Tar balls are atmospheric particles that are abundant in slightly aged biomass burning smoke and have a significant, but highly uncertain, role in Earth’s radiative balance. Tar balls are typically detected using electron microscopy and generally, they are observed as individual spheres. Here, we report new observations of a significant fraction of tar ball aggregates (∼27% by number) from samples collected in a plume of the Whitewater-Baldy Complex fire in New Mexico. The structure of these aggregates is fractal-like and follows a scale invariant power law similar to that of soot particles, despite the considerably larger size and smaller number of monomers. We also present observations of tar ball aggregates from four other geographical locations, including a remote high-elevation site in the North Atlantic Ocean. Aggregation affects the particle optical properties and, therefore, their climatic impact. We performed numerical simulations based on the observed morphology and estimated the effects of aggregation on the optical properties of the tar balls. On the basis of single-particle numerical simulations, we find that aggregates had a single scattering albedo up to 41% and 23% higher than that of individual tar balls at 550 nm and 350 nm, respectively.

We demonstrate precise determination of atmospheric temperature using vibro-rotational Raman (VRR) spectra of molecular nitrogen and oxygen in the range of 292–293 K. We used a continuous wave fiber laser operating at 10 W near 532 nm as an excitation source in conjunction with a multi-pass cell. First, we show that the approximation that nitrogen and oxygen molecules behave like rigid rotors leads to erroneous derivations of temperature values from VRR spectra. Then, we account for molecular non-rigidity and compare four different methods for the determination of air temperature. Each method requires no temperature calibration. The first method involves fitting the intensity of individual lines within the same branch to their respective transition energies. We also infer temperature by taking ratios of two isolated VRR lines; first from two lines of the same branch, and then one line from the S-branch and one from the O-branch. Finally, we take ratios of groups of lines. Comparing these methods, we found that a precision up to 0.1 K is possible. In the case of O2, a comparison between the different methods show that the inferred temperature was self-consistent to within 1 K. The temperature inferred from N2 differed by as much as 3 K depending on which VRR branch was used. Here we discuss the advantages and disadvantages of each method. Our methods can be extended to the development of instrumentation capable of non-invasive monitoring of gas temperature with broad potential applications, for example, in laboratory, ground-based, or airborne remote sensing.

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