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 article, the hygroscopicity of the oxygenated fraction of the organic component, as determined by an Aerodyne aerosol mass spectrometer (AMS), was characterised by two methods.

Abstract: . Cloud condensation nuclei (CCN) concentrations were measured at Egbert, a rural site in Ontario, Canada during the spring of 2007. The CCN concentrations were compared to values predicted from the aerosol chemical composition and size distribution using κ-Kohler theory, with the specific goal of this work being to determine the hygroscopic parameter (κ) of the oxygenated organic component of the aerosol, assuming that oxygenation drives the hygroscopicity for the entire organic fraction of the aerosol. The hygroscopicity of the oxygenated fraction of the organic component, as determined by an Aerodyne aerosol mass spectrometer (AMS), was characterised by two methods. First, positive matrix factorization (PMF) was used to separate oxygenated and unoxygenated organic aerosol factors. By assuming that the unoxygenated factor is completely non-hygroscopic and by varying κ of the oxygenated factor so that the predicted and measured CCN concentrations are internally consistent and in good agreement, κ of the oxygenated organic factor was found to be 0.22±0.04 for the suite of measurements made during this five-week campaign. In a second, equivalent approach, we continue to assume that the unoxygenated component of the aerosol, with a mole ratio of atomic oxygen to atomic carbon (O/C) ≈ 0, is completely non-hygroscopic, and we postulate a simple linear relationship between κorg and O/C. Under these assumptions, the κ of the entire organic component for bulk aerosols measured by the AMS can be parameterised as κorg=(0.29±0.05)·(O/C), for the range of O/C observed in this study (0.3 to 0.6). These results are averaged over our five-week study at one location using only the AMS for composition analysis. Empirically, our measurements are consistent with κorg generally increasing with increasing particle oxygenation, but high uncertainties preclude us from testing this hypothesis. Lastly, we examine select periods of different aerosol composition, corresponding to different air mass histories, to determine the generality of the campaign-wide findings described above.

TL;DR: In this paper, the MI-LAGRO field campaign in Mexico City used a high resolution AMS spectra to identify a biomass burning organic aerosol component, which includes several large plumes that appear to be from forest fires within the region.

Abstract: Submicron aerosol was analyzed during the MI- LAGRO field campaign in March 2006 at the T0 urban su- persite in Mexico City with a High-Resolution Aerosol Mass Spectrometer (AMS) and complementary instrumentation. Positive Matrix Factorization (PMF) of high resolution AMS spectra identified a biomass burning organic aerosol (BBOA) component, which includes several large plumes that appear to be from forest fires within the region. Here, we show

TL;DR: A governing equation was developed to predict the density ρ(org) of organic material composed of carbon, oxygen, and hydrogen using the elemental ratios O:C and H:C as input parameters and has an accuracy of 12% for more than 90% of the 31 atmospherically relevant compounds used in the training set.

Abstract: A governing equation was developed to predict the density ρ(org) of organic material composed of carbon, oxygen, and hydrogen using the elemental ratios O:C and H:C as input parameters: ρ(org) = 1000 [(12 + 1(H:C) + 16(O:C)]/[7.0 + 5.0(H:C) + 4.15(O:C)] valid for 750 < ρ(org) < 1900 kg m(-3). Comparison of the actual to predicted ρ(org) values shows that the developed equation has an accuracy of 12% for more than 90% of the 31 atmospherically relevant compounds used in the training set. The equation was further validated for secondary organic material (SOM) produced by isoprene photo-oxidation and by α-pinene ozonolysis. Depending on the conditions of SOM production, ρ(org/SOM) ranged from 1230 to 1460 kg m(-3), O:C ranged from 0.38 to 0.72, and H:C ranged from 1.40 to 1.86. Atmospheric chemistry models that simulate particle production and growth can employ the developed equation to simulate particle physical properties. The equation can also extend atmospheric measurements presented as van Krevelen diagrams to include estimates of the material density of particles and their components. Use of the equation, however, is restricted to particle components having negligible quantities of additional elements, most notably nitrogen.

