Atmospheric Environment Vol. 27A. No. 9, pp. 1509 1515, 1993. 003~6981/93 $6.00 + 0.00 Printed in Great Britain. cc ~ 1993 Pergamon Press Ltd

S H O R T C O M M U N I C A T I O N

COMPARISON OF H202 AND 03 CONTENT IN ATMOSPHERIC SAMPLES IN THE SAN BERNARDINO MOUNTAINS, SOUTHERN CALIFORNIA*

HIROSHI SAKUGAWA a n d ISAAC R. KAPLAN

Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90024-1567, U.S.A.

(Received 27 July 1992 and in final form 14 October 1992)

Abstraet~oncentrations of atmospheric H20 2 were measured in air, rain, cloud and dew samples in forested areas of the San Bernardino Mountains, southern California, from spring through fall of 1987-1990. 0 3 measurements in air were also conducted for comparison. Typical ranges of H20 2 concentrations measured were 1-3 ppb in air, 10-90 #M in rain and cloud water, and < 1 taM in dew. The results show that gas-phase H20 2 concentrations were slightly higher at rqghttime than at daytime or nearly constant throughout a 24-hr period, whereas 03 concentrations were highest during the afternoon, when polluted air masses from Los Angeles carried by daily sea breezes reached the mountain region. Afternoon concentra- tions of gaseous H20 2 and 0 3 in the mountain region were compared with those measured in Los Angeles urban sites to elucidate the regional variation of these oxidants. The results show that ambient concentrations of H202 and 0 3 were about 50-100% higher at the mountain sites than at the Los Angeles sites.

Key word index: Hydrogen peroxide, ozone, air pollution, decline of forest.

INTRODUCTION

The southern and western slopes of the San Bernardino Mountains (about 120-150 km east of central Los Angeles) which act as an eastern boundary of the Los Angeles Air Basin receive a significant impact of air pollutants from Los Angeles. From the early 1950s, the damage to pine and other coniferous trees were observed in the mountains. The dam- age to trees (symptoms include chlorotic mottle or chlorosis of older needles) are particularly obvious in the southwestern slopes of the mountains. Air pollution was suspected as the possible cause of observed tree damage. Extensive investiga- tions on the effects of air pollution on tree damage were, therefore, conducted by a number of investigators during the 1960s and 1970s. Field and laboratory experiments, includ- ing treatment of trees with charcoal-filtered air and ozone (O3)-added air, and the 0 3 fumigations to seedlings, clearly indicated that 03 is an oxidant responsible for the injury of pine trees (Miller et al., 1963; Richards et al., 1968). In the wake of the significance of the effect of 0 3 on trees, ambient concentrations and distribution patterns of O 3 in the moun- tains were extensively studied during the last decades (Miller, 1983; Miller and Taylor, 1986; Miller et al., 1989).

In this paper, we report the study of atmospheric levels of hydrogen peroxide (H202) concentrations in the forest re- gions of the San Bernardino Mountains in part to examine the possible role of H20 2 as a toxin tO trees. Although several studies were conducted on the distribution of 03 (and its effect on trees in the San Bernardino Mountains as men- tioned above), no reports are available on the levels of atmospheric H202 in the San Bernardino Mountains. Con- sidering that suggestions have been proposed for toxicity of H20 2 on plant cells (Forti and Gerola, 1977; Kaiser, 1979; Robinson et al., 1980; Tanaka et al., 1982, 1985; MacRae and

*Institute ofGeophysicsand Planetary Physics Contfibu- tionNo. 3635.

