Fall 2023 Seminar Series

Zoom Recording

"Insight Into the Changing Sources of Ambient Aerosol in the Troposphere and to Celebrate Richard Honrath's Legacy to Instruction and Research on the Environment"

The study of atmospheric chemistry as a scientific discipline goes back to the eighteenth century, when the principal issue was identifying the major chemical components of the atmosphere, nitrogen, oxygen, water, carbon dioxide, and the noble gases. In the late nineteenth and early twentieth centuries attention turned to the so-called trace gases, species present at less than 1 part per million parts of air by volume (1 µmol per mole). We now know that the atmosphere contains a myriad of trace species, some at levels as low as 1 part per trillion parts of air. The role of trace species is disproportionate to their atmospheric abundance; they are responsible for phenomena ranging from urban photochemical smog, to acid deposition, to stratospheric ozone depletion, to potential climate change. Moreover, the composition of the atmosphere is changing; analysis of air trapped in ice cores reveals a record of striking increases in the long-lived so-called greenhouse gases, carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). Within the last century, concentrations of tropospheric ozone (O3), sulfate (SO42–), and carbonaceous aerosols in the Northern Hemisphere have increased significantly. There is evidence that all these changes are altering the basic chemistry of the atmosphere.

The chemical constituents of the atmosphere do not go through their life cycles independently; the cycles of the various species are linked together in a complex way. Thus a perturbation of one component can lead to significant, and nonlinear, changes to other components and to feedbacks that can amplify or damp the original perturbation.

In many respects, at once both the most important and the most paradoxical trace gas in the atmosphere is ozone (O3). High in the stratosphere, ozone screens living organisms from biologically harmful solar ultraviolet radiation; ozone at the surface, in the troposphere, can produce adverse effects on human health and plants when present at levels elevated above natural. At the urban and regional scale, significant policy issues concern how to decrease ozone levels by controlling the ozone precursors – hydrocarbons and oxides of nitrogen. At the global scale, understanding both the natural ozone chemistry of the troposphere and the causes of continually increasing background tropospheric ozone levels is a major goal.

Aerosols are particles suspended in the atmosphere. They arise directly from emissions of particles and from the conversion of certain gases to particles in the atmosphere. At elevated levels they inhibit visibility and are a human health hazard. There is a growing body of epidemiological data suggesting that increasing levels of aerosols may cause a significant increase in human mortality. For many years it was thought that atmospheric aerosols did not interact in any appreciable way with the cycles of trace gases. We now know that particles in the air affect climate and interact chemically in heretofore unrecognized ways with atmospheric gases.

Historically the study of urban air pollution and its effects occurred more or less separately from that of the chemistry of the Earth's atmosphere as a whole. Similarly, in its early stages, climate research focused exclusively on CO2, without reference to effects on the underlying chemistry of the atmosphere and their feedbacks on climate itself. It is now recognized, in quantitative scientific terms, that the Earth's atmosphere is a continuum of spatial scales in which the urban atmosphere, the remote troposphere, the marine boundary layer, and the stratosphere are merely points from the smallest turbulent eddies and the fastest timescales of free-radical chemistry to global circulations and the decadal timescales of the longest-lived trace gases.

Biography - John Seinfeld is the Louis E. Nohl Professor at Caltech, in the department of Chemical Engineering. Professor Seinfeld graduated in 1967 with a Ph.D. in Chemical Engineering from Princeton University and joined the Caltech faculty in 1967. Professor Seinfeld has graduated over 100 PhD's, many of whom are, themselves, professors. His research has focused on the chemistry and physics of the Earth's atmosphere. In 2023, he was ranked first in the world in publications in the field of Environmental Sciences. He is delighted to have been invited to present the Honrath Memorial Lecture, and to celebrate Richard Honrath's legacy to instruction and research on the environment.

The Richard E. Honrath Memorial Lecture is a Joint EPSSI/Environmental Engineering Graduate Seminar honoring the memory of Richard E. Honrath Jr., who was a faculty member in the Civil, Environmental, and Geospatial Engineering and Geological and Mining Engineering and Sciences Departments. Professor Honrath was a co-founder of Michigan Tech's Atmospheric Sciences Doctoral Degree Program. He died tragically in a kayaking accident in April 2009.

