Judith Weinstein-Lloyd

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%.

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).

PHOTOCHEMISTRY OF OZONE FORMATION IN ATLANTA, GA-MODELS AND MEASUREMENTS*

Chemical measurements made during an air pollution event in Atlanta, GA have been compared with results from several photochemical simulations. Measurements included Os, primary reactive organic gases (ROG), aldehydes, PAN, total reactive nitrogen (NO,,) and HzOz, with vertical profiles for primary ROG. Photochemical models using two different chemical representations and a range of assumptions about winds, vertical mixing and emissions were used to simulate the event. Results show that assumptions about vertical mixing can cause a variation in simulated surface concentrations of primary hydrocarbons of a factor of two or more. A tendency to underestimate isoprene was found in comparison with measured vertical profiles. The models tend to overestimate concentrations of HCHO, H,02 and PAN in comparison with measurements. Peak 0s and concurrent NO, from helicopter measurements was used as a basis for evaluating individual model scenarios. Scenarios were developed with different Op-NO,-ROG sensitivity, but only the NO,-sensitive scenarios are consistent with measured 09, NO, and isoprene. Key word index: Ozone, nitrogen oxides, hydrocarbons, photochemical smog, hydrogen, peroxide.

Measurement of peroxides and related species in the 1993 North Atlantic Regional Experiment

Gaseous peroxides were measured during the North Atlantic Regional Experiment (NARE) aboard the Department of Energys Gulfstream G-1 aircraft in August 1993. Rights originated in Halifax, Nova Scotia and covered a region extending several hundred kilometers from the eastern edge of the North American continent. Total peroxide concentration was dominated by H2O2. Median H2O2 concentration for all flights was 2.5 parts per billion by volume (ppbv), with the highest concentrations observed in spatially extensive layers between 250 and 1750 m above sea level. In the clean free troposphere, H2O2 concentration correlates strongly with the product O3*H2O, consistent with our understanding of the formation mechanism. The median peroxide concentration was 4 ppbv in pollutant plumes, with excursions above 11 ppbv. The high concentration of H2O2 observed, and its covariance with concentrations of anthropogenic species such as O3, NOy, and aerosol particles is attributed to the aging of polluted air masses advected from the continent in stable layers.

Photochemistry of O3 and related compounds over southern Nova Scotia

Photochemical model calculations have been performed for air masses encountered by the National Research Councils Twin Otter aircraft during the 1993 summer North Atlantic Regional Experiment (NARE) intensive. These calculations use observed values of O3, NOy, CO, and hydrocarbons as constraints. NO is determined using the ratio NO/NOy measured from the National Center for Atmospheric Research King Air under comparable circumstances. Measurements over coastal locations indicate photochemically aged air masses with relatively low concentrations of NO and an OH reactivity that is dominated by CO and CH4. Samples over land have higher NO and an OH reactivity that is dominated by isoprene. Ozone production rates and H2O2 concentrations are analyzed using radical budget arguments that are applicable to low NOx conditions. The ozone production rate, P(O3), is predicted to be proportional to Q1/2[NO], where Q is the production rate for free radicals. This relation explains 99% of the variance in P(O3). Over 90% of the variance is explained by [NO] alone. P(O3) in the coastal samples is about a factor of 4 lower than previous estimates for the eastern United States. This is a consequence of low [NO] in the air masses that are advected to Nova Scotia.