Ferguson, 1985), its effects on southern California flora may need to be evaluated. Recent publications show that the reaction of atmospheric 03 with alkenes (especially isoprene and terpenes emitted by vegetation; Becker et al., 1990; Hewitt et al., 1990) and the reaction of nighttime NO 3 radicals with volatile organic compounds (Plattet al., 1990) can produce various free radicals and ultimately lead to the formation of H202 and organic hydroperoxides (ROOHs) which might contribute to the decline of North American and central European forests already weakened by 03 or other pollutants. In plant tissues, dry and wet deposited H202 from the atmosphere pass into (partially damaged) leaves and form liquid-phase free radical species (i.e. OH., HO2. ) (M611er, 1988, 1989). The resultant free radical species as well as H202 may attack nucleic acids, proteins and lipids and in turn lead to tree damage. To date, only a few studies have been conducted to examine the toxic effect of atmospheric H202 on trees (Masuch et al., 1986; Hanson and McLaughlin, 1989; Ennis et al., 1990a, b) and its role in exacerbating the demise of certain California native vegetation is not known. The studies of Hanson and McLaughlin (1989), however, suggest that H202 alone, even at concentrations more than an order of magnitude higher than that present at San Bernardino Mountains, may not be toxic to certain conifer species, e.g. red spruce.

In this paper, we describe ground-level H202 concentra- tion in the San Bernardino Mountain forest atmosphere during the 1980s. Various atmospheric samples (such as air, precipitation, cloud and dew) were collected during the study period to measure H202 concentrations. Results of H202 concentration data obtained in the mountain region were compared with those of 03 and meteorological parameters to elucidate the factors affecting ambient levels of atmospheric H202 at high elevations in the San Bernardino Mountains. Here we report the results of measurements of atmospheric H202 in the San Bernardino Mountains and discuss factors affecting the concentration levels of atmospheric H20 2 in the region.

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METHODOLOGY

1. Samplino sites

The sampling sites for atmospheric HzO2 were at Sky Forest (1790m height above sea level), 5 km southeast of Lake Arrowhead, and at the Crgstline Air Monitoring Station of the South Coast Air Quality Management District (SCAQMD) located at Lake Gregory (1378 m height) in the San Bernardino Mountains (Fig. 1). Mixed conifer tree species, including ponderosa pine and Jeffrey pine, are pre- dominant flora at high elevation ( > 1 km) in the region.

Sky Forest is located on the south slope of the San Bernardino Mountains. This location was chosen as a sampling site in this study because the damage to conifer trees was extensively studied there during the 1970s (Miller and Taylor, 1986). Lake Gregory is located at the west side of the San Bernardino Mountains. Major sources of pollutants at both sites are thought to result from air transport from the coastal areas of the Los Angeles Basin, carried by daily sea breezes (south to southwest winds). A highway and several local streets passing through the mountains may also be a local source of pollutants in the mountain region.

2. Collection o f air, rain, cloud and dew samples

Gas-phase H202 was collected by the method of Saku- gawa and Kaplan (1987). The H2Oz collections were carried out only under clear sky or mildly cloudy days. Data of 03 concentrations and meteorological parameters at Lake Gregory were supplied by SCAQMD.

Rainwater was collected by a Teflon-coated steel funnel (70 cm diameter) with a plastic or glass bottle reservoir. The lid of the collector was opened immediately after the start of a rain event using an automatic trigger system. Two rainwater collectors were set up to obtain duplicate samples for each rain. A mercuric chloride (HgCI2) solution (1 ml of 10 mg ~- t) was added to the (glass bottle) reservoir of a collector before the start of a rain event, to prevent decomposition of organic compounds and H202 by microorganisms during the rain collection. No HgCt2 solution was added into the (plastic

bottle) reservoir of another collector for the control. Rain samples collected by the HgCl2-inoculated collector were subjected to the measurements of concentrations of H202 and organic compounds. Rains collected by the collector without HgClz-inoculation were used to measure major inorganic compounds, pH and conductivity.

Cloud water was collected at Lake Gregory using a ground-based cloud/fog collector (manufactured by Global Geochemistry Corporation, Canoga Park, CA). The col- lector type and details of the collection method were de- scribed by Brewer et al. (1983).

Dew was collected only at night through early morning (before sunrise) by condensing moisture from the air onto a 1.0 x 1.5 m Teflon sheet mounted on a 5 cm thick styrofoam pad and rested on a bench which was set up 1.0 m above the ground surface, using the method of Pierson and Brachaczek (1990). Droplets of dew settling onto the surface were coal- esced and drained into a plastic bottle.