Zoom Recording

"Measurements of small ice particle growth rates to be used in cirrus cloud models"

Cirrus clouds are often composed of small ice crystals and their growth is poorly understood. Many begin as spherical frozen droplets, but develop into complex shapes, or “habits”. For an ice habit to form, water vapor must deposit more efficiently onto some surfaces than others. This growth efficiency is called a deposition coefficient, α. In numerical cloud models, the deposition coefficient is often assumed to be unity, meaning that all vapor molecules incorporate into the crystalline lattice. This would force the small crystals to remain spherical, which contradicts our recent in-cloud observations that showed branched and hollowed particles even at small sizes (maximum dimension < 100 µm). Here, we present data-driven approximations for the deposition coefficient and the processes that produce hollowing and branching. These approximations are derived from almost 300 single-particle growth experiments of small, cirrus-like, ice crystals. The particles grew in a thermal-gradient diffusion chamber at cirrus temperatures (-65 to -40°C) and supersaturations ranging from ice- to liquid-equilibrium. At low supersaturation, the measured growth is well represented by a supersaturation-dependent deposition coefficient function. At high supersaturation the influence of hollows and branches is characterized by a simple power-law. The approximations based on these data can easily be incorporated into cloud and weather forecasting microphysical models.

Zoom Recording

"Deepening Insights into Tropical Convection: Leveraging Satellites, High-Resolution Models, and Data Assimilation Techniques"

Tropical convection plays a major role in Earth’s water and energy cycles. It redistributes energy, moisture, and momentum vertically, and produces tremendous amounts of precipitation in the tropics. Heating released by tropical convection is the major driver of the atmospheric general circulation, and clouds associated with tropical convection modulate considerably the global radiation balance. However, tropical convection often occurs over regions with sparse in situ observations. As a result, our knowledge of the physical processes governing the evolution of tropical convection is still incomplete, and accurate model predictions of tropical convection remain elusive.

In this talk, I will introduce some of our recent work examining the intricate interplay between tropical convection and the large-scale environment. Using a recently developed satellite-based global cloud tracking and classification dataset, I will show that shallow, isolated deep, and organized convection play sequential roles in the precipitation-moisture coupling cycle over the tropics. Organized convection (i.e., mesoscale convective systems, or MCSs) becomes the dominant precipitation type as the troposphere approaches saturation, with a rapid increase in MCS precipitation area. We further found that, in addition to moisture, environmental deep-layer (~surface-400 hPa) wind shear also plays a crucial role in modulating the precipitation of tropical MCSs, mainly by enhancing the heavy precipitating convective activity within MCSs. Using high-resolution models, I will further show how the organization of tropical deep convection will modulate vertical mass, water, and energy transports, and further influence the onset of monsoon systems

In the second half of the talk, I will discuss the limitations of using either satellite observations or numerical models to study tropical convection. To address these limitations, data assimilation is emerging as a valuable statistical approach that integrates information from both observations and model simulations to provide more accurate insights into tropical convection. Using data assimilation, we have recently developed a new reanalysis dataset of tropical convection. This dataset serves as an important tool for our group to study the scale interactions between tropical convection and its environment. I will illustrate this with an interesting case study where gravity waves play an important role in modulating the formation of a tropical cyclone.

Zoom Recording

The Atmospheric Aging and Impacts of Black Carbon

Black carbon (BC) particles originating from fossil fuel combustion and biomass burning contribute substantially to climate warming both directly through the absorption of solar radiation (aerosol-radiation interactions), and indirectly by changing cloud properties such as cloud amount and lifetime (aerosol-cloud interactions). Quantifying the radiative impacts of these BC particles remains a significant challenge that ultimately stems from a poor understanding of the fundamental mechanisms governing the evolution of the structural, chemical, optical, and hygroscopic properties of these particles, and their large spatial and temporal heterogeneity during atmospheric transport. In this seminar, I will highlight some of my past work on the effects of coating distribution and evolving aggregate morphology on the optical properties of aged BC particles (from fossil fuel combustion), and my current work on quantifying the impacts of cloud processing on the size distribution of BC particles formed from biomass burning (wildfires) and the subsequent changes in optical properties resulting from these interactions. Specific results include experimentally constrained modeling simulations of BC optical properties and laboratory measurements of the cloud droplet activation of BC particles.