Aerosol measurements during COPE: composition, size, and sources of CCN and INPs at the interface between marine and terrestrial influences

Abstract

Abstract. Heavy rainfall from convective clouds can lead to devastating flash flooding, and observations of aerosols and clouds are required to improve cloud parameterisations used in precipitation forecasts. We present measurements of boundary layer aerosol concentration, size, and composition from a series of research flights performed over the southwest peninsula of the UK during the COnvective Precipitation Experiment (COPE) of summer 2013. We place emphasis on periods of southwesterly winds, which locally are most conducive to convective cloud formation, when marine air from the Atlantic reached the peninsula. Accumulation-mode aerosol mass loadings were typically 2-3 µg m −3 (corrected to standard cubic metres at 1013.25 hPa and 273.15 K), the majority of which was sulfuric acid over the sea, or ammonium sulfate inland, as terrestrial ammonia sources neutralised the aerosol. The cloud condensation nuclei (CCN) concentrations in these conditions were ∼ 150-280 cm −3 at 0.1 % and 400-500 cm −3 at 0.9 % supersaturation (SST), which are in good agreement with previous Atlantic measurements, and the cloud drop concentrations at cloud base ranged from 100 to 500 cm −3 . The concentration of CCN at 0.1 % SST was well correlated with non-sea-salt sulfate, meaning marine sulfate formation was likely the main source of CCN. Marine organic aerosol (OA) had a similar mass spectrum to previous measurements of sea spray OA and was poorly correlated with CCN.In one case study that was significantly different to the rest, polluted anthropogenic emissions from the southern and central UK advected to the peninsula, with significant enhancements of OA, ammonium nitrate and sulfate, and black carbon. The CCN concentrations here were around 6 times higher than in the clean cases, and the cloud drop number concentrations were 3-4 times higher.Sources of ice-nucleating particles (INPs) were assessed by comparing different parameterisations used to predict INP concentrations, using measured aerosol concentrations as input. The parameterisations based on total aerosol produced INP concentrations that agreed within an order of magnitude with measured first ice concentrations at cloud temperatures as low as −12 • C. Composition-specific parameterisations for mineral dust, fluorescent particles, and sea spray OA were 3-4 orders of magnitude lower than the measured first ice concentrations, meaning a source of INPs was present that was not characterised by our measurements and/or one or more of the composition-specific parameterisations greatly underestimated INPs in this environment.

Aerosol–cloud interactions in mixed-phase convective clouds – Part 1: Aerosol perturbations

“…The selected case (3 August 2013) has been previously analysed from an observational viewpoint with a focus on cloud glaciation (Taylor et al, 2016b) and aerosol concentrations, composition, and sources (Taylor et al, 2016a). Isolated shallow cumulus clouds were scattered across most of the southwestern UK in the early morning.…”

Author responses to Reviewer 2

“…Köhler theory is limited to non-volatile compounds, so it does not consider the effects of compounds of ranging volatility in the atmosphere. It has been shown that SVOCs increase the tendency for activation of CCN, which consequently affects radiative properties of clouds and hence the necessity to quantify their influence (Topping et al, 2013).…”

“…The former process increases the number concentration of aerosol particles while the latter increases the size, and consequently soluble mass, of existing aerosol particles. The enlarged size and altered chemical composition of the particles has a dominant effect on cloud droplet number (Dusek et al, 2006;Topping et al, 2013), and so uncertainties in the amount and composition of secondary organic aerosol mass translate into large uncertainties in cloud properties.…”

Uncertainty in aerosol hygroscopicity resulting from semi-volatile organic compounds

“…The CCN-based approach to calculating κ requires integrating the aerosol size distribution to find the diameter, D d , above which particles activate. The aerosol size distribution, in this case, is measured at subsaturated conditions, typically ≈ 50 % RH (Taylor et al, 2016). The number of particles that activate, however, is controlled Table 3, and the vertical error bars show the 16th and 84th quantiles of our derived values.…”

“…In field measurements, atmospheric aerosol is passed through instruments under subsaturated conditions in order to measure the size distribution and composition (Taylor et al, 2016), while the number of CCN is calculated under supersaturated conditions. Including the production, condensation, evaporation, reaction and oxidation of SVOCs directly in large-scale models is very computationally expensive and is rarely carried out, especially for more than one compound.…”