No abstract

Very recent studies have reported the existence of highly oxidized multifunctional organic compounds (HOMs) with O/C ratios greater than 0.7. Because of their low vapor pressure, these compounds are often referred as extremely low-volatile organic compounds (ELVOCs), and thus, they are able to contribute significantly to organic mass in tropospheric particles. While HOMs have been successfully detected in the gas phase, their fate after uptake into particles remains unclear to date. Hence, the present study was designed to detect HOMs and related oxidation products in the particle phase and, thus, to shed light on their fate after phase transfer. To this end, aerosol chamber investigations of α-pinene ozonolysis were conducted under near environmental precursor concentrations (2.4 ppb) in a continuous flow reactor. The chemical characterization shows three classes of particle constituents: (1) intact HOMs that contain a carbonyl group, (2) particle-phase decomposition products, and (3) highly oxidized organosulfates (suggested to be addressed as HOOS). Besides chamber studies, HOM formation was also investigated during a measurement campaign conducted in summer 2013 at the TROPOS research station Melpitz. During this field campaign, gas-phase HOM formation was found to be correlated with an increase in the oxidation state of the organic aerosol.

Abstract. Gaseous sulphuric acid is a key precursor for new particle formation in the atmosphere. Previous experimental studies have confirmed a strong correlation between the number concentrations of freshly formed particles and the ambient concentrations of sulphuric acid. This study evaluates a body of experimental gas phase sulphuric acid concentrations, as measured by Chemical Ionization Mass Spectrometry (CIMS) during six intensive measurement campaigns and one long-term observational period. The campaign datasets were measured in Hyytiälä,

Abstract. Nucleation experiments starting from the reaction of OH radicals with SO 2 have been performed in the IfT-LFT flow tube under atmospheric conditions at 293±0.5 K for a relative humidity of 13-61%. The presence of different additives (H 2 , CO, 1,3,5-trimethylbenzene) for adjusting the OH radical concentration and resulting OH levels in the range (4-300) ×10 5 molecule cm −3 did not influence the nucleation process itself. The number of detected particles as well as the threshold H 2 SO 4 concentration needed for nucleation was found to be strongly dependent on the counting efficiency of the used counting devices. High-sensitivity particle counters allowed the measurement of freshly nucleated particles with diameters down to about 1.5 nm. A parameterization of the experimental data was developed using power law equations for H 2 SO 4 and H 2 O vapour. The exponent for H 2 SO 4 from different measurement series was in the range of 1.7-2.1 being in good agreement with those arising from analysis of nucleation events in the atmosphere. For increasing relative humidity, an increase of the particle number was observed. The exponent for H 2 O vapour was found to be 3.1 representing an upper limit. Addition of 1.2×10 11 molecule cm −3 or 1.2×10 12 molecule cm −3 of NH 3 (range of atmospheric NH 3 peak concentrations) revealed that NH 3 has a measureable, promoting effect on the nucleation rate under these conditions. The promoting efCorrespondence to: T. Berndt (berndt@tropos.de) fect was found to be more pronounced for relatively dry conditions, i.e. a rise of the particle number by 1-2 orders of magnitude at RH = 13% and only by a factor of 2-5 at RH = 47% (NH 3 addition: 1.2×10 12 molecule cm −3 ). Using the amine tert-butylamine instead of NH 3 , the enhancing impact of the base for nucleation and particle growth appears to be stronger. Tert-butylamine addition of about 10 10 molecule cm −3 at RH = 13% enhances particle formation by about two orders of magnitude, while for NH 3 only a small or negligible effect on nucleation in this range of concentration appeared. This suggests that amines can strongly influence atmospheric H 2 SO 4 -H 2 O nucleation and are probably promising candidates for explaining existing discrepancies between theory and observations.

Particle hygroscopic growth at 90% RH (relative humidity), cloud condensation nuclei (CCN) activity, and size-resolved chemical composition were concurrently measured in the Thüringer Wald mid-level mountain range in central Germany in the fall of 2010. The median hygroscopicity parameter values, κ, of 50, 75, 100, 150, 200, and 250 nm particles derived from hygroscopicity measurements are respectively 0.14, 0.14, 0.17, 0.21, 0.24, and 0.28 during the sampling period. The closure between HTDMA (Hygroscopicity Tandem Differential Mobility Analyzers)-measured (κ_HTDMA) and chemical composition-derived (κ_chem) hygroscopicity parameters was performed based on the Zdanovskii–Stokes–Robinson (ZSR) mixing rule. Using size-averaged chemical composition, the κ values are substantially overpredicted (30 and 40% for 150 and 100 nm particles). Introducing size-resolved chemical composition substantially improved closure. We found that the evaporation of NH₄NO₃, which may happen in a HTDMA system, could lead to a discrepancy in predicted and measured particle hygroscopic growth. The hygroscopic parameter of the organic fraction, κ_org, is positively correlated with the O : C ratio (κ_org = 0.19 × (O : C) − 0.03). Such correlation is helpful to define the κ_org value in the closure study. κ derived from CCN measurement was around 30% (varied with particle diameters) higher than that determined from particle hygroscopic growth measurements (here, hydrophilic mode is considered only). This difference might be explained by the surface tension effects, solution non-ideality, gas-particle partitioning of semivolatile compounds, and the partial solubility of constituents or non-dissolved particle matter. Therefore, extrapolating from HTDMA data to properties at the point of activation should be done with great care. Finally, closure study between CCNc (cloud condensation nucleus counter)-measured (κ_CCN) and chemical composition (κ_{CCN, chem}) was performed using CCNc-derived κ values for individual components. The results show that the κ_CCN can be well predicted using particle size-resolved chemical composition and the ZSR mixing rule