John N. Porter

Ship-Based Sun Photometer Measurements Using Microtops Sun Photometers

Abstract The use of hand-held Microtops II sun photometers (built by Solar Light Inc.) on ship platforms is discussed. Their calibration, filter stability, and temperature effects are also described. It is found that under rough conditions, the ship motion causes the largest error, which can result in a bias toward higher optical depths. In order to minimize this bias, a large number of sun photometer measurements (∼25) should be taken in a short period of time, and the higher values should be discarded. Under rough ocean conditions, it is also best to shorten the Microtops sun photometer sampling period (less than 5 s) and save only a single value (no averaging) and remove the high optical depths in postprocessing. It is found that the Microtops should be turned off frequently to correct for zero drift caused by temperature effects. Calibration is maintained by routine Langley plot calibrations at the Mauna Loa Observatory for each unit or through cross calibration.

Aerosol size distribution models based on in situ measurements

We present an aerosol size distribution model summarizing measurements of sulfate, sea salt, and dust aerosol obtained in the marine boundary layer and marine free troposphere. Stratospheric aerosol cases are not included here. Aerosols were sampled in both both clean background conditions and aged anthropogenic aerosol (>3 days) which had advected over the ocean. Model size distributions are developed for the sulfate accumulation mode, sea-salt coarse mode, and dust coarse mode. The data demonstrate that both the accumulation and coarse mode aerosol gradually shift to larger diameters as the aerosol mass increases. Based on this relationship, it is possible to estimate the size distribution based on aerosol mass. Comparisons with other aerosol models show significant differences, which suggest further measuring and modeling efforts are needed.

Observations of the atmospheric sulfur cycle on SAGA 3

During the Soviet/American Gases and Aerosols (SAGA) 3 program in February and March 1991 we measured a wide variety of sulfur compounds simultaneously in the equatorial Pacific marine boundary layer. We made measurements of atmospheric dimethyl sulfide (DMS), sulfur dioxide (SO2), and size-resolved aerosol non-sea-salt sulfate (NSS), and methane sulfonate (MSA). Some of our observed ratios contradict commonly held views of the marine sulfur cycle: the large DMS/NSS ratio implies that NSS may not be the primary product of DMS oxidation under some conditions. We also found much more DMS than SO2, which may suggest that SO2 is not always an intermediate in DMS oxidation. The small SO2/NSS ratio also supports the idea that most NSS was not formed from SO2. Although our measured ratios of MSA/NSS were similar to previous observations in this region, much of the MSA was contained on supermicron particles, in contrast to both the NSS and the earlier MSA observations at higher latitudes. This implies that MSA/NSS ratios in ice cores may not accurately reflect the MSA/NSS ratios in their source areas.

Atmospheric nuclei and related aerosol fields over the Atlantic: Clean subsiding air and continental pollution during ASTEX

Meteorological conditions in the Atlantic Stratocumulus Transition Experiment (ASTEX) region favored advection of clean air from the central Atlantic during the early part of the experiment that was replaced by polluted air of European origin during the latter part of the experiment. Marked differences in the aerosol size distribution, composition, and state of mixing existed in these air masses. Pronounced differences in their vertical structure also demonstrated that surface measurements often do not represent average boundary layer or column concentrations. Clean subsiding air from the free troposphere had concentrations of condensation nuclei that significantly exceeded concentrations in the boundary layer and had very low mass concentrations and volatility consistent with homogeneous nucleation aloft, supporting the hypothesis that these nuclei can provide a source for “new” nuclei into the marine surface layer. This finding was in contrast to polluted air characterized by high concentrations of aged aerosol but having no evidence for significant recent nuclei formation. Particles in polluted air consisted of more than 90% volatile mass (mostly sulfate) and a refractory residual remaining at 300°C. The refractory mass varied with the concurrently measured light absorption coefficient, associated with combustion-derived soot. In spite of 2 orders of magnitude more volatile sulfate in polluted air, most of the particle number in polluted air remained after volatilization at 300°C, in contrast to less than 10% in clean air. This finding suggests that the number of particles in polluted air may reflect the combustion process producing the primary soot aerosol more than the SO2 emissions responsible for much of the sulfate aerosol mass. The accumulation mode mass mean diameter was found to increase with total accumulation mode mass, suggesting that a size parameterization based upon the more commonly measured sulfate mass could be included in current aerosol models.

Aircraft studies of size-dependent aerosol sampling through inlets

Accurate measurement of atmospheric aerosol from aircraft platforms is of importance to a variety of research interests. Representative measurement of aerosol from aircraft-aspirated systems requires special efforts in order to (1) maintain near isokinetic sampling conditions, (2) estimate aerosol losses in the sample system, and (3) obtain a measurement of sufficient duration to be statistically significant for all sizes of interest. This last point is especially critical for aircraft measurements which typically require fast response times while sampling in clean remote regions. Here we present size-resolved tests, intercomparisons, and analysis of aerosol inlet performance as determined by a custom laser optical particle counter. Measurements discussed here took place during the Global Backscatter Experiment (1988–1989) and the Central Pacific Atmospheric Chemistry Experiment (1988). System configurations are discussed including (1) nozzle design and performance, (2) system transmission efficiency, (3) nonadiabatic effects in the sample line and its effect on sample line relative humidity, and (4) the use and calibration of a virtual impactor.