3. Analytical methods

H202 was analysed by a fluorimetric method using p-hydroxylphenyl acetic acid and peroxidase reagents (Saku- gawa and Kaplan, 1987). The pH in rain, clouds and dew was measured by an electrode using a pH meter. Aldehydes were determined by the method of Steinberg and Kaplan (1984). Organic acids were determined by gas chromatography using the method of Kawamura and Kaplan (1984). In- organic anions were analysed by conventional ion chromato- graphy and inorganic cations were analysed by atomic absorption spectroscopy. The concentration of dissolved organic carbon (DOC) was determined by NDIR spectro- scopy after the conversion of organic compounds to CO2 by UV-assisted persulfate oxidation.

The H202 concentrations in air were measured immedi- ately after collection (within 15 min) because the decay rate of H202 is fast (Lazrus et al., 1985). The pH in rain, clouds and dew was also measured immediately after collection. The concentrations of organic acids and aldehydes were deter- mined within 2 months after rainwater samples were col- lected. Inorganic cations and anions were analysed within 2

Fig. 1. Map of Los Angeles and its adjacent areas. Solid lines indicate the major highway system in the region.

Short Communication 1511

weeks after collection. Concentrations of inorganic anions and cations and DOC in rain, clouds and dew were measured by Global Geochemistry Corporation, Canoga Park, CA. Concentrations of organic acids and aldehydes, and conduct- ivity were determined at the Institute of Geophysics and Planetary Physics, UCLA.

RESULTS AND DISCUSSION

Collections of atmospheric samples for the measurement of H20 2 concentrations were carried out at Sky Forest during 1987-1988 and at Lake Gregory during 1988-1990. Data of Oa concentrations and meteorological parameters were only available at Lake Gregory. At Sky Forest, concen- trations of H20 2 in air, rain and clouds were measured during 1987-1988, whereas at Lake Gregory H202 concen- trations in air, rain, clouds and dew were measured during 1988-1990. Below we present the analytical results.

Concentration measurements of gaseous H20 2 at Sky Forest and Lake Gregory during spring through fall in 1987-1990 show that the afternoon 4-h average (1200-1600 h, Pacific Standard Time) of H20 2 ranged from 0.79 to 3.25 ppb (n=29) and mostly 1-3 ppb. Temporal variation of H202, as well as 03, was investigated at Lake Gregory during May through November in 1989 and 1990 by collect- ing H20 2 every 4 h for 30 h (from noon until 4 p.m. the next day). Hourly concentrations of 0 3 during the study period were measured by SCAQMD at the Lake Gregory air monitoring station. The results of the study of diurnal variations for H20 2 and 0 3 indicate that concentrations of H20 2 are either highest at nighttime (three out of seven cases studied) or nearly constant through a 24-h period (four out of

seven cases studied), whereas the 0 3 concentrations were at a maximum during the afternoon and decreased in the evening through early morning (Figs 2a and 2b).

Diurnal variations of atmospheric dew points and temper- ature during the study periods are shown in Figs 2c and 2d. It is found that, in general, the diurnal pattern of 0 3 concentra- tion at Lake Gregory is associated with that of temperature, whereas no clear relationship was found between H20 2 concentration and temperature or dew point. Association of 0 3 concentrations with temperature in the inversion layer atmosphere has been recognized during several air quality studies in the Los Angeles Basin (Henry and Hidy, 1979; Zeldin et al., 1983; SCAQMD, 1985; Kuntasal and Chang, 1987). On the other hand, a positive correlation of gas-phase H20 2 concentrations with dew point was found at high altitude atmospheres over northeastern U.S.A. (Van Valin et al., 1990) and over central U.S.A. (Daum et al., 1990). In theory, water vapor is involved in the photochemical forma- tion of gas-phase H202, and high water vapor content in air accelerates the formation of gas-phase H20 2 (Calvert and Stockwell, 1983). Our results failed to show a clear relation- ship between H20 2 concentration and dew point. The results indicate that multiple factors, other than dew point, are involved in the regulation of of gaseous H20 2 concentration at Lake Gregory.

