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Abstract. Most precipitation from deep clouds over the continents and in the intertropical convergence zone is strongly influenced by the presence of ice crystals whose formation requires the presence of ice nucleating particles (INPs). Although there are a large number of INP sources, the ice nucleating abilities of aerosol particles originating from oceans, deserts, and wildfires are poorly described at tropical latitudes. To fill this gap in knowledge, the National Autonomous University of Mexico micro-orifice uniform deposit impactor droplet freezing technique (UNAM-MOUDI-DFT) was constructed to measure the ice nucleating activity of aerosol samples that were collected in Sisal and Mérida, Yucatán (Mexico) under the influence of cold fronts, biomass burning (BB), and African dust (AD) intrusions during five short-term field campaigns between January 2017 and July 2018. The three different aerosol types were distinguished by their physicochemical properties. Marine aerosol (MA), BB, and AD air masses were found to contain INPs; the highest concentrations were in AD (from 0.071 to 36.07 L−1 at temperatures between −18 and −27 ∘C), followed by MA (from 0.068 to 18.90 L−1 at temperatures between −15 and −28 ∘C) and BB (from 0.063 to 10.21 L−1 at temperatures between −20 and −27 ∘C). However, MA had the highest surface active site densities (ns) between −15 and −30 ∘C. Additionally, supermicron particles contributed more than 72 % of the total INP concentration independent of aerosol type.

Biomass burning (BB) emissions and African dust (AD) are often associated with poor regional air quality, particularly in the tropics. The Yucatan Peninsula is a fairly pristine site due to predominant trade winds, but occasionally BB and AD plumes severely degrade its air quality. The African Dust And Biomass Burning Over Yucatan (ADABBOY) project (Jan 2017- Aug 2018) was conducted in the Yucatan Peninsula to characterize physical and biological properties of particulate pollution at remote seaside and urban sites. The 18-month long project quantified the large interannual variability in frequency and spatial extent of BB and AD plumes. Remote and urban sites experienced air quality degradation under the influence of these plumes, with up to 200 and 300% increases in coarse particle mass under BB and AD influence, respectively. ADABBOY is the first project to systematically characterize elemental composition of airborne particles as a function of these sources and identify bioaerosol over Yucatan. Bacteria, actinobacteria (both continental and marine) and fungi propagules vary seasonally and interannually and revealed the presence of very different species and genera associated with different sources. A novel contribution of ADABBOY was the determination of the ice-nucleating abilities of particles emitted by different sources within an under-sampled region of the world. BB particles were found to be inefficient ice nucleating particles at temperatures warmer than -20°C, whereas both AD and background marine aerosol activated ice nucleating particles below -10°C.

Abstract. Most precipitation from deep clouds over the continents and in the intertropical convergence zone is strongly influenced by the presence of ice crystals, whose formation requires the presence of ice nucleating particles (INP). Although there are a large number of INP sources, the ice nucleating abilities of aerosol particles emitted from oceans, deserts, and wildfires are poorly described at tropical latitudes. To fill this gap in knowledge, the UNAM-MicroOrifice Uniform Deposit Impactor-Droplet Freezing Technique (UNAM-MOUDI-DFT) was built. Aerosol samples were collected in Sisal and Merida, Yucatan (Mexico) under the influence of cold fronts, biomass burning (BB), and African dust (AD), during five short-term field campaigns between January 2017 and July 2018. The three different aerosol types were distinguished by characterizing their physicochemical properties. Marine aerosol (MA), BB, and AD air masses were found to contain INP; the highest concentrations were found for AD (from 0.071 L−1 to 36.07 L−1), followed by MA (from 0.068 L−1 to 18.90 L−1), and BB (from 0.063 L−1 to 10.21 L−1). However, MA had the highest surface active site density (ns) between −15 °C and −30 °C. Additionally, supermicron particles contributed more than 72 % of the total INP concentration independent of aerosol type; MA had the largest contribution from supermicron particles.

Los núcleos de glaciación (INPs, por su sigla en inglés) presentes en la atmósfera intervienen en la formación de cristales de hielo, los cuales son indispensables para el desarrollo de precipitación en nubes mixtas. Las fuentes y la composición de los INPs son muy variadas: desde el polvo mineral derivado de la erosión de los suelos en los continentes hasta el bioaerosol emitido en la superficie del océano. El dispositivo denominado ensayo de congelación de gotas (DFA, por su sigla en inglés) se construyó recientemente para cuantificar la eficiencia de nucleación de hielo en muestras líquidas mediante la congelación por inmersión. Su funcionamiento se validó comparando los resultados con reportes en la literatura y con los análisis de muestras de la microcapa superficial del océano (SML) y de muestras obtenidas a 1 m de profundidad (BSW) en el Golfo de México (GoM) y en Saanich, frente a la Isla de Vancouver (VI), Canadá. Todas las muestras analizadas contenían INPs en concentraciones moderadas, entre 6.0x10 1 y 1.1x10 5 L-1 de agua, incluso en ausencia de floraciones de fitoplancton. Se estimó la temperatura a la cual se congela el 50% de las gotas (T50) en cada una de las muestras. El valor de T50 fue mayor en muestras de VI SML, seguido por muestras del GoM BSW y del GoM SML, lo que indica que las aguas costeras en latitudes altas tienen un mayor potencial para iniciar la formación de nubes y precipitación.