Aerosol Aging Processes Research Articles

Hygroscopicity and composition of Alaskan Arctic CCN during April 2008

Abstract. We present a comprehensive characterization of cloud condensation nuclei (CCN) sampled in the Alaskan Arctic during the 2008 Aerosol, Radiation, and Cloud Processes affecting Arctic Climate (ARCPAC) project, a component of the POLARCAT and International Polar Year (IPY) initiatives. Four distinct air mass types were sampled including a cleaner Arctic background and a relatively pristine sea ice boundary layer as well as biomass burning and anthropogenic pollution plumes. Despite differences in chemical composition, inferred aerosol hygroscopicities were fairly invariant and ranged from κ = 0.1–0.3 over the atmospherically-relevant range of water vapor supersaturations studied. Organic aerosols sampled were found to be well-oxygenated, consistent with long-range transport and aerosol aging processes. However, inferred hygroscopicities are less than would be predicted based on previous parameterizations of biogenic oxygenated organic aerosol, suggesting an upper limit on organic aerosol hygroscopicity above which κ is less sensitive to the O:C ratio. Most Arctic aerosols act as CCN above 0.1 % supersaturation, although the data suggest the presence of an externally-mixed, non-CCN-active mode comprising approximately 0–20% of the aerosol number. CCN closure was assessed using measured size distributions, bulk chemical composition, and assumed aerosol mixing states; CCN predictions tended toward overprediction, with the best agreement (±0–20 %) obtained by assuming the aerosol to be externally-mixed with soluble organics. Closure also varied with CCN concentration, and the best agreement was found for CCN concentrations above 100 cm−3 with a 1.5- to 3-fold overprediction at lower concentrations.

Design of a new multi-phase experimental simulation chamber for atmospheric photosmog, aerosol and cloud chemistry research

Abstract. A new simulation chamber has been built at the Interuniversitary Laboratory of Atmospheric Systems (LISA). The CESAM chamber (French acronym for Experimental Multiphasic Atmospheric Simulation Chamber) is designed to allow research in multiphase atmospheric (photo-) chemistry which involves both gas phase and condensed phase processes including aerosol and cloud chemistry. CESAM has the potential to carry out variable temperature and pressure experiments under a very realistic artificial solar irradiation. It consists of a 4.2 m3 stainless steel vessel equipped with three high pressure xenon arc lamps which provides a controlled and steady environment. Initial characterization results, all carried out at 290–297 K under dry conditions, concerning lighting homogeneity, mixing efficiency, ozone lifetime, radical sources, NOy wall reactivity, particle loss rates, background PM, aerosol formation and cloud generation are given. Photolysis frequencies of NO2 and O3 related to chamber radiation system were found equal to (4.2 × 10−3 s−1) for JNO2 and (1.4 × 10−5 s−1) for JO1D which is comparable to the solar radiation in the boundary layer. An auxiliary mechanism describing NOy wall reactions has been developed. Its inclusion in the Master Chemical Mechanism allowed us to adequately model the results of experiments on the photo-oxidation of propene-NOx-Air mixtures. Aerosol yields for the α-pinene + O3 system chosen as a reference were determined and found in good agreement with previous studies. Particle lifetime in the chamber ranges from 10 h to 4 days depending on particle size distribution which indicates that the chamber can provide high quality data on aerosol aging processes and their effects. Being evacuable, it is possible to generate in this new chamber clouds by fast expansion or saturation with or without the presence of pre-existing particles, which will provide a multiphase environment for aerosol-droplet interaction.

Comparing polybrominated diphenyl ethers (PBDEs) in airborne particles in Guangzhou and Hong Kong: sources, seasonal variations and inland outflow

The historical application/usage and management of chemicals in Hong Kong have been distinctively different from mainland China. In the present study, polybrominated diphenyl ethers (PBDEs) were measured in year-round atmospheric particle samples collected from urban Hong Kong and Guangzhou for comparison. The concentrations of BDE-209 and Sigma9PBDEs (defined as the sum of BDE-28, -47, -66, -100, -99, -154, -153, -138 and -183) in Guangzhou ranged from 758 to 21,900 pg m(-3) and from 31.8 to 3320 pg m(-3), respectively, and in Hong Kong ranged from 8.5 to 895 pg m(-3) and from 1.0 to 386 pg m(-3), respectively. Elevated concentrations of PBDEs were observed in Guangzhou, showing significant atmospheric PBDE pollution. BDE-209, -47, and -99 were the dominant congeners in all the samples, suggesting that the widely used commercial penta- and deca-BDE products were the original sources. Distinct seasonal patterns were observed in the PBDE concentrations of aerosols in Hong Kong, higher during the winter monsoon period, and lower during summertime. The less distinct seasonal variations of PBDE concentrations in the aerosols of Guangzhou suggested the dominance of local pollution sources around the city. Significant correlations were found between BDE-209 and organic carbon (OC) or elemental carbon (EC) in the two cities, suggesting that combustion may be an important pathway introducing BDE-209 to the atmosphere. The lower BDE-209 concentrations along with higher OC/EC ratios implied that a quick loss of BDE-209 may occur during the aerosol aging processes. Back trajectory analysis showed that the high PBDE concentrations observed in Hong Kong may be related to the outflows from the inland area of the Pearl River Delta (PRD) by prevailing the northeast or northwest wind in winter.

