Peter H. Daum

Anthropogenic aerosols. Indirect warming effect from dispersion forcing.

Anthropogenic aerosols enhance cloud reflectivity by increasing the number concentration of cloud droplets, leading to a cooling effect on climate that is referred to as the Twomey effect. Here we show that anthropogenic aerosols exert an additional effect on cloud properties that is derived from changes in the spectral shape of the size distribution of cloud droplets in polluted air and acts to diminish this cooling. This finding could help to improve our understanding of the indirect aerosol effect and its treatment in climate modelling.

Measurements of the chemical composition of stratiform clouds

Abstract Measurements are reported of the chemical composition of the liquid water and interstitial air in warm (> 0°C), non-precipitating stratus and strato-cumulus clouds at various locations in the eastern United States. Inorganic ionic composition of the cloud water was generally dominated by H + , NH 4 + , NO 3 − and SO 4 2− , similar to the composition of precipitation in this region of the U.S. Concentrations of the corresponding interstitial aerosol species and gaseous HNO 3 were invariably low in comparison to concentrations of the respective ionic species in cloudwater. In contrast, the concentration of NO x (i.e. NO + NO 2 + organic nitrates) was invariably comparable to or in excess of that of cloudwater nitrate. Sulfur dioxide was found at varying concentrations relative to cloudwater sulfate. In many cases, the SO 2 concentration was quite low ( 4 2− (> 1 ppb equivalent gas-phase concentration), suggesting large fractional conversion and incorporation into cloudwater. In other cases, in which dilute SO 2 plumes (pso, > 5 ppb) were observed in the cloud interstitial air, the gaseous SO 2 concentration substantially exceeded the cloudwater sulfate concentration. Concentrations of H 2 O 2 in cloudwater were found to exhibit strong inverse correlation with interstitial SO 2 . Appreciable concentrations of SO 2 in cloud interstitial air and H 2 O 2 in cloudwater were only rarely observed to coexist, for the most part only one or the other being present above the limit of detection. These observations are consistent with aqueous-phase oxidation of SO 2 by H 2 O 2 , as has been inferred previously on the basis of laboratory kinetic studies, and with the hypothesis that depending on relative concentrations, either of these species can be a limiting reagent for in-cloud SO 2 oxidation. The uptake of NO x as cloudwater nitrate is less complete than the uptake of SO 2 as sulfate, and evidence for the occurrence of similar in-cloud processes causing the conversion of NO or NO 2 to cloudwater nitrate has not been found.

Parameterization of the Autoconversion Process. Part I: Analytical Formulation of the Kessler-Type Parameterizations

Abstract Various commonly used Kessler-type parameterizations of the autoconversion of cloud droplets to embryonic raindrops are theoretically derived from the same formalism by applying the generalized mean value theorem for integrals to the general collection equation. The new formalism clearly reveals the approximations and assumptions that are implicitly embedded in these different parameterizations. A new Kessler-type parameterization is further derived by eliminating the incorrect and/or unnecessary assumptions inherent in the existing Kessler-type parameterizations. The new parameterization exhibits a different dependence on liquid water content and droplet concentration, and provides theoretical explanations for the multitude of values assigned to the tunable coefficients associated with the commonly used parameterizations. Relative dispersion of the cloud droplet size distribution (defined as the ratio of the standard deviation to the mean radius of the cloud droplet size distribution) is explici...

Dependence of ozone production on NO and hydrocarbons in the troposphere

An expression for the production rate of 03, P(O 3), is derived based on a radical budget equation applicable to low and high NOx conditions. Differentiation of this equation with respect to NO or hydrocarbons (HC) gives an approximate analytic formula in which the relative sensitivity of P(O3) to changes in NO or HC depends only on the fraction of radicals which are removed by reactions with NOx. This formula is tested by comparison with results from a photochemical calculation driven by trace gas observations from the 1995 Southern Oxidants Study (SOS) campaign in Nashville, Tennessee.

