CCN closure study: Effects of aerosol chemical composition and mixing state

TL;DR: In this article, the effects of chemical composition (bulk and size resolved) and mixing state (internal and external) on CCN activity of aerosols were investigated during the winter season in Kanpur.

Abstract: This study presents a detailed cloud condensation nuclei (CCN) closure study that investigates the effects of chemical composition (bulk and size resolved) and mixing state (internal and external) on CCN activity of aerosols. Measurements of the chemical composition, aerosol size distribution, total number concentration, and CCN concentration at supersaturation (SS = 0.2–1.0%) were performed during the winter season in Kanpur, India. Among the two cases considered here, better closure results are obtained for case 1 (low total aerosol loading, 49.54 ± 26.42 μg m−3, and high O:C ratio, 0.61 ± 0.07) compared to case 2 (high total aerosol loading, 101.05 ± 18.73 μg m−3, and low O:C ratio, 0.42 ± 0.06), with a maximum reduction of 3–81% in CCN overprediction for all depleted SS values (0.18–0.60%). Including the assumption that less volatile oxidized organic aerosols represent the soluble organic fraction reduced the overprediction to at most 40% and 129% in the internal and external mixing scenarios, respectively. At higher depleted SS values (0.34–0.60%), size-resolved chemical composition with an internal mixing state performed well in CCN closure among all organic solubility scenarios. However, at a lower depleted SS value (0.18%), closure is found to be more sensitive to both the chemical composition and mixing state of aerosols. At higher SS values, information on the solubility of organics and size-resolved chemical composition is required for accurate CCN predictions, whereas at lower SS values, information on the mixing state in addition to the solubility of organics and size-resolved chemical composition is required. Overall, κtotal values are observed to be independent of the O:C ratio [κtotal = (0.36 ± 0.01) × O:C − (0.03 ± 0.01)] in the range of 0.2

TL;DR: In this article, the mean hygroscopicity parameters (κs) of 50, 100, 150, 200, and 250 nm particles were respectively 0.16, 0.19, p.07 and 0.10, showing an increasing trend with increasing particle size.

Abstract: . Simultaneous measurements of particle number size distribution, particle hygroscopic properties, and size-resolved chemical composition were made during the summer of 2014 in Beijing, China. During the measurement period, the mean hygroscopicity parameters (κs) of 50, 100, 150, 200, and 250 nm particles were respectively 0.16 p 0.07, 0.19 p 0.06, 0.22 p 0.06, 0.26 p 0.07, and 0.28 p 0.10, showing an increasing trend with increasing particle size. Such size dependency of particle hygroscopicity was similar to that of the inorganic mass fraction in PM1. The hydrophilic mode (hygroscopic growth factor, HGF > 1.2) was more prominent in growth factor probability density distributions and its dominance of hydrophilic mode became more pronounced with increasing particle size. When PM2.5 mass concentration was greater than 50 μg m−3, the fractions of the hydrophilic mode for 150, 250, and 350 nm particles increased towards 1 as PM2.5 mass concentration increased. This indicates that aged particles dominated during severe pollution periods in the atmosphere of Beijing. Particle hygroscopic growth can be well predicted using high-time-resolution size-resolved chemical composition derived from aerosol mass spectrometer (AMS) measurements using the Zdanovskii–Stokes–Robinson (ZSR) mixing rule. The organic hygroscopicity parameter (κorg) showed a positive correlation with the oxygen to carbon ratio. During the new particle formation event associated with strongly active photochemistry, the hygroscopic growth factor or κ of newly formed particles is greater than for particles with the same sizes not during new particle formation (NPF) periods. A quick transformation from external mixture to internal mixture for pre-existing particles (for example, 250 nm particles) was observed. Such transformations may modify the state of the mixture of pre-existing particles and thus modify properties such as the light absorption coefficient and cloud condensation nuclei activation.

TL;DR: In this paper, the effects of NO x and SO 2 on secondary organic aerosol (SOA) formation from photooxidation of α-pinene and limonene at ≥ 0.05 to 15.5ppb were investigated.

