The Amazon Basin is a unique region to study atmospheric aerosols, given their relevance for the regional hydrological cycle and the large uncertainty of their sources. Multi-year datasets are crucial when contrasting periods of natural conditions and periods influenced by anthropogenic emissions. In the wet season, biogenic sources and processes prevail, and the Amazonian atmospheric composition resembles preindustrial conditions. In the dry season, the basin is influenced by widespread biomass burning emissions. This work reports multi-year observations of high time resolution submicrometer (10-600 nm) particle number size distributions at a rain forest site in Amazonia (TT34 tower, 60 km NW from Manaus city), between 2008 and 2010 and 2012 and 2014. The median particle number concentration was 403 cm(-3) in the wet season and 1254 cm(-3) in the dry season. The Aitken mode (similar to 30-100 nm in diameter) was prominent during the wet season, while the accumulation mode (similar to 100-600 nm in diameter) dominated the particle size spectra during the dry season. Cluster analysis identified groups of aerosol number size distributions influenced by convective downdrafts, nucleation events and fresh biomass burning emissions. New particle formation and subsequent growth was rarely observed during the 749 days of observations, similar to previous observations in the Amazon Basin. A stationary 1-D column model (ADCHEM Aerosol Dynamics, gas and particle phase CHEMistry and radiative transfer model) was used to assess the importance of the processes behind the observed diurnal particle size distribution trends. Three major particle source types are required in the model to reproduce the observations: (i) a surface source of particles in the evening, possibly related to primary biological emissions; (ii) entrainment of accumulation mode aerosols in the morning; and (iii) convective downdrafts transporting Aitken mode particles into the boundary layer mostly during the afternoon. The latter process has the largest influence on the modeled particle number size distributions. However, convective downdrafts are often associated with rain and, thus, act as both a source of Aitken mode particles and a sink of accumulation mode particles, causing a net reduction in the median total particle number concentrations in the surface layer. Our study shows that the combination of the three mentioned particle sources is essential to sustain particle number concentrations in Amazonia.

Although elevated surface ozone (O-3) concentrations are observed in many areas within southern Africa, few studies have investigated the regional atmospheric chemistry and dominant atmospheric processes driving surface O-3 formation in this region. Therefore, an assessment of comprehensive continuous surfaceO(3) measurements performed at four sites in continental South Africa was conducted. The regional O-3 problem was evident, with O-3 concentrations regularly exceeding the South African air quality standard limit, while O-3 levels were higher compared to other background sites in the Southern Hemisphere. The temporal O-3 patterns observed at the four sites resembled typical trends for O-3 in continental South Africa, with O-3 concentrations peaking in late winter and early spring. Increased O-3 concentrations in winter were indicative of increased emissions of O-3 precursors from household combustion and other low-level sources, while a spring maximum observed at all the sites was attributed to increased regional biomass burning. Source area maps of O-3 and CO indicated significantly higher O-3 and CO concentrations associated with air masses passing over a region with increased seasonal open biomass burning, which indicated CO associated with open biomass burning as a major source of O-3 in continental South Africa. A strong correlation between O-3 on CO was observed, while O-3 levels remained relatively constant or decreased with increasing NOx, which supports a VOC-limited regime. The instantaneous production rate of O-3 calculated at Welgegund indicated that similar to 40 % of O-3 production occurred in the VOC- limited regime. The relationship between O-3 and precursor species suggests that continental South Africa can be considered VOC limited, which can be attributed to high anthropogenic emissions of NOx in the interior of South Africa. The study indicated that the most effective emission control strategy to reduce 03 levels in continental South Africa should be CO and VOC reduction, mainly associated with household combustion and regional open biomass burning.

In September 2017, we conducted a proton-transfer-reaction mass-spectrometry (PTR-MS) intercomparison campaign at the CESAR observatory, a rural site in the central Netherlands near the village of Cabauw. Nine research groups deployed a total of 11 instruments covering a wide range of instrument types and performance. We applied a new calibration method based on fast injection of a gas standard through a sample loop. This approach allows calibrations on timescales of seconds, and within a few minutes an automated sequence can be run allowing one to retrieve diagnostic parameters that indicate the performance status. We developed a method to retrieve the mass-dependent transmission from the fast calibrations, which is an essential characteristic of PTR-MS instruments, limiting the potential to calculate concentrations based on counting statistics and simple reaction kinetics in the reactor/drift tube. Our measurements show that PTR-MS instruments follow the simple reaction kinetics if operated in the standard range for pressures and temperature of the reaction chamber (i.e. 1-4 mbar, 30-120 degrees, respectively), as well as a reduced field strength E/N in the range of 100-160 Td. If artefacts can be ruled out, it becomes possible to quantify the signals of uncalibrated organics with accuracies better than +/- 30 %. The simple reaction kinetics approach produces less accurate results at E/N levels below 100 Td, because significant fractions of primary ions form water hydronium clusters. Deprotonation through reactive collisions of protonated organics with water molecules needs to be considered when the collision energy is a substantial fraction of the exoergicity of the proton transfer reaction and/or if protonated organics undergo many collisions with water molecules.