J. Dommen

Cloud forming potential of secondary organic aerosol under near atmospheric conditions

An Aerodyne quadrupole aerosol mass spectrometer (QAMS) was used to provide on-line, quantitative measurements of the chemical composition and mass size distributions of the non-refractory fraction of the SOA particles at a temporal resolution of two minutes. In brief, the AMS utilizes an aerodynamic lens [Zhang et al., 2004, 2002] to produce a collimated particle beam that impacts on a porous tungsten surface heated typically to 600◦C under high vacuum (∼10−8 Torr), causing the non-refractory fraction of the particles to flash vaporize. The vapor plume is immediately ionized using a 70 eV electron impact (EI) ionization source, and a quadrupole mass spectrometer (QMA 410, Balzers, Liechtenstein) is used to analyze the resultant ions with unit mass-to-charge (m/z ) resolution. More detailed descriptions of the AMS measurement principles and various calibrations [Jayne et al., 2000], its modes of operation [Jimenez et al., 2003] and data processing and analysis [Allan et al., 2004, 2003] are available in other publications.

Photochemical oxidant formation over southern Switzerland: 1. Results from summer 1994

We present ground-based and aircraft measurements of photochemically relevant trace gases from the southern part of Switzerland from summer 1994. The region is adjacent to the Po Valley and exhibits the highest ozone concentrations within Switzerland. O3 concentrations of up to 166 parts per billion by volume (ppbv) were measured. Isoprene was 3 ppbv on the ground and 0.3 ppbv at 1000 m altitude on average in the afternoon and dominates the volatile organic compound (VOC) reactivity toward OH at the ground. Measured HONO concentrations of 0.2 ppbv in the afternoon are much higher, as can be explained from gas phase reactions alone. Radical concentrations are derived from steady state calculations using ground-based measurements in the afternoon. The resulting concentrations for OH, HO2, and RO2 are 3.1×106, 1.1×109, and 1.3×109 molecules cm−3, respectively. Isoprene and NO have the largest influence on the estimated OH concentration, followed by O3, photolysis frequency of O3, and HONO. HO2 and RO2 concentrations are most sensitive to changes in HONO, O3, and photolysis frequency of O3. The estimated radical production is larger than the NOx emissions, suggesting low-NOx chemistry. We calculated indicators for NOx or VOC sensitivity and show that reduction of NOx emissions would be more efficient in reducing O3 than VOC reduction upwind of the measurement site.

Effect of ions on sulfuric acid-water binary particle formation: 2. Experimental data and comparison with QC-normalized classical nucleation theory

We report comprehensive, demonstrably contaminant-free measurements of binary particle formation rates by sulfuric acid and water for neutral and ion-induced pathways conducted in the European Organization for Nuclear Research Cosmics Leaving Outdoor Droplets chamber. The recently developed Atmospheric Pressure interface-time of flight-mass spectrometer was used to detect contaminants in charged clusters and to identify runs free of any contaminants. Four parameters were varied to cover ambient conditions: sulfuric acid concentration (10^5 to 10^9  mol cm^(−3)), relative humidity (11% to 58%), temperature (207 K to 299 K), and total ion concentration (0 to 6800 ions cm^(−3)). Formation rates were directly measured with novel instruments at sizes close to the critical cluster size (mobility size of 1.3 nm to 3.2 nm). We compare our results with predictions from Classical Nucleation Theory normalized by Quantum Chemical calculation (QC-normalized CNT), which is described in a companion paper. The formation rates predicted by the QC-normalized CNT were extended from critical cluster sizes to measured sizes using the UHMA2 sectional particle microphysics model. Our results show, for the first time, good agreement between predicted and measured particle formation rates for the binary (neutral and ion-induced) sulfuric acid-water system. Formation rates increase with RH, sulfuric acid, and ion concentrations and decrease with temperature at fixed RH and sulfuric acid concentration. Under atmospheric conditions, neutral particle formation dominates at low temperatures, while ion-induced particle formation dominates at higher temperatures. The good agreement between the theory and our comprehensive data set gives confidence in using the QC-normalized CNT as a powerful tool to study neutral and ion-induced binary particle formation in atmospheric modeling.

Diurnal variations of volatile organic compounds and local circulation systems in an Alpine valley

Volatile organic compounds (VOCs) were measured during one summer month on the floor of the deep Swiss Alpine Mesolcina valley, accompanied by aircraft-based VOC measurements during six intensive observation days. In the late morning, a strong decrease of VOC concentrations despite an increasing traffic intensity was observed on the valley floor. This occurred when clean air masses were advected during the preceding night from high altitudes north of the Alps. During these nights the average VOC concentration was 40% lower in the nocturnal surface layer than in cases of southerly advection. Nights with southerly advection led to high VOC concentrations in the valley on the following morning and no decrease of VOC concentrations between 9 and 12 h. Only during these nights high ozone concentrations produced in the Po Basin of northern Italy could reach the Alpine crest and yielded ozone concentrations above 100 ppb there. The diurnal cycle of VOCs, influenced by the various meteorological features characteristic for deep Alpine valleys, is discussed in detail.

Photochemical production and aging of an urban air mass

Aircraft measurements of O3, NO, NO2, NOy, HCHO, and H2O2 over the Swiss Plateau during 4 days in July 1993 were analyzed. Special emphasis was put on the urban plume of Zurich. An effective photochemical age for an urban plume was introduced, which accounts for a background NOz concentration. Only the effective photochemical age of the urban air mass increased with the time of transport between Zurich and the flight leg, where the aircraft crossed the plume. Ozone and NOz were strongly correlated. Ozone production rates ranged from 2.4 to 6.5 ozone molecules produced per NOx processed. The production of ozone per NOx molecule was lowest when the effective photochemical age was lowest and vice versa. Good correlations between HCHO and NOy have been found in the urban plume of Zurich as well as over the Swiss Plateau. Between H2O2 and NOz, a negative correlation was observed. On the basis of Sillmans [1995] indicator species, the ozone production in the Zurich plume and other air masses over the Swiss Plateau is in the NOx-sensitive range. However, there remains some uncertainty in this approach regarding the influence of biogenic emissions and the initial concentration of indicator species. Taking the low emissions of biogenic hydrocarbons compared to Sillmans model calculation into consideration, the central portion of the Zurich plume falls in the transition or even reactive organic gases (ROG)- limited range of all indicators. Estimates of production rates of HNO3 and peroxides support a ROG-sensitive ozone production in the most polluted portion of the plume.

Two-dimensional volatility basis set modeling of pinanediol oxidation in the CLOUD experiment

We describe a 2D-VBS model of organic oxidation in the CERN CLOUD Experiment. Specifically, we model the steady-state supersaturations of organic compounds formed during the oxidation of pinanediol by OH. The steady state is controlled in the model almost entirely by production from OH oxidation and deposition to the chamber walls.

Evolution of nanoparticle composition in CLOUD in presence of sulphuric acid, ammonia and organics

In this study, we investigate the composition of nucleated nanoparticles formed from sulphuric acid, ammonia, amines, and oxidised organics in the CLOUD chamber experiments at CERN. The investigation is carried out via analysis of the particle hygroscopicity (size range of 15-63 nm), ethanol affinity (15-50nm), oxidation state (<50 nm), and ion composition (few nanometers). The organic volume fraction of particles increased with an increase in particle diameter in presence of the sulphuric acid, ammonia and organics. Vice versa, the sulphuric acid volume fraction decreased when the particle diameter increased. The results provide information on the size-dependent composition of nucleated aerosol particles.