Abstract: . Anthropogenic emissions such as NO x and SO 2 influence the biogenic secondary organic aerosol (SOA) formation, but detailed mechanisms and effects are still elusive. We studied the effects of NO x and SO 2 on the SOA formation from the photooxidation of α -pinene and limonene at ambient relevant NO x and SO 2 concentrations (NO x : 2 : < 0.05 to 15 ppb). In these experiments, monoterpene oxidation was dominated by OH oxidation. We found that SO 2 induced nucleation and enhanced SOA mass formation. NO x strongly suppressed not only new particle formation but also SOA mass yield. However, in the presence of SO 2 which induced a high number concentration of particles after oxidation to H 2 SO 4 , the suppression of the mass yield of SOA by NO x was completely or partly compensated for. This indicates that the suppression of SOA yield by NO x was largely due to the suppressed new particle formation, leading to a lack of particle surface for the organics to condense on and thus a significant influence of vapor wall loss on SOA mass yield. By compensating for the suppressing effect on nucleation of NO x , SO 2 also compensated for the suppressing effect on SOA yield. Aerosol mass spectrometer data show that increasing NO x enhanced nitrate formation. The majority of the nitrate was organic nitrate (57–77 %), even in low-NO x conditions ( ∼ 1 ppb). Organic nitrate contributed 7–26 % of total organics assuming a molecular weight of 200 g mol −1 . SOA from α -pinene photooxidation at high NO x had a generally lower hydrogen to carbon ratio (H ∕ C), compared to low NO x . The NO x dependence of the chemical composition can be attributed to the NO x dependence of the branching ratio of the RO 2 loss reactions, leading to a lower fraction of organic hydroperoxides and higher fractions of organic nitrates at high NO x . While NO x suppressed new particle formation and SOA mass formation, SO 2 can compensate for such effects, and the combining effect of SO 2 and NO x may have an important influence on SOA formation affected by interactions of biogenic volatile organic compounds (VOCs) with anthropogenic emissions.

TL;DR: Aerosol composition varied largely in different regions, but was overall dominated by organic aerosols (OA, 32-75%), especially in south and southeast Asia due to the impact of biomass burning, and secondary OA was a ubiquitous and dominant aerosol component in all regions.

Abstract: Anthropogenic emissions in Asia have significantly increased during the last two decades; as a result, the induced air pollution and its influences on radiative forcing and public health are becoming increasingly prominent The Aerodyne Aerosol Mass Spectrometer (AMS) has been widely deployed in Asia for real-time characterization of aerosol chemistry In this paper, we review the AMS measurements in Asia, mainly in China, Korea, Japan, and India since 2001 and summarize the key results and findings The mass concentrations of non-refractory submicron aerosol species (NR-PM1) showed large spatial distributions with high mass loadings occurring in India and north and northwest China (602-813 μg m-3), whereas much lower values were observed in Korea, Japan, Singapore and regional background sites (75-151 μg m-3) Aerosol composition varied largely in different regions, but was overall dominated by organic aerosols (OA, 32-75%), especially in south and southeast Asia due to the impact of biomass burning While sulfate and nitrate showed comparable contributions in urban and suburban regions in north China, sulfate dominated inorganic aerosols in south China, Japan and regional background sites Positive matrix factorization analysis identified multiple OA factors from different sources and processes in different atmospheric environments, eg, biomass burning OA in south and southeast Asia and agricultural seasons in China, cooking OA in urban areas, and coal combustion in north China However, secondary OA (SOA) was a ubiquitous and dominant aerosol component in all regions, accounting for 43-78% of OA The formation of different SOA subtypes associated with photochemical production or aqueous-phase/fog processing was widely investigated The roles of primary emissions, secondary production, regional transport, and meteorology on severe haze episodes, and different chemical responses of primary and secondary aerosol species to source emission changes and meteorology were also demonstrated Finally, future prospects of AMS studies on long-term and aircraft measurements, water-soluble OA, the link of OA volatility, oxidation levels, and phase state were discussed

TL;DR: In this article, a detailed time-resolved chemical characterization of ambient nonrefractory submicron aerosols (NR-PM1) was conducted for the first time in India.

