Xianliang Zhou

Ozone formation at a rural site in the southeastern United States

Trace gas measurements pertinent to understanding the transport and photochemical formation of O3 were made at a surface site in rural Georgia as part of the Southern Oxidant Study during the summer of 1991. It was found that there was a strong correlation between O3 and the oxidation products of NOx: O3(ppb) = 27 + 11.4 (NOy(ppb) − NOx(ppb)), r2 = 0.78. This fit is similar to that observed at other rural sites in eastern North America and indicates a nominal background O3 level of 27 ppb; values higher than 27 ppb are due to photochemical production in the recent past, which varied from near zero to ≈50 ppb. The origin of the O3 above background was investigated by using a free radical budget equation to calculate an in situ O3 production rate in terms of measured concentrations of NO and free radical precursors (O3, HCHO, peroxides, and other carbonyls). A comparison of observed and predicted diurnal trends in O3 indicates significant O3 production in the afternoon at a time when O3 concentration is either steady or decreasing. The afternoon near-surface layer is thereby a source region for O3 which can be exported. In situ production accounts for approximately one half of the morning increase in O3 concentration on days with high O3; the remainder is due to entrainment of dirty air aloft by the growing convective boundary layer. Additional evidence for the role of vertical transport in controlling the hour-to-hour changes in O3 is found in the diurnal cycles of SO2 and HNO3 which also have rapid increases in the morning. The day-to-day variability of O3 was investigated using a back trajectory model. NOy concentration at the measurement site could be reasonably accounted for by considering NOx emission sources located within 1-day transport distance. In as much as there is a strong correlation between O3 and NOy, the coincidence between trajectory location and NOx emission sources appears to be an important factor influencing midday O3 concentration. Hydrocarbon measurements are consistent with NOx being the limiting factor for formation of O3.

Determination of photochemically produced hydroxyl radicals in seawater and freshwater

A variety of short-lived, reactive chemical species (i.e. free radicals and excited state species) are known to be photochemically produced in natural waters. Some of these transients may strongly affect chemical and biological processes, and they have been implicated in the degradation of organic pollutants and natural organic compounds in aqueous environments. Previous studies demonstrated that the highly reactive hydroxyl radical (OH) is photochemically formed in seawater. However, the quantitative importance of this key species in the sea has not been previously studied because of past analytical limitations. By using a highly sensitive probe based on α-H atom abstraction from methanol, we were able to measure production rates and steady-state concentrations of photochemically produced OH radicals in coastal and open ocean seawater and freshwaters. The validity of the method was tested by intercalibrating with an independent, OH-specific reaction, hydroxylation of benzoic acid, and also by competition kinetics experiments. Our OH production rates and steady-state concentrations for freshwaters are in excellent agreement with those measured by previous investigators for similar waters. In contrast, for seawater, the values we measured are 1–3 orders of magnitude higher than previously predicted by models, indicating that there is a major unknown photochemical OH source (s) in seawater.

Photochemical production of low-molecular-weight carbonyl compounds in seawater and surface microlayer and their air-sea exchange

Abstract Coastal and oceanic surface microlayer samples were collected using a stainless steel screen, along with subsurface bulk seawater, and were analyzed for low-molecular-weight (LMW) carbonyl compounds, including formaldehyde, acetaldehyde, propanal, glyoxal, methylglyoxal, glyoxylic acid and pyruvic acid. The enrichment factor in surface microlayer compared to corresponding subsurface seawater ranged from 1.2 to 21. A time-series measurement at a coastal site showed strong diurnal variations in concentrations of the LMW carbonyl compounds in the surface microlayer and in the enrichment factor, with maxima in the early afternoon and minima in the early morning. Exposure of samples to sunlight resulted in the higher yields of these compounds in the surface microlayer than in the bulk seawater, by a factor of 1.1–25, suggesting that the higher photoproduction rate of LMW carbonyl compounds in the surface microlayer accounts for the majority of the observed enrichment in these samples. Potential sinks include biological uptake and mixing. Air-sea exchange may be a source for soluble compounds and a sink for less soluble compounds. The enrichment of the LMW carbonyl in surface microlayer may alter their net air-sea exchange direction e.g., from the ocean as a potential sink to a source for atmospheric acetaldehyde and acetone. The residence times of the LMW carbonyl compounds in the microlayer were estimated to be on the order of tens of seconds to minutes using a modified two-layer model. However, to maintain the observed microlayer enrichment factor, the residence time should be on the order of ~ 1 hour. This prolonged residence time may be due to organic enrichment in the surface microlayer (‘organic film’) which inhibited molecular transfer of carbonyl compounds into and out of the microlayer. The deviated behavior from model prediction may also be due to changes in the apparent partition coefficients of these species as a result of thier physical and chemical interactions with organic matrix in the surface microlayer.