The relation of oxidant concentration with wind direction was studied. 0 3 concentration was highest when south- southwest winds were predominant, whereas 0 3 concentra- tion during other wind directions was nearly constant at a low level (Fig. 3). The south-southwest winds carry air pollutants which originate from Los Angeles, San Bernar- dino and Riverside areas, whereas winds from other direc- tions which have originated from ocean, desert or mountain areas of southern California carry only few pollutants. Other

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1512 Short Communication

investigators have shown that an afternoon 03 peak in the mountain region occurs slightly later (1-2 h) than that in San Bernardino City, at the foot of the mountains, when south- southwest winds predominate (Miller and Taylor, 1986).

The south-southwest winds predominate in summer after- noons at Lake Gregory. For example, in a typical summer month (August 1988), the south-southwest winds dominated throughout the afternoon (1200-1800 h) for 23 out of 31

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days studied, whereas the remainder were east-south winds (4 days) and southwest-west winds (4 days) (SCAQMD, unpub- lished reports). At nighttime, the south-southwest winds diminish and then east-south or southwest-west winds dom- inate from late evening through early morning when the measured concentration of O3 is low. This diurnal pattern of change of wind direction significantly affects the concentra- tion levels of 03 in the San Bernardino Mountains.

The diurnal change of wind direction in the study area appears to only produce a minor effect on concentration levels of H202. No clear relation was found between H202 concentrations and wind direction (Fig. 3), suggesting that wind direction is a secondary factor in regulating the concen- tration of H202 at Lake Gregory. Clearly, more studies will be required to elucidate the factors controlling the concentra- tion of gaseous H202 at Lake Gregory.

To determine the regional variation of atmospheric H202 levels throughout the Los Angeles Basin and the San Bernardino Mountains, time-equivalent measurements of gaseous H202 as well as 03 were conducted during spring through fall of 1987-1990 at Westwood (near coast) and Duarte (inland) in the Los Angeles Basin and at Sky Forest and Lake Gregory in the San Bernardino Mountains (Fig. 1). These H 2 0 2 and 03 measurements were carried out only during midday (1200-1600 h) using time-equivalent measure- ments at multiple locations under clear sky or under partially cloudy conditions. Results of the time-equivalent measure- ments show that H202 concentrations measured at the two sampling sites in the San Bernardino Mountains were (on overall average) about twice as high as those measured at Westwood in Los Angeles (Table I). Similarly, 03 concentra- tions measured at the mountain sites were about 50% higher than those at Westwood.

Higher H202 concentrations observed in the San Bernar- dino Mountains may be explained by the following two processes. (1) Lower NO~ and SO2 concentrations in the mountain atmosphere which facilitate higher concentrations of H202 because NOx and SO2 in air promotes the removal or scavenging of H202 from the air (Sakugawa et al., 1990). (2) Alternatively, emissions of natural hydrocarbons (e.g. isoprene or terpenes) from trees, in addition to anthropogenic hydrocarbons, may favor the generation of H202 in the mountain atmosphere. The reaction of natural hydrocarbons emitted by trees with O3 and NO3 radicals can produce H202 and organic peroxides as mentioned earlier. Simult-

Table 1. Afternoon 4-hr mean (12~, p.m.) concentrations of gaseous H202 and 03 at Westwood (W) and Duarte (D) in the Los Angeles Basin, and Lake Gregory (LG) and

Sky Forest (SF) in the San Bernardino Mountains

Concentrations Sites Mean (range) Mean (range) simultaneously measured* Number of samples H202 (#g m- 3) ? O3(/ag m- a)

W 11 0.47 (0.15-0.91) 165 (43-321) and D 0.59 (0.23--0.97) 223 (64-493)

W 17 0.82 (0.18-1.37) 152 (55-315) and LG 1.59 (0.27-2.96) 239 (91-344)

W 9 0.70 (0.26-1.42) 124 (43-235) and SF 1.44 (0.55-1.86) NA

NA: not analysed, * Simultaneous measurements of ambient concentrations of H202 and 03 were

performed from March to October in 1987-1990 at each two locations such as Westwood and Duarte, Westwood and Lake Gregory, or Westwood and Sky Forest.