Abstract: A detailed time-resolved chemical characterization of ambient nonrefractory submicron aerosols (NR-PM1) was conducted for the first time in India. The measurements were performed during the winter (November 2011 to January 2012) in a heavily polluted city of Kanpur, which is situated in the Indo-Gangetic Plain. Real-time measurements provided new insights into the sources and evolution of organic aerosols (OA) that could not be obtained using previously deployed filter-based measurements at this site. The average NR-PM1 loading was very high (>100 µg/m3) throughout the study, with OA contributing approximately 70% of the total aerosol mass. Source apportionment of the OA using positive matrix factorization revealed large contributions from fresh and aged biomass burning OA throughout the entire study period. A back trajectory analysis showed that the polluted air masses were affected by local sources and distant source regions where the burning of paddy residues occurs annually during winter. Several fog episodes were encountered during the study, and the OA composition varied between foggy and nonfoggy periods, with higher oxygen to carbon (O/C) ratios during the foggy periods. The evolution of OA and their elemental ratios (O:C and H:C) were investigated for the possible effects of fog processing.

TL;DR: In this paper, a new method to parameterize H:C of OOA in terms of the ratio of m/z 43, mostly C_2H_3O^+ to total signal in the component mass spectrum is presented.

Abstract: Organic aerosols (OA) can be separated with factor analysis of aerosol mass spectrometer (AMS) data into hydrocarbon-like OA (HOA) and oxygenated OA (OOA). We develop a new method to parameterize H:C of OOA in terms of f_(43)(ratio of m/z 43, mostly C_2H_3O^+, to total signal in the component mass spectrum). Such parameterization allows for the transformation of large database of ambient OOA components from the f_(44) (mostly CO^+_2, likely from acid groups) vs. f_(43) space ("triangle plot") (Ng et al., 2010) into the Van Krevelen diagram (H:C vs. O:C) (Van Krevelen, 1950). Heald et al. (2010) examined the evolution of total OA in the Van Krevelen diagram. In this work total OA is deconvolved into components that correspond to primary (HOA and others) and secondary (OOA) organic aerosols. By deconvolving total OA into different components, we remove physical mixing effects between secondary and primary aerosols which allows for examination of the evolution of OOA components alone in the Van Krevelen space. This provides a unique means of following ambient secondary OA evolution that is analogous to and can be compared with trends observed in chamber studies of secondary organic aerosol formation. The triangle plot in Ng et al. (2010) indicates that f_(44) of OOA components increases with photochemical age, suggesting the importance of acid formation in OOA evolution. Once they are transformed with the new parameterization, the triangle plot of the OOA components from all sites occupy an area in Van Krevelen space which follows a ΔH:C/ΔO:C slope of ~ −0.5. This slope suggests that ambient OOA aging results in net changes in chemical composition that are equivalent to the addition of both acid and alcohol/peroxide functional groups without fragmentation (i.e. C-C bond breakage), and/or the addition of acid groups with fragmentation. These results provide a framework for linking the bulk aerosol chemical composition evolution to molecular-level studies.

TL;DR: Organic particles are abundant in the troposphere and important for air quality and climate, but what are their sources and why are they important?

Abstract: Organic particles are abundant in the troposphere and important for air quality and climate, but what are their sources?

TL;DR: Particulate sulfate and nitrate and gaseous nitric acid concentrations were relatively higher in the daytime than those in the nighttime during the both seasons, which indicates that the droplet phase reactions and gas phase reactions are important for the oxidation of SO2 to sulfate.

TL;DR: In this article, the effective hygroscopicity parameters describing the influence of chemical composition on the CCN activ- ity of aerosol particles varied in the range of 0.1-0.4 (0.16±0.06 arithmetic mean and standard deviation).

