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Studies of physicochemical processes in atmospheric particles and acid deposition

Citation

Pandis, Spyros N. (1991) Studies of physicochemical processes in atmospheric particles and acid deposition. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-05062004-154106

Abstract

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Atmospheric particles, or particulate matter, can be solid or liquid with diameters varying from around 0.002[micrometers] to roughly 100[micrometers]. Atmospheric aerosol sources can be classified as primary or secondary, with the primary aerosol being directly emitted from the corresponding sources and the secondary particles being formed in the atmosphere, for example, from gas-phase chemical reactions that produce condensable vapors. At the same time aerosol particles are ultimately connected with the formation of water droplets and equivalently with the formation of clouds and fogs in the atmosphere.

The first part of this thesis concerns the mathematical modeling of wet and dry acid deposition and of the relevant physicochemical processes. Acid deposition consists of the delivery of acidic substances, principally sulfuric and nitric acid, from the atmosphere to the earth's surface. Upon emission to the atmosphere, SO2 and NOx, are photochemically oxidized, yielding sulfuric and nitric acid vapors. Sulfuric acid is rapidly incorporated into aerosol particles, while nitric acid may be scavenged by particles or droplets or remain in the gas phase. Even in the absence of an aqueous phase (no clouds or fog), the acidic gases and dry particles can be transported to and deposited at ground level; this process is called dry deposition. When an aqueous phase is present (inside a cloud or a fog), gas-phase species like SO2, HNO3, NH3 and aerosol particles are scavenged by water droplets resulting in a solution that can be significantly acidic. Additional cloudwater or fogwater acidity beyond that attained purely from scavenging of gases and particles results from aqueous-phase chemistry, most notably oxidation of dissolved SO2 to sulfuric acid. These acidic droplets can reach the earth's surface either as precipitation or as impacted cloud and fogwater, in the processes termed wet deposition. If they are not rained or deposited out the aqueous droplets can evaporate leaving as residue new aerosol particles that may themselves undergo dry deposition to the earth's surface. The effects of acid deposition include soil and lake acidification, forest decline and deterioration of cultural monuments.

Mathematical models are a major tool in our effort to understand and ultimately control acid deposition. The development of such a mathematical model represents a major challenge as it requires the ability to describe the entire range of atmospheric physicochemical phenomena.

As a first step in the modeling, a comprehensive chemical mechanism for aqueous-phase atmospheric chemistry was developed and its detailed sensitivity analysis was performed. The main aqueous-phase reaction pathways for the system are the oxidation of S(IV) to S(VI) by H2O2, OH, HO2, O2 (catalysed by Fe3+ and Mn2+), O3, and [...]. The dominant pathway for HNO3(aq) acidity is scavenging of nitric acid from the gas phase. HCOOH is produced because of the reaction of HCHO(aq) with OH(aq). The gas-phase concentrations of SO2, H2O2, HO2, OH, O3, HCHO, NH3, HNO3, and HCl are of primary importance. Increase of the liquid water content of the cloud results in a decrease of the sulfate concentration, but an increase of the total sulfate amount in the aqueous-phase. On the basis of the sensitivity analysis, a condensed mechanism was derived.

The next step was the development of a model that actually predicts the amount of liquid water in the atmosphere solving the energy balance. This Lagrangian model combines for the first time a detailed description of gas and aqueous-phase atmospheric chemistry with a treatment of the dynamics of radiation fog, that is the fog that is created due to the radiative cooling of the earth's surface to the space during the night. The model was evaluated against a well documented radiation fog episode in Bakersfield in the San Joaquin Valley of California over the period January 4-5, 1985. This application showed that the model predictions for temperature profile, fog development, liquid water content, gas-phase concentrations of SO2, HNO3, and NH3, pH, aqueous-phase concentrations of [...], [...], and [...], and finally deposition rates of the above ions match well the observed values. The fog was found to lead to a drastic increase of deposition rates over those in its absence for the major ionic species, with most notable being the increase of sulfate deposition. Several important differences were found to exist between the characteristics of a radiation fog and a representative cloud environment. Radiation fogs typically develop under stable conditions (very low wind speed) resulting in weak mixing and significant vertical gaseous species concentration gradients. Because of the proximity of the fog to ground-level sources of pollutants like SO2 and NOx, the corresponding gas-phase concentrations can reach much higher levels that in a cloud. In such a case, pathways for aqueous-phase sulfate production that are of secondary importance in a cloud environment may become significant in a fog.

