerosol mass spectrometer (AMS) measurements (κAMS) were compared. The accumulation mode particles were found to be more hygroscopic (κCCN = 0.24) than Aitken mode (κCCN = 0.13), perhaps due to increased ratio of inorganic to organic mass fraction. The activation diameter calculated from size-resolved CCN activity measurements at 5 different supersaturation (SS) levels varied in the range of 115 nm–42 nm with κCCN = 0.13–0.23 (avg = 0.18 ± 0.10 (±1σ)). Further, κAMS>κCCN was observed possibly due to the fact that organic and inorganic mass present in the Aitken mode was not correctly represented by bulk chemical composition and size-resolved fractional contribution of oxidized OA was not accurately accounted. Better correlation of organic fraction (forg) and κCCN at lower SS explained this behaviour. The decrease in κCCN with the time of the day was more pronounced at lower SS because of the relative mass reduction of soluble inorganic species by ∼17%.Despite the large differences between κ measured from two approaches, less over-prediction (up to 18%) between measured and predicted CCN concentration suggested lower impact of chemical composition and mixing state at higher SS. However, at lower SS, presences of externally mixed CCN-inactive aerosols lead to CCN over-prediction reflecting the significance of aerosol mixing state information. Further examination of the effect of biomass burning aerosols (∼35% in least oxidized biomass burning organic aerosol (BBOA-2 fraction) on hygroscopicity and CCN activity showed increase in the concentration of all AMS measured species (except NH4+ and SO42−), less O:C ratio, and organic mass fraction (forg) peak shift to lower diameter range caused ∼13% change in critical diameter (Da) and ∼40% change in κCCN. Increased deviation of ∼100% between κCCN and κAMS due to sudden influx of internally mixed BBOA caused suppressed hygroscopic growth. This study finally suggests the assumption of pure internally mixed aerosol does not completely hold true for this anthropogenically polluted site.

Hygroscopic Properties of Atmospheric Aerosol Measured with an HTDMA in an Urban Background Site in Madrid

The observation of high aerosol hygroscopic growth in Madrid is mainly limited to specific atmospheric conditions, such as local stagnation episodes, which take place in winter time. One of these episodes was identified in December 2014 and the hygroscopic growth factor (GF) measurements obtained in such episode were analysed in order to know the influence of the meteorological conditions on aerosol hygroscopic properties.The prevailing high atmospheric stability triggered an increase of the particle total concentration during the study period, with several peaks that exceeded 4.0 104 particles cm-3, as well as an increase in the inorganic fraction of the aerosol, the NO3− concentration, which in this case corresponded to 25% of the total PM1 non-refractory composition.The aerosol hygroscopic growth distribution was bimodal during the episode, with an average GF around 1.2 for the five dry particle sizes measured and an average GF spread ≥ 0.15. In addition, it is important to note that when a reduction in the concentrations of NO3− is observed, it coincides with a decrease of the GF and its spread. These data suggest, on the one hand, a high degree of external mixing state of the aerosol during the episode and, on the other hand, a notable association between the GF and the inorganic fraction of the aerosol.

Long‐range transport across the Atlantic in summertime does not enhance the hygroscopicity of African mineral dust

AbstractWe present the first direct evidence that the hygroscopic properties of super micron (>1 µm) African dust particles did not change despite undergoing long‐range transport across the Atlantic toward the Caribbean. Concurrent measurements of chemical composition show that most of mineral dust was chemically unprocessed and externally mixed. A minor portion of mineral dust was internally mixed with sulfate and chloride (~13–24% by number) or aggregated with sea‐salt particles (~3–6%). Only dust particles aggregated with sea salt showed significant hygroscopic growth above 75% relative humidity (RH), resulting in a decrease in extinction mass efficiency by up to a factor 2.2. All other dust particles did not take up significant amounts of water when exposed to up to 94% RH. These results demonstrate that the direct radiative effect of African dust in this region remained independent of RH and an external mixing state could be considered for evaluating the climate effects of dust.

