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Abstract. In situ observations of spectrally-resolved aerosol extinction coefficients (300–700 nm at ~ 0.8 nm resolution) from the May–June 2016 Korea U.S. – Ocean Color (KORUS-OC) oceanographic field campaign are reported. Measurements were made with the custom-built Spectral Aerosol Extinction (SpEx) instrument that previously has been characterized only using laboratory-generated aerosols of known size and composition. Here, the performance of SpEx under realistic operating conditions in the field was assessed by comparison to extinction coefficients derived from commercial instruments that measured scattering and filter-based absorption coefficients at three discrete visible wavelengths. Good agreement was found between these two sets of extinction coefficients with slopes near unity for all 3 wavelengths within the SpEx measurement error (±5 Mm−1). The meteorological conditions encountered during the cruise fostered diverse ambient aerosol populations with varying sizes and composition at concentrations spanning two orders of magnitude. The sampling inlet had a 50 % size cut of 1.3 µm diameter particles such that the in situ aerosol sampling suite deployed aboard ship measured fine mode aerosols only. The extensive hyperspectral extinction data set acquired revealed that nearly all measured spectra exhibited curvature in logarithmic space, such that Ångström exponent (α) power law fits led to large errors compared to measured values, especially in the ultraviolet (UV) wavelength range. This problem was particularly acute for α values calculated over only visible wavelengths, then extrapolated to the UV, highlighting the need for measurements in this wavelength range. Second-order polynomial fits to the logarithmically-transformed data provided a much better fit to the measured spectra than the linear fits of power laws. Building on previous studies that used total column AOD observations to examine the information content of spectral curvature, the relationship between α and the second order polynomial fit coefficients (a1 and a2) was shown to depend on the characteristic wavelength (λch) of any given spectral measurement, such that differing curvature among aerosol size distributions with the same α will map to a line in (a1,a2) space with a slope related to λch. Thus, spectral curvature represented by (a1,a2) may provide more detailed aerosol size distribution information than α alone.

<p><strong>Abstract.</strong> The AErosol RObotic NETwork (AERONET) program over the past 24 years has provided highly accurate remote sensing characterization of aerosol optical and physical properties for an increasingly extensive geographic distribution that includes all continents and many island sites. The measurements and retrievals from the AERONET global network have addressed satellite and model validation needs very well, but there have been challenges in making comparisons to similar parameters from in situ surface and airborne measurements. Additionally, with improved spatial and temporal satellite remote sensing of aerosols, there is a need for higher spatial resolution ground-based remote sensing networks. An effort to address this need resulted in a number of field campaign networks called Distributed Regional Aerosol Gridded Observation Networks (DRAGONs) that were designed to provide a database for in situ and remote sensing comparison and analysis of local to meso-scale variability of aerosol properties. This paper describes the networks that that have contributed and will continue to contribute to that body of research. The research presented in this special issue illustrates the diversity of topics that has resulted from the application of data from these networks.</p>

Abstract— Fluorescence yields (πf,'s) and polarizations (P) are measured for aqueous 5‐methylcytosine (˜ 0.1 mM) at 20°C as a function of pH over the range 2–14. Both properties change abruptly and in parallel at pH's corresponding to the known pKa values. Polarizations were also obtained for the 5‐methylcytosine cation, neutral and anion species in ethylene glycol water glass at ˜180K. The weak fluorescence of the neutral and cation species at 20°C was polarized almost as highly as at low temperature. When the fluorescence lifetimes are assumed to be correctly given by the product of calculated radiative lifetimes and quantum yields, the polarizations are found to be consistent with rotational diffusion rates ˜4 times faster than predicted from Stokes‐Einstein models for the neutral and anion species. The cation seemed to rotate about two times more slowly than the neutral and anion species. It was also shown that the properties of the three species are such that a plot of 1/P vs apparent πf in the pH range 2–11 is fortuitously linear.

Figure S1. Observations versus the no-het model. Scatter plots of observed versus modeled OH (upper right), HO2 (upper left), HO2/OH (lower left), and OH reactivity (lower right). Gray points are one-minute averages. Dashed red lines are factors of 1/1.4 and 1.4 times the fitted line (solid red).