Atmospheric chemistry research
Alicat has been cited in over 1,000 peer-reviewed research papers. The following papers focus on atmospheric chemistry and emerging technologies in that field. Contact us if you’d like your research to be highlighted.
The Southern Ocean region is one of the most pristine in the world and serves as an important proxy for the pre-industrial atmosphere. Improving our understanding of the natural processes in this region is likely to result in the largest reductions in the uncertainty of climate and earth system models. While remoteness from anthropogenic and continental sources is responsible for its clean atmosphere, this also results in the dearth of atmospheric observations in the region. Here we present a statistical summary of the latitudinal gradient of aerosol (condensation nuclei larger than 10 nm, CN10) and cloud condensation nuclei (CCN at various supersaturations) concentrations obtained from five voyages spanning the Southern Ocean between Australia and Antarctica from late spring to early autumn (October to March) of the 2017/18 austral seasons.
Three main regions of influence were identified: the northern sector (40–45◦ S), where continental and anthropogenic sources coexisted with background marine aerosol populations; the mid-latitude sector (45–65◦ S), where the aerosol populations reflected a mixture of biogenic and sea-salt aerosol; and the southern sector (65–70◦ S), south of the atmospheric polar front, where sea-salt aerosol concentrations were greatly reduced and aerosol populations were primarily biologically derived sulfur species with a significant history in the Antarctic free troposphere. The northern sector showed the highest number concentrations with median (25th to 75th percentiles) CN10 and CCN0.5 concentrations of 681 (388–839) cm−3 and 322 (105–443) cm−3 , respectively. Concentrations in the mid-latitudes were typically around 350 cm−3 and 160 cm−3 for CN10 and CCN0.5, respectively. In the southern sector, concentrations rose markedly, reaching 447 (298–446) cm−3 and 232 (186–271) cm−3 for CN10 and CCN0.5, respectively. The aerosol composition in this sector was marked by a distinct drop in sea salt and increase in both sulfate fraction and absolute concentrations, resulting in a substantially higher CCN0.5/CN10 activation ratio of 0.8 compared to around 0.4 for mid-latitudes.
Long-term measurements at land-based research stations surrounding the Southern Ocean were found to be good representations at their respective latitudes; however this study highlighted the need for more long-term measurements in the region. CCN observations at Cape Grim (40◦390 S) corresponded with CCN measurements from northern and mid-latitude sectors, while CN10 observations only corresponded with observations from the northern sector. Measurements from a simultaneous 2-year campaign at Macquarie Island (54◦300 S) were found to represent all aerosol species well. The southernmost latitudes differed significantly from both of these stations, and previous work suggests that Antarctic stations on the East Antarctic coastline do not represent the East Antarctic sea-ice latitudes well. Further measurements are needed to capture the long-term, seasonal, and longitudinal variability in aerosol processes across the Southern Ocean.
Humphries, R. S., Keywood, M. D., Gribben, S., McRobert, I. M., Ward, J. P., Selleck, P., Taylor, S., Harnwell, J., Flynn, C., Kulkarni, G. R., Mace, G. G., Protat, A., Alexander, S. P., & McFarquhar, G. (2021). Southern Ocean latitudinal gradients of cloud condensation nuclei. Atmospheric Chemistry and Physics, 21(16), 12757–12782. https://doi.org/10.5194/acp-21-12757-2021
The Particle Analysis by Laser Mass Spectrometry – Next Generation (PALMS-NG) instrument has been designed and tested for use on NASA’s ER-2, a stratospheric research aircraft, to characterize in-situ the chemical composition of particles. The strong convective storms that generally occur during the summertime over North America carry out particles from the troposphere to the stratosphere. The NASA DCOTSS research project aims to get a better understanding of the chemistry and composition of the stratosphere seeded with water and pollutants originally from the troposphere. PALMS-NG is an evolution of the existing laboratory and aircraft PALMS instruments. The main new feature consists of a bipolar time-of-flight mass spectrometer, which has a dual S shape and is named sTOF. In addition, optical improvements have led to a higher detection efficiency and larger size range of measurable particles than the original instruments. It can detect particles below 150 nm and up to several micrometers in diameter.
PALMS-NG can be decomposed in two distinct parts: (1) particle detection and size estimation and (2) chemical analysis. A particle enters the instrument under vacuum through a multiple stage inlet tube to allow control of particle speed while maximizing particle transmission. First, two 405nm continuous laser beams enable an estimate of the particle size from the scattering signal analysis. Second, a 193 nm ultraviolet (UV) pulsed laser ionizes the particle. PALMS-NG bi-polar mass spectrometer has two high voltage electrodes – one positive and one negative – that separate and extract the positive and negative ions resulting from the UV pulse hit. The ions enter the sTOF mass spectrometers and ultimately impact microchannel detection plates located at each side of the instrument. The measured time-of-flight of the ions gives a mass spectrum for the identification of the chemical components that composed the detected particle.
Jacquot, J., Shen, X., Slovacek, K., Schill, G., Lawler, M., Thomson, D., Froyd, K., Murphy, D., & Cziczo, D. J. (2021). Chemical characterization of stratospheric particles with the next generation of airborne laser mass spectrometer: PALMS NG. The American Association for Aerosol Research 39th Annual Conference. https://www.researchgate.net/publication/356289854_Chemical_characterization_of_ stratospheric_particles_with_the_next_generation_of_airborne_laser_mass_spectrometer_PALMS-NG
There is evidence that black carbon (BC) particles may affect cirrus formation and hence global climate by acting as potential ice nucleating particles (INPs) in the troposphere. Nevertheless, the ice nucleation (IN) ability of bare BC and BC coated with secondary organic aerosol (SOA) material remains uncertain. We have systematically examined the IN ability of 100-400 nm size-selected BC particles with different morphologies and different SOA coatings representative of anthropogenic (toluene and n-dodecane) and biogenic (β-caryophyllene) sources in the cirrus regime (-46 to -38 °C). Several aerosolized BC proxies were selected to represent different particle morphologies and oxidation levels. Atmospheric aging was further replicated with exposure of SOA-coated BC to OH.
The results demonstrate that the 400 nm hydrophobic BC types nucleate ice only at or near the homogeneous freezing threshold (-42 to -46 ˚C). Deposition IN, as opposed to purely homogeneous freezing, was observed to occur for some BC types between 100-200 nm within the investigated temperature range. More fractal BC particles did not consistently act as superior deposition INPs over more spherical ones. SOA coating generated by oxidizing β-caryophyllene with O3 did not seem to affect BC IN ability. However, SOA coatings generated from OH oxidation of various organic species did exhibit higher IN onset supersaturation ratio with respect to ice (SSi) compared with bare BC particles, with toluene SOA coating showing an increase of SSi by 0.1-0.15 while still below the homogeneous threshold. ndodecane and β-caryophyllene-derived SOA only froze in the homogeneous regime. We attribute the inhibition of IN ability to the filling of the pores on the BC surface by the SOA material coating. OH exposure levels of all SOA coating experiments, from an equivalent atmospheric 10 days to 90 days, did not render significant differences in IN potential. Our study suggests that BC particles with large sizes and/or oxidized surfaces generally exhibit better IN ability, and that the organic coating materials can inhibit ice formation.
Zhang, C., Zhang, Y., Wolf, M. J., Nichman, L., Shen, C., Onasch, T. B., Chen, L., & Cziczo, D. J. (2020). The effects of morphology, mobility size, and SOA material coating on the ice nucleation activity of black carbon in the cirrus regime. Atmospheric Chemistry and Physics. https://doi.org/10.5194/acp-2020-809