THE EFFECT OF REFRACTIVE INDEX ON SIZE DISTRIBUTIONS AND LIGHT SCATTERING COEFFICIENTS DERIVED FROM OPTICAL PARTICLE COUNTERS

Abstract The effect of refractive index on particle size distributions measured by optical particle counters is examined. Similar to previous investigations, it is found that optical counters undersize ambient particles because the refractive index of these particles is generally lower than that of the latex particles commonly used for the calibration of optical counters. The maximum undersizing is found to occur when particle sizes are comparable to the wavelength of light used in the measurement. A new approach for modeling the effect of refractive index on the sizing of optical counters is presented. Previously derived optical response functions are compared and a generalized formulation is proposed which includes the existing response functions as special cases. Algorithms are presented for correcting size distributions measured by optical counters for the difference between the refractive index of ambient and calibration particles. Data collected by a Passive Cavity Aerosol Spectrometer (PCASP) and by an integrating nephelometer are compared. Light scattering coefficients calculated from the optical probe data uncorrected for the effect of refractive index differ from those measured by the integrating nephelometer by a factor of 2. An iterative procedure that adjusts the PCASP-measured size distribution for the effect of refractive index is used to derive the best agreement between calculated and observed light scattering coefficients. The refractive indices of aerosols at wavelength of 0.45 μ m that best fit the data vary between 1.3 and 1.5, with an average of 1.41. The relative importance of the underestimation of light scattering coefficients calculated from the PCASP-measured size distributions due to the refractive index and the size truncation effect are evaluated. The former is found to be more important than the latter. Implications of this study for addressing aerosol shortwave radiative forcing and potential uncertainties relevant to this study are discussed.

Chemical and physical properties of plumes of anthropogenic pollutants transported over the North Atlantic during the North Atlantic Regional Experiment

Plumes of photochemical pollutants transported from the industrialized regions of the northeast United States and Canada were sampled over the North Atlantic Ocean at distances up to 1000 km from the coast. The plumes were found in well defined layers up to 1 km thick and were usually isolated from the surface by a low altitude inversion. Plume composition was consistent with the occurrence of extensive photochemical processing during transit from source regions as indicated by high 03 concentrations (03 maximum -150 parts per billion by volume (ppbv)), generally high fractional conversion (>85%) of NOx to its oxidation products, and high peroxide concentrations (median 3.6 ppbv; maximum 11 ppbv). These observations are in accord with processing times estimated from back trajectory analysis. CO and 03 concentrations were well correlated (r 2 = 0.64) with a slope (0.26) similar to previous measurements in photochemically aged air. Good correlations were also observed between CO and accumulation mode particle number densities (r 2 = 0.64), and CO and NOy (r 2 = 0.67). 03 was found to depend nonlinearly on the NOx oxidation product concentration. At low values of (NOy-NOx), the slope (14) was within the range of values measured previously in photochemically aged air masses, at higher concentrations the slope was much lower (4.6). The low slope at high concentrations is attributed to minimization of losses of NOx oxidation products in spatially well-defined plumes during transport. A strong linear correlation (r 2 = 0.73) was found between 03, and the concentration of radical sink species as represented by the quantity ((NOy-NOx) + 2H202).

Ozone production in the New York City urban plume

In the summer of 1996 the Department of Energy G-1 aircraft was deployed in the New York City metropolitan area as part of the North American Research Strategy for Tropospheric Ozone-Northeast effort to determine the causes of elevated O3 levels in the northeastern United States. Measurements of O3, O3 precursors, and other photochemically active trace gases were made upwind and downwind of New York City with the objective of characterizing the O3 formation process and its dependence on ambient levels of NOx and volatile organic compounds (VOCs). Four flights are discussed in detail. On two of these flights, winds were from the W-SW, which is the typical direction for an O3 episode. On the other two flights, winds were from the NW, which puts a cleaner area upwind of the city. The data presented include plume and background values of O3, CO, NOx, and NOy concentration and VOC reactivity. On the W-SW flow days O3 reached 110 ppb. According to surface observations the G-1 intercepted the plume close to the region where maximum O3 occurred. At this point the ratio NOx/NOy was 20–30%, indicating an aged plume. Plume values of CO/NOy agree to within 20% with emission estimates from the core of the New York City metropolitan area. Steady state photochemical calculations were performed using observed or estimated trace gas concentrations as constraints. According to these calculations the local rate of O3 production P(O3) in all four plumes is VOC sensitive, sometimes strongly so. The local sensitivity calculations show that a specified fractional decrease in VOC concentration yields a similar magnitude fractional decrease in P(O3). Imposing a decrease in NOx, however, causes P(O3) to increase. The question of primary interest from a regulatory point of view is the sensitivity of O3 concentration to changes in emissions of NOx and VOCs. A qualitative argument is given that suggests that the total O3 formed in the plume, which depends on the entire time evolution of the plume, is also VOC sensitive. Indicator ratios O3/NOz and H2O2/NOz mainly support the conclusion that plume O3 is VOC sensitive.

