Aerosol and Trace Gases

Atmospheric aerosol particles cover a wide size range from ca. 1 nm (0.001 µm) to 100 µm. The smallest of these particles originate from a process termed new particle formation (NPF) or nucleation, which starts with the formation of small clusters consisting only of a few molecules. Most of the times these clusters evaporate rapidly but depending on the conditions they can also form thermodynamically stable particles that can eventually grow and become seeds for cloud droplets (cloud condensation nuclei, CCN, with a diameter above ca. 50 nm). NPF is therefore an important factor when assessing the climatic impact of aerosol particles due to the aerosol-cloud-climate interaction. Model calculations suggest that around half of the CCN originate from new particle formation on a global scale, whereas the other half is due to primary emissions (mainly from sea salt, desert dust, biomass and fossil fuel burning, plant debris, etc.). The health effects of nanoparticles are a second major topic of active research as the tiny particles can penetrate deep into the human body, which can make them potentially more harmful compared with larger particles. However, atmospheric new particle formation and growth is not fully understood yet despite its importance for climate and human health.

The most important factors influencing the new particle formation rate (number of new particles produced per volume and time) include the presence of suitable trace gases, their concentrations, temperature, and, in the case of ion-induced nucleation, also the ionization rate. Compounds that have been identified to be involved in nucleation are mainly sulfuric acid, ammonia, amines, water vapor, iodine oxides, and highly oxygenated organic compounds. However, in most cases one compound alone cannot efficiently nucleate at atmospheric concentrations. Therefore, mixtures of gases (e.g., the ternary system of sulfuric acid, water vapor and ammonia) are generally required and it is not sufficient to measure just one compound in order to characterize NPF. Another important factor that complicates research on atmospheric nucleation is the fact that the relevant trace gases occur only at tiny concentrations. Peak sulfuric acid mixing ratios are usually only around 0.1 pptv (pptv = parts per trillion by volume, i.e., 0.1 pptv corresponds to 1 molecule in 1013 other air molecules). Yet such a minuscule concentration can have a strong impact regarding the health and climate aspects (see above).

The nitrate CI-APi-TOF is a time-of-flight mass spectrometer with chemical ionisation using the nitrate ion. This device has a mass resolution of up to 12,000 and a mass range of up to 2000 Da. The ion source uses a corona discharge to generate the primary ions (HNO₃)_0–2NO₃^– from nitric acid, which can ionise trace gases with particularly low volatilities, e.g. sulfuric acid, methanesulfonic acid, iodic acid or highly oxidised organic molecules (HOM), and thus make them measurable in the device. The nitrate CI-APi-TOF is calibrated for the measurement of gaseous sulfuric acid (Kürten et al. 2012) and a transmission calibration enables the derivation of concentrations for other masses (Heinritzi et al. 2016).

This is achieved by mixing defined amounts of nitrogen, oxygen, sulfur dioxide and water vapor; the gas mixture flows through a quartz glass tube that can be illuminated with filtered UV light (185 nm from a Hg-lamp) of known intensity. A simple chemistry model is used to calculate the generated sulfuric acid concentration from reactions initiated by the photolysis of water (producing OH radicals). The calibration set-up is portable and can be used during field and laboratory experiments. The estimated systematic error of the calibration factor derived for the nitrate CI-APi-TOF is ~30%.

The calibration factor for sulfuric acid can also be applied for the measurement of highly oxygenated organic molecules (HOM). Since the HOM measured by the mass spectrometer cover a wide mass range (from ca. 200 to 650 Th) a second calibration needs be performed, which takes into account that the transmission efficiency of the mass spectrometer depends on the mass-to-charge ratio of the measured ions. This requires the calibration method described by Heinritzi et al. (2016). It is emphasized, however, that this second calibration is independent of the sulfuric acid calibration and only needed for HOM measurements.

• Ultrafine Condensation Particle Counter (CPC) using butanol as condensing fluid
• Particle Size Magnifier 
• Nano Differential Mobility Analyzer (nano DMA)
• Nano and long Scanning Mobility Particle Sizer (SMPS)
• Optical Particle Counter (OPC)
• Aerodynamic Particle Sizer (APS)
• Parallel-Plate High Resolution DMA 
• Trace gas monitors for SO2, O3, NOx, CO2 and CO

• BACCHUS
• CLOUD-ITN
• CLOUD-TRAIN
• INUIT
• PARADE
• REACT-EU