Direct aerosol chemical composition measurements to evaluate the physicochemical differences between controlled sea spray aerosol generation schemes

Abstract. Controlled laboratory studies of the physical and chemical properties of sea spray aerosol (SSA) must be under-pinned by a physically and chemically accurate representation of the bubble-mediated production of nascent SSA particles. Bubble bursting is sensitive to the physico-chemical properties of seawater. For a sample of seawater, any important differences in the SSA production mechanism are projected into the composition of the aerosol particles produced. Using direct chemical measurements of SSA at the single-particle level, this study presents an intercomparison of three laboratory-based, bubble-mediated SSA production schemes: gas forced through submerged sintered glass filters ("frits"), a pulsed plunging-waterfall apparatus, and breaking waves in a wave channel filled with natural seawater. The size-resolved chemical composition of SSA particles produced by breaking waves is more similar to particles produced by the plunging waterfall than those produced by sintered glass filters. Aerosol generated by disintegrating foam produced by sintered glass filters contained a larger fraction of organic-enriched particles and a different size-resolved elemental composition, especially in the 0.8–2 μm dry diameter range. Interestingly, chemical differences between the methods only emerged when the particles were chemically analyzed at the single-particle level as a function of size; averaging the elemental composition of all particles across all sizes masked the differences between the SSA samples. When dried, SSA generated by the sintered glass filters had the highest fraction of particles with spherical morphology compared to the more cubic structure expected for pure NaCl particles produced when the particle contains relatively little organic carbon. In addition to an intercomparison of three SSA production methods, the role of the episodic or "pulsed" nature of the waterfall method on SSA composition was under-taken. In organic-enriched seawater, the continuous operation of the plunging waterfall resulted in the accumulation of surface foam and an over-expression of organic matter in SSA particles compared to those produced by a pulsed plunging waterfall. Throughout this set of experiments, comparative differences in the SSA number size distribution were coincident with differences in aerosol particle composition, indicating that the production mechanism of SSA exerts important controls on both the physical and chemical properties of the resulting aerosol with respect to both the internal and external mixing state of particles. This study provides insight into the inextricable physicochemical differences between each of the bubble-mediated SSA generation mechanisms tested and the aerosol particles that they produce, and also serves as a guideline for future laboratory studies of SSA particles.

Mixing State of Size-Selected Submicrometer Particles in the Arctic in May and September 2012

Aerosols have been associated with large uncertainties in estimates of the radiation budget and cloud formation processes in the Arctic. This paper reports the results of a study of in situ measurements of hygroscopicity, fraction of volatile species, mixing state, and off-line morphological and elemental analysis of Aitken and accumulation mode particles in the Arctic (Ny-Ålesund, Svalbard) in May and September 2012. The accumulation mode particles were more abundant in May than in September. This difference was due to more air mass flow from lower latitude continental areas, weaker vertical mixing, and less wet scavenging in May than in September, which may have led to a higher amount of long-range transport aerosols entering the Arctic in the spring. The Aitken mode particles observed intermittently in May were produced by nucleation, absent significant external mixing, whereas the accumulation mode particles displayed significant external mixing. The occurrence of an external mixing state was observed more often in May than in September and more often in accumulation mode particles than in Aitken mode particles, and it was associated more with continental air masses (Siberian) than with other air masses. The external mixing of the accumulation mode particles in May may have been caused by multiple sources (i.e., long-range transport aerosols with aging and marine aerosols). These groups of externally mixed particles were subdivided into different mixing structures (internal mixtures of predominantly sulfates and volatile organics without nonvolatile species and internal mixtures of sulfates and nonvolatile components, such as sea salts, minerals, and soot). The variations in the mixing states and chemical species of the Arctic aerosols in terms of their sizes, air masses, and seasons suggest that the continuous size-dependent measurements observed in this study are useful for obtaining better estimates of the effects of these aerosols on climate change.