Spectral dispersion of cloud droplet size distributions and the parameterization of cloud droplet effective radius

Parameterization of effective radius (re) as proportional (Pontikis and Hicks, 1992; Bower and Choularton, 1992; Bower et to the cube root of the ratio of cloud liquid water content (L) to al., 1994; Martin et al., 1994; Liu and Hallett, 1997; Reid et al., Hicks (1992) and by Liu and Hallet (1997) that account for the dependence of ot on the spectral dispersion are compared to each where re is the effective radius intm, L the liquid water content in other and to cloud microphysical data collected during two recent gm -3, N the total droplet concentration in cm -3, and ot the prefactor. field studies. The expression of Liu and Hallet describes the A key issue in use of this parameterization is the specification spectral dependence of ot (or re) more accurately than the Pontikis of or. Here we explore the dependence of ot on the spectral disper- and Hicks relation over the observed range of spectral dispersions. sion of cloud droplet size distributions. Values of ot derived from The comparison shows that the different treatments of ot as a func- other studies are compared to those derived from the data collected tion of spectral dispersion alone can result in substantial differ- during two Intensive Observation Periods (IOP) conducted at the ences in re estimated from different parameterization schemes, Atmospheric Radiation Measurements (ARM) program Southern suggesting that accurately representing re in climate models Great Plain (SGP) site in Oklahoma, in the spring and fall of 1997. requires predicting ot in addition to L and N. This analysis suggests the necessity and possibility of improving

Efficient scavenging of aerosol sulfate by liquid-water clouds☆

Abstract Continuous aircraft measurements of aerosol sulfate concentration, [SO 4 2− ], (flame-photometric detector) and light-scattering coefficient, b scat , (integrating nephelometer) in cloud interstitial air and in associated clear air are reported for several locations in the eastern United States. Measurements were made in situations where, on the basis of other physical and chemical measurements, it can be argued that the cloud air and the clear air have similar history and composition. It is found in cloud interstitial air that [SO 4 2− ] approaches zero and b scat approaches the Rayleigh scattering coefficient of air. Sulfate concentration and b scat in successive subsaturated regions along a given flight path at a given altitude were frequently nearly constant or slowly varying, the pattern presented appearing to be that of a fairly uniform concentration of sulfatecontaining, light-scattering aerosol from which segments had been removed in regions corresponding to the presence of cloud. The strong decrease in both [SO 4 2− ] and b scat within cloud interstitial air relative to clear air is attributed to nearly complete uptake of sulfate-containing and light-scattering aerosol particles into cloud droplets. From comparison of the in-cloud signal to the non-cloud signal it is possible to set an upper bound to the fraction of unscavenged sulfate aerosol (frequently as low as 0.25) and to b scat (frequently as low as 0.1), although for both quantities the measurements would be consistent with essentially complete scavenging. Additionally, the observation in these situations that no cloud-free regions exhibited the low values of [SO 4 2− ] and b scat characteristic of the in-cloud regions is indicative of the return of the dissolved material to the air as clear-air aerosol upon cloud evaporation.

Model correlations for ozone, reactive nitrogen, and peroxides for Nashville in comparison with measurements : Implications for O3-NOx-hydrocarbon chemistry

We present an analysis of correlations between O 3 , NO x reaction products (NO 2 ), and peroxides in photochemical models for polluted environments in comparison with measurements in the vicinity of Nashville, Tennessee. This analysis is associated with the use of O 3 /NO z , H 2 O 2 /NO z , and similar ratios as indicators for the relative impact of NO x and hydrocarbons (volatile organic compounds, VOC) on ozone formation. The measurements are used both to evaluate the NO x -VOC indicator theory and to identify NO x -VOC chemistry in Nashville. Results show that a linear correlation exists between O 3 and the sum 2H 2 O 2 +NO z , consistent between models and measurements. The ratio O 3 /2H 2 O 2 +NO z has a near-constant value in both the Nashville urban plume and surrounding rural area. A similar correlation is found with total peroxides (O 3 versus 2peroxides+NO z ) but with greater scatter. The correlations between O 3 , NO z , and peroxides are consistent with an assumed dry deposition rate of 5 cm s -1 for H 2 O 2 and HNO 3 . Changes in dry deposition and RO 2 reaction rates cause minor adjustments in the NO x -VOC-indicator analysis for H 2 O 2 /NO z . Measured indicator ratios for Nashville are close to the NO x -VOC transition predicted by models and form an intermediate point between previous measurements for Atlanta (NO x sensitive) and Los Angeles (VOC sensitive). The model ozone production efficiency (P(O 3 )/L(NO x )) is 3-4, significantly lower than would be derived from the O 3 -NO z slope (5-8).