No abstract

Abstract. We evaluate the sensitivity of the size calibrations of two commercially available, high-resolution optical particle sizers to changes in aerosol composition and complex refractive index (RI). The Droplet Measurement Technologies Ultra-High Sensitivity Aerosol Spectrometer (UHSAS) and the TSI, Inc. Laser Aerosol Spectrometer (LAS) are two commonly used instruments for measuring the portion of the aerosol size distribution with diameters larger than nominally 60–90 nm. Both instruments illuminate particles with a laser and relate the single-particle light scattering intensity and count rate measured over a wide range of angles to the size-dependent particle concentration. While the optical block geometry and flow system are similar for each instrument, a significant difference between the two models is the laser wavelength (1054 nm for the UHSAS and 633 nm for the LAS) and intensity (about 100 times higher for the UHSAS), which may affect the way each instrument sizes non-spherical or absorbing aerosols. Here, we challenge the UHSAS and LAS with laboratory-generated, mobility-size-classified aerosols of known chemical composition to quantify changes in the optical size response relative to that of ammonium sulfate (RI of 1.52+0i at 532 nm) and NIST-traceable polystyrene latex spheres (PSLs with RI of 1.59+0i at 589 nm). Aerosol inorganic salt species are chosen to cover the real refractive index range of 1.32 to 1.78, while chosen light-absorbing carbonaceous aerosols include fullerene soot, nigrosine dye, humic acid, and fulvic acid standards. The instrument response is generally in good agreement with the electrical mobility diameter. However, large undersizing deviations are observed for the low-refractive-index fluoride salts and the strongly absorbing nigrosine dye and fullerene soot particles. Polydisperse size distributions for both fresh and aged wildfire smoke aerosols from the recent Fire Influence on Regional to Global Environments Experiment and Air Quality (FIREX-AQ) and the Cloud, Aerosol, and Monsoon Processes Philippines Experiment (CAMP2Ex) airborne campaigns show good agreement between both optical sizers and contemporaneous electrical mobility sizing and particle time-of-flight mass spectrometric measurements. We assess the instrument uncertainties by interpolating the laboratory response curves using previously reported RIs and size distributions for multiple aerosol type classifications. These results suggest that, while the optical sizers may underperform for strongly absorbing laboratory compounds and fresh tailpipe emissions measurements, sampling aerosols within the atmospherically relevant range of refractive indices are likely to be sized to better than ±10 %–20 % uncertainty over the submicron aerosol size range when using instruments calibrated with ammonium sulfate.

Refractive index and optical properties of biogenic and anthropogenic secondary organic aerosol (SOA) particles were investigated. Aerosol precursors, namely longifolene, a-pinene, 1-methylnaphthalene, phenol, and toluene were oxidized in a Teflon chamber to produce SOA particles under different initial hydrocarbon concentrations and hydroxyl radical sources, reflecting exposures to different levels of nitrogen oxides (NO x). The real and imaginary components (n and k, respectively) of the refractive index at 375 nm and 632 nm were determined by Mie theory calculations through an iterative process, using the v 2 function to evaluate the fitness of the predicted optical parameters with the measured scattering, absorption, and extinction coefficients from a Photoacoustic Extinctiometer and Cavity Attenuated Phase Shift Spectrometer. Single scattering albedo (SSA) and bulk mass absorption coefficient (MAC) at 375 nm were calculated. SSA values of SOA particles from biogenic precursors (longifolene and a-pinene) were $0.98-0.99 ($6.3% uncertainty), reflecting purely scattering aerosols regardless of the NO x regime. However, SOA particles from aromatic precursors were more absorbing and displayed NO x-dependent SSA values. For 1-methylnaphthalene SOA particles, SSA values of 0.92-0.95 and $0.75-0.90 ($6.1% uncertainty) were observed under intermediate-and high-NO x conditions, respectively, reflecting the absorbing effects of SOA particles and NO x chemistry for this aromatic system. In mixtures of longifolene and phenol or longifolene and toluene SOA under intermediate-and high-NO x conditions, k values of the aromatic-related component of the SOA mixture were higher than that of 1-methylnaphthalene SOA particles. With the increase in OH exposure, k phenol decreased from 0.10 to 0.02 and 0.22 to 0.05 for intermediateand high-NO x conditions, respectively. A simple relative radiative forcing calculation for urban environments at k ¼ 375 nm suggests the influence of absorbing SOA particles on relative radiative forcing at this wavelength is most significant for aerosol sizes greater than 0.4 mm.

No abstract

Improved understanding of the optical properties of secondary organic aerosol (SOA) particles is needed to better predict their climate impacts. Here, SOA was produced by reacting 1-methylnaphthalene or longifolene with hydroxyl radicals (OH) under variable ammonia (NH3), nitrogen oxide (NO x ), and relative humidity (RH) conditions. In the presence of NH3 and NO x , longifolene-derived aerosols had relatively high single scattering albedo (SSA) values and low absorption coefficients at 375 nm independent of RH, suggesting that the longifolene SOA is mostly scattering. In 1-methylnaphthalene experiments, the resulting SSA and SOA mass absorption coefficient (MACorg) values suggest the formation of light-absorbing SOA, and the addition of high NO x and high NH3 enhanced the SOA absorption. Under intermediate-NO x dry conditions, the MACorg values increased from 0.13 m2 g–1 in NH3-free conditions to 0.28 m2 g–1 in high-NH3 conditions. Under high-NH3 conditions, the MACorg value further increased to 0.36 m2 g–1 with an increase in RH. Under dry high-NO x conditions, the MACorg value increased from 0.42 to 0.67 m2 g–1 with the addition of NH3, while with elevated RH, the MACorg value reached 0.70 m2 g–1. The time series of MACorg showed increasing trends only in the presence of NH3. Composition analysis of SOA suggests that organonitrates, nitroorganics, and other nitrogen-containing organic compounds (NOCs) are potential chromophores in the 1-methylnaphthalene SOA. Significant formation of NOCs was observed in the presence of high-NO x and NH3 and was enhanced under elevated RH.

No abstract

No abstract

an Aerograph 1520 chromatograph with flame ionization detector and a 20% silicone elastomer (D.C. 550) on Gas-Chrom P column (7 ft X 0.05 in.). The column temperature was raised during 50 min from 185 to 225°. Under these conditions lb did not rearrange. The chromatograph was calibrated using pure sample with 4-bromobiphenyl as an internal standard. Reactions were followed to at least 95 % conversion of lb. First-order rate constants were found by a least-squares fit to the individual points.B. Nematic Solvent. A mixture (% by weight) of lb (2.14), phenyl benzoate (0.853) as internal standard, and 3 (97.007) was heated to 202°{i.e., above the nematic -> liquid transition), vigorously stirred, and then rapidly cooled. The resulting solid was pulverized and homogenized. The resulting mixture was homogeneous to glc and the nematic -> liquid transition temperatures of various samples were identical (200°). The reactions were carried out as before with samples (2 mg) in evacuated ampoules.In the case of the experiments in a magnetic field, the samples were held between the pole pieces of an electromagnet in an aluminum container through which thermostated oil was circulated. The samples were dissolved in benzene for analysis by glc as before.Clathrate of lb in 21. The clathrate was prepared from a mixture of lb (0.25 g) and 22 (0.75 g) by crystallization from acetone (125 ml) forming dark yellow prisms. Anal. Caled for Cf,3H<6N8Oi7