Volatility and mixing states of ultrafine particles from biomass burning

Fine and ultrafine carbonaceous aerosols produced from burning biomasses hold enormous importance in terms of assessing radiation balance and public health hazards. As such, volatility and mixing states of size-selected ultrafine particles (UFP) emitted from rice straw, oak, and pine burning were investigated by using volatility tandem differential mobility analyzer (VTDMA) technique in this study. Rice straw combustion produced unimodal size distributions of emitted aerosols, while bimodal size distributions from combustions of oak (hardwood) and pine (softwood) were obtained. A nearness of flue gas temperatures and a lower CO ratio of flaming combustion (FC) to smoldering combustion (SC) were characteristic differences found between softwood and hardwood. SC emitted larger mode particles in higher numbers than smaller mode particles, while the converse was true for FC. Rice straw open burning UFPs exhibited a volatilization behavior similar to that between FC and SC. In addition, internal mixing states were observed for size-selected UFPs in all biomasses for all combustion conditions, while external mixing states were only observed for rice straw combustion. Results for FC and open burning suggested there was an internal mixing of volatile organic carbon (OC) and non-volatile core (e.g., black carbon (BC)), while the SC in rice straw produced UFPs devoid of non-volatile core. Also, it was found that volatility of constituting OC in FC and SC particles was different.

Influence of mineral dust mixing-state and reaction probabilities on size-resolved sulfate formation in Northeast Asia

Significant differences were found between two particulate sulfate size-distributions measured using a MOUDI impactor at Gosan, Jeju Island, Korea, and simulated via the Lagrangian photochemical model under a condition of aerosol internal mixing between mineral dust and urban pollution particles. It was suspected that these differences might have resulted from: (1) the assumption of aerosol internal mixing and (2) the uses of identical reaction probabilities (γ) of the gaseous sulfate precursors (SO2 and H2SO4) onto both urban pollution particles and mineral dust in the Lagrangian photochemical model simulations. In this study; therefore, some cases of aerosol external mixing between urban-derived pollution aerosols and mineral dust were investigated, with different magnitudes of γ for the gas-phase sulfate precursors onto the two different condensing media. The model simulations under the external mixing condition, with different magnitudes of the reaction probabilities (γi,urban and γi,dust) onto urban pollution particles and mineral dust, successfully reproduced the size-dependent particulate sulfate formation measured at the Gosan station. Further attempts were made to approximate the magnitudes of γSO2,urban and γH2SO4,dust under external mixing state conditions with the fixed γSO2,dust and γH2SO2,urban values of 10−4 and 1.0. The best-estimates of γSO2,urban and γH2SO4,dust found in this study were in the orders of 10−4–10−5 and 10−2–10−3, respectively.

Elucidating determinants of aerosol composition through particle-type-based receptor modeling

Abstract. An aerosol time-of-flight mass spectrometer (ATOFMS) was deployed at a semi-rural site in southern Ontario to characterize the size and chemical composition of individual particles. Particle-type-based receptor modelling of these data was used to investigate the determinants of aerosol chemical composition in this region. Individual particles were classified into particle-types and positive matrix factorization (PMF) was applied to their temporal trends to separate and cross-apportion particle-types to factors. The extent of chemical processing for each factor was assessed by evaluating the internal and external mixing state of the characteristic particle-types. The nine factors identified helped to elucidate the coupled interactions of these determinants. Nitrate-laden dust was found to be the dominant type of locally emitted particles measured by ATOFMS. Several factors associated with aerosol transported to the site from intermediate local-to-regional distances were identified: the Organic factor was associated with a combustion source to the north-west; the ECOC Day factor was characterized by nearby local-to-regional carbonaceous emissions transported from the south-west during the daytime; and the Fireworks factor consisted of pyrotechnic particles from the Detroit region following holiday fireworks displays. Regional aerosol from farther emissions sources was reflected through three factors: two Biomass Burning factors and a highly chemically processed Long Range Transport factor. The Biomass Burning factors were separated by PMF due to differences in chemical processing which were in part elucidated by the passage of two thunderstorm gust fronts with different air mass histories. The remaining two factors, ECOC Night and Nitrate Background, represented the night-time partitioning of nitrate to pre-existing particles of different origins. The distinct meteorological conditions observed during this month-long study in the summer of 2007 provided a unique range of temporal variability, enabling the elucidation of the determinants of aerosol chemical composition, including source emissions, chemical processing, and transport, at the Canada-US border. This paper presents the first study to elucidate the coupled influences of these determinants on temporal variability in aerosol chemical composition using single particle-type-based receptor modelling.