The differences of the size distribution of polycyclic aromatic hydrocarbons (PAHs) between urban and rural sites of Guangzhou, China

Size distribution aerosol samples collected at two urban locations (Liwan and Wushan) and a suburban location (Xinken) in Guangzhou (South China) in autumn using a MOUDI (Micro-orifice Uniform Deposit Impactor) have been analyzed for 13 polycyclic aromatic hydrocarbons (PAHs) by gas chromatography with mass selective detection (GC-MS). Different PAH size distribution models were found for urban locations and a suburban location. The particle size distributions of PAHs tend to shift toward a larger size in a suburban location than in an urban location caused by aerosol aging processes. The ratios of total-PAH/TSP changed greatly during different sampling periods, even though the samplings were conducted consecutively. This can be the result of reactions between PAHs and atmospheric photooxidants, and the formation of SOAs (secondary organic aerosols) during the aging process. Adsorption behavior on available aerosol surface area, absorption behavior in available aerosol organic mass and multilayer adsorptive accumulation in coarse mode aerosols were the three mechanisms which control the size distribution of PAHs.

Measurements of non-volatile fractions of pollution aerosols with an eight-tube volatility tandem differential mobility analyzer (VTDMA-8)

A volatility tandem differential mobility analyzer (VTDMA) was developed that offers high size resolution with high temporal resolution: the eight heating columns of the VTDMA-8 allow rapid measurement of the non-volatile component of aerosol particles within a narrow particle size range in about 15 min . The non-volatile composition of these quasi-monodisperse particles is described in terms of number fraction Φ N and volume fraction Φ V of the non-volatile material. Laboratory studies were made to determine the transfer efficiency and the volatile behavior of sulfate particles in the VTDMA-8. Its applicability to atmospheric aerosols was confirmed during three field measurements. Since the distribution of carbonaceous material depends on the distance to combustion sources and varies with region (i.e., urban or rural) and with time, we made continuous surface measurements in air masses polluted to different levels: (a) in Falkenberg, a moderately polluted, rural region in north-eastern Germany, (b) in the city of Leipzig, a strongly polluted environment, and (c) directly at a combustion source in the exhaust of a diesel-fueled vehicle. The results show that the refractory material contained in 150 nm atmospheric particles (at Falkenberg and Leipzig) ranged between 8% and 25% in volume and represents an external mixture of particles containing various amounts of non-volatile material. However, the vehicle-emitted 50 nm particles at a single combustion source were internally mixed, and that non-volatile fraction ranged between 75% and 98% in volume, depending on the operating conditions of the vehicle (speed, engine revolution). The results show that throughout all measurements the total number of particles remained unchanged after evaporation of the volatile material. Our results demonstrate that the VTDMA-8 can rapidly measure the non-volatile fractions of size-segregated pollution aerosols and that it is applicable to study the mixing state of aerosols and the effect of carbonaceous material on aerosol aging processes.

Characteristic parameters of particle size distributions of primary organic constituents of ambient aerosols

Particle size distributions have been measured for nonvolatile organic constituents of ambient aerosols, sampled by Sierra and Andersen Hi-Vol cascade impactors, in suburban, rural, and seashore areas in Belgium and the Netherlands during the four seasons. The distributions of polycyclic and aza-heterocyclic aromatic hydrocarbons, n-aliphatic hydrocarbons, and carboxylic acids determined by gas chromatographic-mass spectrometric analysis are compared by using different formats. Results are discussed with respect to (1) generation and aerosol incorporation processes, which show condensation in the accumulation mode (<2.5 ..mu..m) of anthropogenic combustion related compounds and natural contributions of the biosphere in the dispersion mode, and (2) aerosol aging processes, which show that the particle size distributions of its constituents tend to shift toward a larger size within the accumulation mode upon atmospheric transport. This is reflected in the mass median equivalent diameters of the partial cumulative distributions for the accumulation mode.