? Unit in concentrations of H202 and O3 used is #g m- 3 to compare directly those concentrations among Los Angeles Basin and high elevation mountain sites.

Short Communication 1513

aneous measurements of concentrations of atmospheric oxi- dants, natural hydrocarbons such as isoprene and terpenes, and NO 3 radicals may further help in understanding the effect of natural hydrocarbons on H202 formation in forest regions of the San Bernardino Mountains.

Table 2 presents the results of chemical analysis of rain, clouds and dew collected at Sky Forest and Lake Gregory in the San Bernardino Mountains during the 1987-1990 study period. Due to the semi-arid condition throughout spring to fall, there were six or fewer rain, cloud and dew samples collected during the study period. Rainwater data from Sky Forest (n =2) and Lake Gregory (n=4) were combined into one. Other than measurements of H202, concentrations of major inorganic compounds and aldehydes (formaldehyde + acetaldehyde + glyoxal + methylglyoxal), organic acids (formic acid+acetic acid) and DOC in the atmospheric samples collected were also determined.

Concentrations of H202 in rain and cloud measured in this study ranged from 3.5 to 90 #M (volume-weighted mean 22 pM) and from 6 to 62 #M (vol-wt mean 24 pM), respect- ively, whereas H~O2 concentration in dew was low ( < 1.4/zM). These concentrations are comparable with those collected at Los Angeles urban sites. Los Angeles rain and clouds contain 0.01-145 pM (vol-wt mean 4.4#M) and 1-167 ,uM of H202, respectively (Richards, 1989; Sakugawa and Kaplan, 1992), whereas Los Angeles dew contains less than 1 #M of H~O2 (Pierson and Brachaczek, 1990). Similar concentration ranges of H202 in rain and cloudwaters to those in San Bernardino Mountains were measured by others at high elevations on Whiteface Mountain (Mohnen and Kadlececk, 1989) and on Whitetop Mountain (Olszyna et al., 1988) in eastern North America•

The concentration of other organic and inorganic com- pounds in rain ranged from micromolar to tens of micro- molar, whereas concentrations of these chemical compounds in clouds were tens to above one thousand micromolar (Table 2). Nitrate, sulfate and organic acids were the major contributors to the free acidity of rain and cloudwater collected and these ionic species accounted for 54, 28 and 18%, respectively, of the free acidity of rainwater, and 59, 27 and 15%, respectively, of the free acidity of cloudwaters. By contrast, in Los Angeles rain and clouds, both SO~- and NO~- contribute about equal amounts to the acidity (Young et al., 1986; California Air Resources Board (CARB), 1988; Sakugawa and Kaplan, 1992).

The pH of rain (5.3) and clouds (6.2) collected in the mountain region were not as low as those collected in Los Angeles (Young et al., 1986; CARB, 1988; Sakugawa and Kaplan, 1992). Relatively high concentrations of NH,~ and Ca ~+ ions in the mountain rain and clouds resulted in elevating the pH of the rain.

In summary, we believe this is the first report of the atmospheric concentration levels of H202 in forest regions of the San Bernardino Mountain, southern California. Our data indicate that the concentrations of gas-phase H202 and 03, at high elevations of the mountain region, appear to be higher than in Los Angeles urban sites. These data may help plant pathologists assess whether the H202 concentrations in the San Bernardino Mountains atmospheric burden con- tribute to the decline in conifer population.

A c k n o w l e d g e m e n t s - - W e thank the U.S. Environmental Pro- tection Agency and the National Center for Intermedia Transport Research at UCLA for supporting this project• Although the information in this document has been funded by EPA under assistance agreement CR-812771 to the National Center for Intermedia Transport Research at UCLA, it does not necessarily reflect the views of the Agency and no official endorsement should be inferred. We also thank Mr William Bope, atmospheric measurements man- ager, and other staff in SCAQMD for the measurements of atmospheric H202 at the Crestline Air Monitoring Station.

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1514 Short Communication

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Short Communication 1515

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