Observations of new particle formation in enhanced UV irradiance zones near cumulus clouds

Abstract

Abstract. During the CARRIBA (Cloud, Aerosol, Radiation and tuRbulence in the trade wInd regime over BArbados) campaign, the interaction between aerosol particles and cloud microphysical properties was investigated in detail, which also includes the influence of clouds on the aerosol formation. During two intensive campaigns in 2010 and 2011, helicopter-borne measurement flights were performed to investigate the thermodynamic, turbulent, microphysical, and radiative properties of trade-wind cumuli over Barbados. During these flights, 91 cases with increased aerosol particle number concentrations near clouds were detected. The majority of these cases are also correlated with enhanced irradiance in the ultraviolet (UV) spectral wavelength range. This enhancement reaches values up to a factor of 3.3 greater compared to background values. Thus, cloud boundaries provide a perfect environment for the production of precursor gases for new particle formation. Another feature of cloud edges is an increased turbulence, which may also enhance nucleation and particle growth. The observed events have a mean length of 100 m, corresponding to a lifetime of less than 300 s. This implies that particles with diameters of at least 7 nm grew several nanometers per minute, which corresponds to the upper end of values in the literature (Kulmala et al., 2004). Such high values cannot be explained by sulfuric acid alone; thus extremely low volatility organic compounds (ELVOCs) are probably involved here.

Aerosol arriving on the Caribbean island of Barbados: physical properties and origin

“…But based on the modeling results by Korhonen et al (2008) and Merikanto et al (2009), it can be assumed that the recently nucleated particles likely originated in the free troposphere, at least to a large extent. Alternatively, they were formed in the vicinity of trade wind cumuli, as suggested by Wehner et al (2015). According to Merikanto et al (2009), these small particles will eventually grow sufficiently so that they add to keeping up N CCN , to which they contribute roughly half of all particles in the marine aerosol (Merikanto et al, 2009).…”

“…Growth rates observed in the tropical and subtropical MBL were reported to be roughly 1 to 6 nm h −1 in a review by Kulmala et al (2004). Based on measurements made during the CARRIBA campaigns, Wehner et al (2015) reported additional events of new particle formation with much higher growth rates of up to several nanometers per minute for particles with diameters of roughly 10 nm. These events occurred just outside of clouds, in isolated air parcels with an extent of several tens of meters.…”

Atmospheric new particle formation and growth: review of field observations

“…Several explanations for this phenomenon were suggested, including enhanced UV irradiance and turbulence in the vicinity of clouds (e.g. Wehner et al 2015).…”

Introductory lecture: atmospheric chemistry in the Anthropocene

“…Given their very low vapor pressures, ELVOC are believed to be important in the earliest stages of particle formation and growth in air, while somewhat higher volatility compounds contribute more to their subsequent growth. 353,363,366,367,369,374,[380][381][382][383] However, analysis of SOA composition in some eld studies suggests that more volatile products can be found in SOA than expected based on their volatility. 384 This may be related to the particle phase and growth mechanisms.…”