Abstract: Atmospheric aerosol particles serving as cloud condensation nuclei (CCN) are key elements of the hydro- logical cycle and climate. We have measured and charac- terized CCN at water vapor supersaturations in the range of S=0.10-0.82% in pristine tropical rainforest air during the AMAZE-08 campaign in central Amazonia. The effective hygroscopicity parameters describing the influence of chemical composition on the CCN activ- ity of aerosol particles varied in the range of 0.1-0.4 (0.16±0.06 arithmetic mean and standard deviation). The overall median value of 0.15 was by a factor of two lower than the values typically observed for continental aerosols in other regions of the world. Aitken mode particles were less hygroscopic than accumulation mode particles ( 0.1 at D 50 nm; 0.2 at D 200 nm), which is in agreement with earlier hygroscopicity tandem differential mobility ana- lyzer (H-TDMA) studies. The CCN measurement results are consistent with aerosol mass spectrometry (AMS) data, showing that the organic mass fraction (forg) was on average as high as 90% in the Aitken mode (D 100 nm) and decreased with increas- ing particle diameter in the accumulation mode ( 80% at D 200 nm). The values exhibited a negative linear cor-

TL;DR: In this article, the authors measured and characterized CCN in polluted air and biomass burning smoke during the PRIDE-PRD2006 campaign from 1-30 July 2006 at a rural site ~60 km northwest of the mega-city Guangzhou in southeastern China.

Abstract: . Atmospheric aerosol particles serving as Cloud Condensation Nuclei (CCN) are key elements of the hydrological cycle and climate. We measured and characterized CCN in polluted air and biomass burning smoke during the PRIDE-PRD2006 campaign from 1–30 July 2006 at a rural site ~60 km northwest of the mega-city Guangzhou in southeastern China. CCN efficiency spectra (activated fraction vs. dry particle diameter; 20–290 nm) were recorded at water vapor supersaturations (S) in the range of 0.068% to 1.27%. The corresponding effective hygroscopicity parameters describing the influence of particle composition on CCN activity were in the range of κ≈0.1–0.5. The campaign average value of κ=0.3 equals the average value of κ for other continental locations. During a strong local biomass burning event, the average value of κ dropped to 0.2, which can be considered as characteristic for freshly emitted smoke from the burning of agricultural waste. At low S (≤0.27%), the maximum activated fraction remained generally well below one, indicating substantial portions of externally mixed CCN-inactive particles with much lower hygroscopicity – most likely soot particles (up to ~60% at ~250 nm). The mean CCN number concentrations (NCCN,S) ranged from 1000 cm−3 at S=0.068% to 16 000 cm−3 at S=1.27%, which is about two orders of magnitude higher than in pristine air. Nevertheless, the ratios between CCN concentration and total aerosol particle concentration (integral CCN efficiencies) were similar to the ratios observed in pristine continental air (~6% to ~85% at S=0.068% to 1.27%). Based on the measurement data, we have tested different model approaches for the approximation/prediction of NCCN,S. Depending on S and on the model approach, the relative deviations between observed and predicted NCCN,S ranged from a few percent to several hundred percent. The largest deviations occurred at low S with a simple power law. With a Kohler model using variable κ values obtained from individual CCN efficiency spectra, the relative deviations were on average less than ~10% and hardly exceeded 20%, confirming the applicability of the κ-Kohler model approach for efficient description of the CCN activity of atmospheric aerosols. Note, however, that different types of κ-parameters must be distinguished for external mixtures of CCN-active and -inactive aerosol particles (κa, κt, κcut). Using a constant average hygroscopicity parameter (κ=0.3) and variable size distributions as measured, the deviations between observed and predicted CCN concentrations were on average less than 20%. In contrast, model calculations using variable hygroscopicity parameters as measured and constant size distributions led to much higher deviations: ~70% for the campaign average size distribution, ~80% for a generic rural size distribution, and ~140% for a generic urban size distribution. These findings confirm earlier studies suggesting that aerosol particle number and size are the major predictors for the variability of the CCN concentration in continental boundary layer air, followed by particle composition and hygroscopicity as relatively minor modulators. Depending on the required and applicable level of detail, the information and parameterizations presented in this study should enable efficient description of the CCN activity of atmospheric aerosols in detailed process models as well as in large-scale atmospheric and climate models.