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Project at a Glance

Project Status: complete

Report Published February 1989:

Title: Fog, cloud, and dew chemistry : final report for the California Air Resources Board

Principal Investigator / Author(s): Hoffman, Michael R

Contractor: California Institute of Technology

Contract Number: A4-075-32

Research Program Area: Atmospheric Processes

Topic Areas: Acid Deposition


In our previous studies, we had established that fog and cloudwater in Southern California was consistently acidic (pH 1.7 to 4.0). As a continuation of that work, we have expanded both the intensity of our sampling effort and the detail of our chemical characterization. The principal objectives of this research project were:

1 .to develop refined instrumentation for the collection of cloud- and fogwater and to automate the fogwater collection process,
2. to carry out multiple site sampling and analysis of aerosol, gases, clouds, and fogs in the Los Angeles (LA) coastal zone, in the Riverside area, at elevated sites in the Los Angeles basin, in the Santa Barbara Channel area, and in the Southern San Joaquin Valley,
3. to characterize dew chemistry
4. and to correlate cloud and fogwater data with standard air quality measurements.

Our first principal objective was to construct an automated fogwater collector, which would allow fogwater monitoring to be done more conveniently. This objective has been achieved; the development effort resulted in the granting of a United States Patent (No. 4,697,462) for the design of a fog- and cloudwater collector with an associated automatic collection and field storage system. The results of this work are presented in Chapters 1, 2, and 3 of the final report.

With the aid of automated collectors, we were able to determine the spatial and temporal variation of fog / cloud chemistry within large areas of fog occurrence. The second array of principal objectives were all achieved; detailed reports of these efforts are given in Chapters 4, 5, 6, 7, 12, 13, 14, 15, 16, 17 and 18 of the final report. After an preliminary investigation of dew chemistry in a variety of locations in the LA Basin, further investigations were halted because the relative importance of dew as a depositional pathway for atmospheric acidity was established to be negligible. Other investigators have confirmed our assessment of the importance of dew chemistry/deposition relative to dry deposition, wet deposition, and fog-derived deposition. This work has been reported in the Ph.D. thesis of J. M. Waldman (Ph.D., Caltech, 1986). With the encouragement and approval of the CARB project officers, we shifted our focus from dew to a preliminary investigation of the cloudwater chemistry in the Sierra Nevada Mountains. Results of this work are reported in Chapter 10.

Research objective number 4 was explored for fogs in the San Joaquin Valley, in the Los Angeles basin, and in the Santa Barbara Channel area., Using the techniques of multiple linear regression, a significant correlation was observed between the average nighttime cloud fogwater loading of H+ and NO3- and. the maximum levels of O3, NO and NO recorded during the preceding days. Regression equations that were obtained were found to be reliable for predicting fogwater loading of H+ and NO3-.

Regions of high fog / cloud frequency that have been studied in detail include the Los Angeles coastal area, where previous work suggests that the highest acidity occurs; the Riverside-San Bernardino area, which is subject to fog during the height of the smog season; the Santa Barbara Channel area; and the Southern San Joaquin Valley, in which the occurrence of highly acidic fogs is held in check by substantial ammonia emissions. The limited ventilation documented by previous tracer studies of the Santa Barbara Channel and presence of fog and high humidity there suggest that rapid S(IV) (i.e. total SO₂ aqueous) oxidation in the Channel results in highly acidic fog. As predicted pH values in the vicinity of the Santa Barbara Channel have been found to range from 2.1 to 3.5. Preliminary investigations of mountain-top cloud chemistry were performed in Sequoia National Park. In conjunction with fog and cloud sampling, we have performed more detailed measurements of deposition during fog.

In addition to determining the inorganic speciation of fog and cloudwater, we have determined the role of carboxylic and sulfonic acids in fog acidification. Our previous work has shown the covariance of very high levels of S( IV) and formaldehyde, suggesting the presence of aldehyde-S (IV) adducts (i.e. sulfonic acids). We have determined quantitatively the higher aldehydes and a series of dicarbonyls such as glyoxal, methyl glyoxal and biacetyl; furthermore we have identified a variety of sulfonic acid salts formed by reaction of S(IV) with aldehydes in the droplet phase.


For questions regarding this research project, including available data and progress status, contact: Research Division staff at (916) 445-0753

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