Single particle analysis of ambient aerosols in Shanghai during the World Exposition, 2010: two case studies

ATSI Model 3800 aerosol time-of-flight mass spectrometer (ATOFMS) was deployed for single-particle analysis in Shanghai during the World Exposition (EXPO), 2010. Measurements on two extreme cases: polluted day (1st May) and clean day (25th September) were compared to show how meteorological conditions affected the concentration and composition of ambient aerosols. Mass spectra of 90496 and 50407 particles were analyzed respectively during the two sampling periods. The ART-2a neural network algorithm was applied to sort the collected particles. Seven major classes of particles were obtained: dust, sea salt, industrial, biomass burning, organic carbon (OC), elementary carbon (EC), and NH4-rich particles. Number concentration of ambient aerosols showed a strong anti-correlation with the boundary layer height variation. The external mixing states of aerosols were quite different during two sampling periods because of different air parcel trajectories. Number fraction of biomass burning particles (43.3%) during polluted episode was much higher than that (21.6%) of clean time. Air parcels from the East China Sea on clean day diluted local pollutant concentration and increased the portion of sea salt particle dramatically (13.3%). The large contribution of biomass burning particles in both cases might be an indication of a constant regional background of biomass burning emission. Mass spectrum analysis showed that chemical compositions and internal mixing states of almost all the particle types were more complicate during polluted episode compared with those observed in clean time. Strong nitrate signals in the mass spectra suggested that most of the particles collected on polluted day had gone through some aging processes before reaching the sampling site.

Influence of the external mixing state of atmospheric aerosol on derived CCN number concentrations

We derived the range of particle hygroscopicities (κ) that occurs in the atmosphere, based on literature data of measured hygroscopic growth or based on chemical composition. The derived κ‐values show that the atmospheric aerosol often is an external mixture with respect to hygroscopicity. Mean κ were derived for urban, rural, and marine aerosols for the different hygroscopic modes. Using these κ and exemplary particle number size distributions for the different aerosols, the number concentration of cloud condensation nuclei (NCCN) was derived for two cases, (1) accounting for the less hygroscopic fraction of particles and (2) assuming all particles to have κ of the more hygroscopic mode. NCCN derived from measured particle hygroscopicity is overestimated for case (2). Overestimation of NCCN is largest for fresh continental aerosol and less pronounced for marine aerosol. With κ derived from bulk aerosol composition data, only the hygroscopicity of more soluble aerosol particles is captured. Bulk or even size‐resolved composition data will be insufficient to predict NCCN under many conditions unless independent information about particle mixing state is available.

On geometric optics and surface waves for light scattering by spheres

A geometric optics approach including surface wave contributions has been developed for homogeneous and concentrically coated spheres. In this approach, a ray-by-ray tracing program was used for efficient computation of the extinction and absorption cross sections. The present geometric-optics surface-wave ( GOS) theory for light scattering by spheres considers the surface wave contribution along the edge of a particle as a perturbation term to the geometric-optics core that includes Fresnel reflection–refraction and Fraunhofer diffraction. Accuracies of the GOS approach for spheres have been assessed through comparison with the results determined from the exact Lorenz–Mie ( LM) theory in terms of the extinction efficiency, single-scattering albedo, and asymmetry factor in the size–wavelength ratio domain. In this quest, we have selected a range of real and imaginary refractive indices representative of water/ice and aerosol species and demonstrated close agreement between the results computed by GOS and LM. This provides the foundation to conduct physically reliable light absorption and scattering computations based on the GOS approach for aerosol aggregates associated with internal and external mixing states employing spheres as building blocks.