Lagrangian evolution of an aerosol column during the Atlantic Stratocumulus Transition Experiment

Two Lagrangian experiments were carried out during the Atlantic Stratocumulus Transition Experiment (ASTEX) with the intent of looking at aerosol evolution in the marine boundary layer (MBL). The second Lagrangian (L2) took place below broken stratus clouds and was more successful since little precipitation reached the surface. Aerosol below the inversion was primarily an aged pollution aerosol from central Europe. Vertical variability in aerosol concentrations was generally characterized by highest concentrations in the moist surface and transition layers. Concentrations were a factor of 2 or more lower in the dry subcloud layer and cloud layer and dropped to less than one tenth of surface values in the free troposphere above cloud. This behavior reflected the decoupled boundary layer below the main inversion and complicated the assessment of Lagrangian aerosol evolution. No evidence for new particle formation was observed during L2, and aerosol evolution proceeded only by mixing, coagulation, and removal mechanisms. An entrainment rate of about 0.6 cm s -1 from the free troposphere into the MBL was a key parameter affecting aerosol evolution and resulted in about a 35% column dilution during the 34-hour L2 measurement period. Aerosol evolution in the decoupled subcloud and surface marine layer is consistent with an entrainment rate of about 0.45 cm s -1 into the surface layer and about 0.25 cm s -1 out of the layer. The ability to resolve the effects of separate processes influencing both gas and aerosol during Lagrangian evolution will depend upon (1) an adequate assessment of the variability in the air mass, (2) the ability to characterize this variability relative to the uncertainties in resampling the air mass, and (3) the extent to which the substantial changes due to entrainment alone can be reliably determined.

Single-pulse standoff Raman detection of chemicals from 120 m distance during daytime.

The capability to analyze and detect the composition of distant samples (minerals, organics, and chemicals) in real time is of interest for various fields including detecting explosives, geological surveying, and pollution mapping. For the past 10 years, the University of Hawaii has been developing standoff Raman systems suitable for measuring Raman spectra of various chemicals in daytime or nighttime. In this article we present standoff Raman spectra of various minerals and chemicals obtained from a distance of 120 m using single laser pulse excitation during daytime. The standoff Raman system utilizes an 8-inch Meade telescope as collection optics and a frequency-doubled 532 nm Nd: YAG laser with pulse energy of 100 mJ/pulse and pulse width of 10 ns. A gated intensified charge-coupled device (ICCD) detector is used to measure time-resolved Raman spectra in daytime with detection time of 100 ns. A gate delay of 800 ns (equivalent to target placed at 120 m distance) was used to minimize interference from the atmospheric gases along the laser beam path and near-field scattering. Reproducible, good quality single-shot Raman spectra of various inorganic and organic chemicals and minerals such as ammonium nitrate, potassium perchlorate, sulfur, gypsum, calcite, benzene, nitrobenzene, etc., were obtained through sealed glass vials during daytime. The data indicate that various chemicals could easily be identified from their Raman fingerprint spectra from a far standoff distance in real time using single-shot laser excitation.

Repetitive genetic inversion of optical extinction data

We describe a genetic method of deriving aerosol size distributions from multiwavelength extinction measurements. The genetic inversion searches for log-normal size distribution parameters whose calculated extinctions best fit the data. By repetitively applying the genetic inversion using different random number seeds, we are able to generate multiple solutions that fit the data equally well. When these solutions are similar, they lend confidence to an interpretation, whereas when they vary widely, they demonstrate nonuniqueness. In this way we show that, even in the case of a single log-normal distribution, many different distributions can fit the same set of extinction data unless the misfit is reduced below typical measurement error levels. In the case of a bimodal distribution, we find many dissimilar size distributions that fit the data to within 1% at six wavelengths. To recover the original bimodal distribution satisfactorily, we found that extinctions at ten wavelengths must be fitted to within 0.5%. Our results imply that many size distributions recovered from existing extinction measurements can be highly nonunique and should be treated with caution.

Using Horizontal and Slant Lidar Measurements to Obtain Calibrated Aerosol Scattering Coefficients from a Coastal Lidar in Hawaii

Abstract Sea salt aerosol concentrations in the clean marine boundary layer can be considered spatially homogeneous when averaged over space and time. Using this assumption, horizontal and slant lidar measurements are carried out at a Hawaii coastal site allowing accurate retrieval of the marine aerosol scattering coefficient. It is found that when the lidar calibration or the aerosol phase function is adjusted so that the derived scattering coefficients are constant with range, then the derived aerosol scattering coefficients are correct and agree with nephelometer measurements (within 25%) and sun photometer (within 7%). Examples of this calibration process are given.

Vertical and horizontal aerosol scattering fields over Bellows Beach, Oahu, during the SEAS experiment

Scanning lidar measurements were carried out during the Shoreline Environment Aerosol Study (SEAS) experiment (19‐30 April 2000) to characterize the aerosol scattering fields in the coastal marine boundary layer at Bellows Beach on the southeast side of Oahu, Hawaii. The sea salt was found to be well mixed throughout the mixed layer, although the depth of the trade wind mixed layer was found to vary significantly over short timescales. As expected, the frequency distribution of aerosol scatter had a lognormal distribution, with the exception of regions downwind of breaking waves, where the frequency distribution was bimodal. A spatial statistical study revealed that the island-blocking effects cause low-level clouds to develop as they approach the island, with enhanced drizzle near the coastline reaching all the way to the surface. The spray from waves breaking on an outer reef was found to be intermittent and contained to heights of 20 m (on average) for the average wind speed of 7 m s21. Sea-salt concentrations and fluxes from the breaking waves were estimated from the lidar measurements and found to be within the range of values reported by other investigators.