TL;DR: In this paper, the authors present a comprehensive 1 year (January 2007-March 2008) data set on the chemical composition of ambient aerosols collected from an urban location (Kanpur) in the Indo-Gangetic Plain (IGP) and suggest that the varying strength of the regional emission sources, boundary layer dynamics, and formation of secondary aerosols all contribute significantly to the temporal variability in the mass concentrations of elemental carbon (EC), organic carbon (OC), and water-soluble OC (WSOC).

Abstract: [1] This study presents a comprehensive 1 year (January 2007–March 2008) data set on the chemical composition of ambient aerosols collected from an urban location (Kanpur) in the Indo-Gangetic Plain (IGP) and suggests that the varying strength of the regional emission sources, boundary layer dynamics, and formation of secondary aerosols all contribute significantly to the temporal variability in the mass concentrations of elemental carbon (EC), organic carbon (OC), and water-soluble OC (WSOC). On average, carbonaceous aerosols contribute nearly one third of the PM10 mass during winter, whereas their fractional mass is only ∼10% during summer. A three- to four-fold increase in the OC and K+ concentrations during winter and a significant linear relation between them suggest biomass burning (wood fuel and agricultural waste) emission as a dominant source. The relatively high OC/EC ratio (average: 7.4 ± 3.5 for n = 66) also supports that emissions from biomass burning are overwhelming for the particulate OC in the IGP. The WSOC/OC ratios vary from 0.21 to 0.70 over the annual seasonal cycle with relatively high ratios in the summer, suggesting the significance of secondary organic aerosols. The long-range transport of mineral aerosols from Iran, Afghanistan, and the Thar Desert (western India) is pronounced during summer months. The temporal variability in the concentrations of selected inorganic constituents and neutralization of acidic species (SO42− and NO3−) by NH4+ (dominant during winter) and Ca2+ (in summer) reflect conspicuous changes in the source strength of anthropogenic emissions.

TL;DR: In this article, an Aerodyne high-resolution time-of-flight aerosol mass spectrometer was deployed at an urban site in the Hong Kong-Shenzhen metropolitan area between 25 October and 2 December 2009.

Abstract: [1] The Pearl River Delta (PRD) region in South China is one of the most economically developed regions in China while also noted for its severe air pollution, especially in the urban environments. In order to understand in depth the aerosol chemistry and the emission sources in PRD, an Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) was deployed at an urban site in the Hong Kong–Shenzhen metropolitan area between 25 October and 2 December 2009. Ten minute–resolved measurement data were analyzed, and an average mass concentration of 44.5 ± 34.0 μg m−3 was calculated for the entire campaign. On average, organic matter was the most abundant PM1 component accounting for 39.7% of the total mass, followed by sulfate (24.5%), black carbon (measured by aethalometer, 14.0%), ammonium (10.2%), nitrate (10.0%), and chloride (1.6%). Moreover, organic matter comprised an increasing fraction of the PM1 loading as the PM1 loading increased, denoting its key role in particulate pollution in this region. Calculations of organic elemental composition based on the high-resolution organic mass spectra obtained indicated that C, H, O, and N on average contributed 33.8%, 55.1%, 10.2%, and 0.9%, respectively, to the total atomic numbers of organic aerosol (OA), which corresponded to an OM/OC ratio (the ratio of organic matter mass/organic carbon mass) of 1.57 ± 0.08. Positive matrix factorization analysis was then conducted on the high-resolution organic mass spectral data set. Four OA components were identified, including a hydrocarbon-like (HOA), a biomass burning (BBOA), and two oxygenated (LV-OOA and SV-OOA) components, which on average accounted for 29.5%, 24.1%, 18.8%, and 27.6%, respectively, of the total organic mass. The HOA was found to have contributions from both fossil fuel combustion and cooking emissions, while the BBOA was well correlated with acetonitrile, a known biomass burning marker. The LV-OOA and SV-OOA corresponded to more aged and fresher secondary organic aerosol, respectively. The diurnal variations of the LV-OOA and SV-OOA showed significant increase in concentration in the daytime, denoting their substantial photochemical formation. Back trajectory analysis indicated that the short-range regional transport from the northeast was the key factor leading to severe submicron aerosol pollution in this area. The HR-ToF-AMS measurement results in this campaign are completely compared with a previous paper that reports the HR-ToF-AMS measurement results at a rural site in PRD in the same season, based on which the regional pollution characteristics of submicron particle in PRD were analyzed.