Peroxy radical concentration and ozone formation rate at a rural site in the southeastern United States

As part of the Southern Oxidants Study, Brookhaven National Laboratory operated an intensive measurement site near Metter, Georgia, during parts of the summers of 1991 and 1992. Measurements were made of photochemically active trace gases and meteorological parameters relevant to determining causes for elevated ambient ozone concentration. The 1992 data set was used to calculate peroxy radical concentration and ozone formation rate based on determining the departure from the photostationary state (PSS) and based on a radical budget equation, such as applied previously to the 1991 data set. Averaged over the 28-day experimental period, we find maximum radical production occurring near noon at 2.5 ppb h−1, maximum peroxy radical concentration also occurring near noon at 80 ppt, and maximum ozone production of 8 ppb h−1 occurring near 1000 EST. Ozone photolysis accounts for 55% of radical production, HCHO and other carbonyl compounds about 40%. The radical budget and PSS methods depend in different ways on atmospheric photochemistry and a comparison between them affords a test of our understanding of the photochemical production of O3. We find that these methods agree to the extent expected based on uncertainty estimates. For the data set as a whole, the median estimate for fractional error in hourly average peroxy radical concentration determined from the radical budget method is approximately 30% and from the PSS method, 50%. Error estimates for the PSS method are highly variable, becoming infinite as peroxy radical concentration approaches zero. This behavior can be traced back to the difference form of the PSS equations. To conduct a meaningful comparison between the methods, the data set was segregated into subsets based on PSS uncertainty estimates. For the low-uncertainty subset, consisting of a third of the whole data set, we find that the ratio of peroxy radical concentration predicted from the PSS method to that predicted from the radical budget method to be 1.22±32%.

Tropospheric formaldehyde concentration at the Mauna Loa Observatory during the Mauna Loa Observatory Photochemistry Experiment 2

The concentration of formaldehyde at Mauna Loa Observatory, Hawaii, was determined during four Mauna Loa Observatory Photochemistry Experiment 2 (MLOPEX 2) measurement intensives between September 1991 and August 1992. The observed diurnal variations, 200–900 parts per trillion by volume (pptv) during daytime and 60–200 pptv during nighttime, resulted mainly from the local air circulation pattern whereby island modified marine boundary layer air prevailed during the day and free tropospheric air dominated during the night. A seasonal variation was also observed; the median/mean values of all data points are: 149/196, 129/149, 143/178, and 181/211 pptv for autumn, winter, spring, and summer intensives, respectively. During nighttime downslope flow periods which brought in free tropospheric air to the measurement site, the formaldehyde concentrations (median/mean) were 122/123, 110/112, 120/125, and 140/137 pptv for autumn, winter, spring, and summer, respectively. This seasonal dependence may be attributable to changes in solar insolation and NO concentrations. A simple box model calculation constrained by the experimentally determined concentrations of CH3OOH yielded a formaldehyde concentration (without/with heterogeneous removal) for free tropospheric air, at 7°C, of 155/140, 125/115, 210/195, and 220/205 pptv for autumn, winter, spring and summer, respectively. The calculated values are in good agreement with the measured concentrations for winter (within 27/15%, without/with heterogeneous removal) and fall (within 14/5%), but are significantly higher for spring (75/63%) and summer (57/46%).

Carbonyl compounds in the lower marine troposphere over the Caribbean Sea and Bahamas

Formaldehyde, acetaldehyde, acetone, and butanone were measured in 59 samples of marine air. Average clean air background concentrations were about 0.55, 0.50, 0.38, and 0.03 ppbv, respectively, in agreement with past measurements. The formaldehyde concentration is also in agreement with that predicted from photooxidation of methane and other locally derived organic matter. Formaldehyde and acetaldehyde showed strong diurnal variations throughout the 12-day sampling period. Photochemical oxidation of locally derived organic matter, such as nonmethane hydrocarbons and long-chained lipids, appears to be the major source for both formaldehyde and acetaldehyde in the lower marine boundary layer. Acetone, on the other hand, showed weaker diurnal fluctuations, consistent with its significantly longer lifetime in the trophosphere. Sinks for carbonyl compounds in the lower marine boundary layer are less clearly known. The results suggest that photolysis, reaction with free radicals, and deposition at the sea surface are minor, short-term sinks during the study. The main sink appears to be vertical mixing, probably followed by photolysis in the upper marine boundary layer and free troposphere. 34 refs., 6 figs., 2 tabs.