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Abstract Aircraft observations have revealed ubiquitous new particle formation in the tropical upper troposphere over the Amazon^1,2and the Atlantic and Pacific oceans^3,4. Although the vapours involved remain unknown, recent satellite observations have revealed surprisingly high night-time isoprene mixing ratios of up to 1 part per billion by volume (ppbv) in the tropical upper troposphere⁵. Here, in experiments performed with the CERN CLOUD (Cosmics Leaving Outdoor Droplets) chamber, we report new particle formation initiated by the reaction of hydroxyl radicals with isoprene at upper-tropospheric temperatures of −30 °C and −50 °C. We find that isoprene-oxygenated organic molecules (IP-OOM) nucleate at concentrations found in the upper troposphere, without requiring any more vapours. Moreover, the nucleation rates are enhanced 100-fold by extremely low concentrations of sulfuric acid or iodine oxoacids above 10⁵ cm⁻³, reaching rates around 30 cm⁻³ s⁻¹at acid concentrations of 10⁶ cm⁻³. Our measurements show that nucleation involves sequential addition of IP-OOM, together with zero or one acid molecule in the embryonic molecular clusters. IP-OOM also drive rapid particle growth at 3–60 nm h⁻¹. We find that rapid nucleation and growth rates persist in the presence of NO_xat upper-tropospheric concentrations from lightning. Our laboratory measurements show that isoprene emitted by rainforests may drive rapid new particle formation in extensive regions of the tropical upper troposphere^1,2, resulting in tens of thousands of particles per cubic centimetre. 

Abstract. Due to its remote location and extreme weather conditions, atmospheric in situmeasurements are rare in the Southern Ocean. As a result, aerosol–cloudinteractions in this region are poorly understood and remain a major source ofuncertainty in climate models. This, in turn, contributes substantially topersistent biases in climate model simulations such as the well-known positiveshortwave radiation bias at the surface, as well as biases in numericalweather prediction models and reanalyses. It has been shown in previousstudies that in situ and ground-based remote sensing measurements across theSouthern Ocean are critical for complementing satellite data sets due to theimportance of boundary layer and low-level cloud processes. These processesare poorly sampled by satellite-based measurements and are often obscured bymultiple overlying cloud layers. Satellite measurements also do not constrainthe aerosol–cloud processes very well with imprecise estimation of cloudcondensation nuclei. In this work, we present a comprehensive set of ship-basedaerosol and meteorological observations collected on the 6-weekSouthern Ocean Ross Sea Marine Ecosystem and Environment voyage(TAN1802) voyage of RV Tangaroa across the Southern Ocean, from Wellington, New Zealand, tothe Ross Sea, Antarctica. The voyage was carried out from 8 February to21 March 2018. Many distinct, but contemporaneous, data sets were collectedthroughout the voyage. The compiled data sets include measurements from arange of instruments, such as (i) meteorological conditions at the sea surfaceand profile measurements; (ii) the size and concentration of particles; (iii)trace gases dissolved in the ocean surface such as dimethyl sulfide andcarbonyl sulfide; (iv) and remotely sensed observations of low clouds. Here,we describe the voyage, the instruments, and data processing, and provide a briefoverview of some of the data products available. We encourage the scientificcommunity to use these measurements for further analysis and model evaluationstudies, in particular, for studies of Southern Ocean clouds, aerosol, andtheir interaction. The data sets presented in this study are publiclyavailable at https://doi.org/10.5281/zenodo.4060237 (Kremser et al., 2020).