The next level of treatment beyond assuming that all the water droplets have the size and chemical composition is to explicitly model the size-composition distribution of droplets as a result of nucleation on aerosol particles. A third model was developed to study the distribution of acidity and solute concentration among the various droplet sizes in a fog or a cloud. The major finding of this study was that significant solute concentration differences can occur in aqueous droplets inside a fog or a cloud. For the fog simulated, during the period of dense fog, the solute concentration in droplets larger than 10[micrometers] diameter increased with size, in such a way that droplets of diameter 20[micrometers] attain a solute concentration that is a factor of 3.6 larger than that in the 10[micrometer] droplets. Chemical processes tend to decrease the total solute mass concentration differences among the various droplet sizes. Low cooling rates of the system also tend to decrease these concentration differences while high cooling rates have exactly the opposite effect. The mass/size distribution of the condensation nuclei influences quantitatively, but not qualitatively, the above concentration differences.

The effects of equilibration processes on wet and dry deposition were then investigated and furthermore the accuracy of the currently used modelling approaches of these phenomena was examined. Atmospheric equilibration processes between two phases with different deposition velocities have the potential to affect significantly the amount of total material deposited on the ground. The magnitude of the effects of the equilibration processes depends primarily on the ratio of the deposition velocities of the two phases, on the production/emission rate of the gas-phase species, and on the initial distribution of species between the two phases.

At this point all the tools were available for the detailed investigation of the cyclical relationship between the aerosol and aqueous droplets; a polluted atmosphere with high aerosol concentration assists the formation of the aqueous phase which itself appears to enhance smog production, visibility reduction and aerosol sulfate levels after its dissipation. A model including descriptions of aerosol and droplet microphysics, gas and aqueous-phase chemistry and deposition was used to study the transformation of aerosol to fog droplets and back to aerosol in an urban environment. Fogs in polluted environments have the potential to increase aerosol sulfate concentrations, but at the same time to cause reductions in the aerosol concentration of nitrate, chloride, ammonium and sodium as well as in the total aerosol mass concentration. The sulfate produced during fog episodes favors the aerosol particles that have access to most of the fog liquid water which are usually the large particles. Aerosol scavenging efficiencies of around 80% were calculated for urban fogs. Sampling and subsequent mixing of fog droplets of different sizes may result in measured concentrations that are not fully representative of the fogwater chemical composition and can introduce errors in the reported values of the ionic species deposition velocities. Differences in the major ionic species deposition velocities can be explained by their distribution over the aerosol size spectrum and can be correlated with the species average diameter.

The second part of this work was focused on the experimental study of the mechanisms of formation of secondary aerosol particles due to the atmospheric photooxidation of hydrocarbons. In this smog chamber the aerosol forming potential of natural hydrocarbons was investigated. Natural hydrocarbons like the monoterpenes C10H16 and isoprene C5H8 are emitted by various trees and plants in significant quantities. Isoprene and [Beta]-pinene, at concentration levels ranging from a few ppb to a few ppm were reacted photoehemically with NOx, in the Caltech outdoor smog chamber facility. Aerosol formation from the isoprene photooxidation was found to be negligible even under extreme ambient conditions due to the relatively high vapor pressure of its condensable products. Aerosol carbon yield from the [Beta]-pinene photooxidation is as high as 8% and depends strongly on the initial HC/NOx ratio. The average vapor pressure of the [Beta]-pinene aerosol is estimated to be 37 [plus or minus] 24 ppt at 31?c. Monoterpene photooxidation can be a significant source of secondary aerosol in rural environments and in urban areas with extended natural vegetation.

Item Type:Thesis (Dissertation (Ph.D.))
Degree Grantor:California Institute of Technology
Division:Chemistry and Chemical Engineering
Major Option:Chemical Engineering
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Seinfeld, John H.
Thesis Committee:
  • Seinfeld, John H. (chair)
  • Gavalas, George R.
  • Cass, Glen Rowan
  • Flagan, Richard C.
Defense Date:1 July 1990
Record Number:CaltechETD:etd-05062004-154106
Persistent URL:http://resolver.caltech.edu/CaltechETD:etd-05062004-154106
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:1649
Collection:CaltechTHESIS
Deposited By: Imported from ETD-db
Deposited On:10 May 2004
Last Modified:26 Dec 2012 02:40

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