Satellite characterization of urban aerosols: Importance of including hygroscopicity and mixing state in the retrieval algorithms

This model study examines the sensitivity of the calculated optical properties of urban aerosols to (1) hygroscopicity and (2) internal or external mixing state, and it further investigates the associated implications for the accuracy of satellite retrievals of aerosol optical thickness (τ) and aerosol effective radius (reff). State‐of‐the‐art retrieval algorithms widely omit variable hygroscopicity and mixing state. For the study described herein, the modeled urban aerosols are composed of water‐soluble sulfates and water‐insoluble black carbon (BC) in the fine mode and of water‐insoluble compounds in the coarse mode. The calculations show that external compared to internal mixing of black carbon and sulfate not only significantly affects the single‐scattering albedo but also alters the diagnostic relationship of the Angstrom exponent (α) to the aerosol effective radius. The implication is that over a dark surface of visible reflectance less than 0.1, satellite retrievals of urban aerosols having a BC/sulfate mass ratio of 5% can differ in τ and reff by as much as 60% and 0.2 μm, respectively, depending upon the retrieval algorithm's assumptions regarding hygroscopicity and mixing state. For surface reflectances greater than 0.1 or BC/sulfate mass ratios larger than 5%, the retrieval bias, including the possibility of unphysical retrievals, increases further. The calculations also show that hygroscopic growth at elevated relative humidity increases the single‐scattering albedo of urban aerosols, decreases their backscattering, and as a consequence reduces the influence of mixing state on τ and reff. These results suggest that current operational retrieval algorithms lead to a possibly systematic underestimate of aerosol optical thickness when ambient BC/sulfate aerosols are internally mixed at mass ratios greater than 3%. This study's recommendation is that aerosol retrieval algorithms, when applied to urban aerosols, incorporate in situ knowledge of relative humidity, mixing state, and BC/sulfate mass ratios, either from ground‐based measurements or by auxiliary use of chemical transport models.

Sensitivity of photodissociation rate coefficients and O3 photochemical tendencies to aerosols and clouds

We examine the sensitivity of the daily integrated photodissociation rate (DIPR) coefficients of O3(O1D) and NO2, the daily averaged O3 photochemical tendency, and OH concentration to variations of clouds and of four aerosol components: black carbon (BC), mineral dust, sea salt, and sulfate. Clouds and BC aerosols are found to be the most important. A comparison between the sensitivity study and some representative observations showed that the global average cloud reduction of DIPR at the surface level is ∼20% and it is ∼30% in storm track zones. At the surface level the average negative BC impact is ∼10% in urban areas and could reach ∼40% in some heavily polluted urban areas. Mineral dust aerosol, which is the next most important, can reduce the photochemistry at the surface level by over 17% in some seasons over the desert or along its long‐range transport paths. The negative impact of sulfate aerosols is around 2% at the surface level, and the impact is as positive as 4% at the top of the sulfate aerosol layer in some urban areas. BC, sulfate, and mineral dust all have much smaller impacts away from their source regions. The impact of sea‐salt aerosol is generally less than 1%. While the fractional impacts on O3 production and destruction and OH concentration do not depend much on NOx, the magnitude of the impact on O3 chemical tendency and OH concentration depends strongly on the concentration of NOx: When NOx is very low, the impacts are also very small even if DIPRs are strongly affected. Different mixing states of absorbing and scattering aerosol components, external, coating, and internal, are studied. It is found that with the same amount of each component, the external mixing state produces the weakest impact while the internal mixing state produces the strongest impact. Coating has almost the same impact as internal mixing. There is very little synergy between cloud and absorbing aerosols when clouds are located above the aerosol. The aerosol absorption is strengthened when clouds and aerosol are located in the same layer. The synergy is strong when clouds are located below the absorbing aerosol, where the impact of absorbing aerosol dominates.