E. J. Williams

Indications of photochemical histories of Pacific air masses from measurements of atmospheric trace species at Point Arena, California

Measurements of light hydrocarbons, ozone, peroxyacetyl nitrate (PAN), HNO3, NO3−, NOx, NOy, and meteorological parameters were made during a 10-day period in April and May 1985 at Point Arena, a coastal inflow site on the Pacific Ocean in northern California. The meteorological measurements indicate that during this study the sampled air was usually from the marine boundary layer with little land influence on the meteorological parameters. In this marine air the mixing ratios of the alkanes, ozone, PAN, and HNO3 showed strong correlations coincident with variations in the origins of calculated air parcel trajectories and with variations in the ratios of the light alkanes. This variation in the ratios is attributed to different degrees of photochemical aging of the alkanes that are generally consistent with the calculated trajectories. This behavior indicates that the alkane levels are determined by transport to the marine area from continental sources, most likely Asian, followed by photochemical removal over the Pacific Ocean. Since the concentrations of PAN and ozone correlate well with the alkane ratios, it is concluded that the observed PAN and ozone were dominated by continental sources and removal processes in the marine areas. This and other marine studies have observed a strong correlation of PAN and ozone, and it is suggested that production over the continents, transport to the marine areas, and parallel removal processes account for much of the observed correlation. From the correlation of these two species with the measured alkane ratios, approximate net lifetimes of PAN and ozone in the marine troposphere of ≤2.5 and ≥19 days, respectively, are derived. The primary conclusion is that the alkanes, ozone, and PAN in these air parcels from the Pacific marine troposphere are dominated by transport from continental sources and removal by photochemical processes. Direct emissions of the alkanes and in situ photochemical production of PAN and ozone from precursors emitted into the marine region from the surface or the stratosphere must play less important roles. Similar indications of continental influence in marine areas have been seen in other studies of ozone, the sulfur cycle, oxidized nitrogen, and hydrocarbons. It is suggested that the ratios of the light alkanes provide photochemical “clocks” that are useful for gauging the importance of continental influence in a particular marine measurement.

An efficient photolysis system for fast‐response NO2 measurements

A new photolytic converter for NO2 measurement is described and its performance assessed using laboratory, ground-, and aircraft-based field data. Focusing the output of a 200-W short-arc Hg lamp into a photolytic cell attains conversion fractions of NO2 to NO greater than 0.70 in cell residence times of less than a second. Limiting lamp output to wavelengths greater than 350 nm by means of optical filters increases specificity for NO2, affording a peroxyacetyl nitrate conversion fraction of less than 0.006 and negligible conversion of HNO3. Unwanted (artifact) signal in clean synthetic air is also greatly minimized through the use of optical filters. Fast instrument response is achieved by minimizing NO2 inlet line and photolysis cell residence times. NO and NO2 sample residence times are matched in a multichannel instrument so that signal from ambient NO may be easily subtracted from the total signal and ambient NO2 calculated by difference at high time resolution. Induced change in the ambient ratio of NO to NO2, due to reaction of ozone and other oxidants with NO during sampling, is minimized in the new design. This configuration permits simple and accurate retrieval of NO2 concentrations in aircraft transects of power plant plumes, where ambient NO concentrations can change over several orders of magnitude in seconds. At lower concentrations found in the planetary boundary layer, agreement between calculated and observed NO2 is within ±(40 pptv+7%) for a 10-s average. The new converter consumes less power, is more efficient, and is less expensive to operate than previous photolysis designs.

Photochemical modeling of hydroxyl and its relationship to other species during the Tropospheric OH Photochemistry Experiment

Because of the extremely short photochemical lifetime of tropospheric OH, comparisons between observations and model calculations should be an effective test of our understanding of the photochemical processes controlling the concentration of OH, the primary oxidant in the atmosphere. However, unambiguous estimates of calculated OH require sufficiently accurate and complete measurements of the key species and physical variables that determine OH concentrations. The Tropospheric OH Photochemistry Experiment (TOHPE) provides an extremely complete set of measurements, sometimes from multiple independent experimental platforms, that allows such a test to be conducted. When the calculations explicitly use observed NO, NO2, hydrocarbons, and formaldehyde, the photochemical model consistently overpredicts in situ observed OH by ∼50% for the relatively clean conditions predominantly encountered at Idaho Hill. The model bias is much higher when only CH4-CO chemistry is assumed, or NO is calculated from the steady state assumption. For the most polluted conditions encountered during the campaign, the model results and observations show better agreement. Although the comparison between calculated and observed OH can be considered reasonably good given the ±30% uncertainties of the OH instruments and various uncertainties in the model, the consistent bias suggests a fundamental difference between theoretical expectations and the measurements. Several explanations for this discrepancy are possible, including errors in the measurements, unidentified hydrocarbons, losses of HOx to aerosols and the Earths surface, and unexpected peroxy radical chemistry. Assuming a single unidentified type of hydrocarbon is responsible, the amount of additional hydrocarbon needed to reduce theoretical OH to observed levels is a factor of 2 to 3 greater than the OH-reactivity-weighted hydrocarbon content measured at the site. Constraints can be placed on the production and yield of various radicals formed in the oxidation sequence by considering the observed levels of certain key oxidation products such as formaldehyde and acetaldehyde. The model results imply that, under midday clean westerly flow conditions, formaldehyde levels are fairly consistent with the OH and hydrocarbon observations, but observed acetaldehyde levels are a factor of 4 larger than what is expected and also imply a biogenic source. Levels of methacrolein and methylvinylketone are much lower than expected from steady state isoprene chemistry, which implies important removal mechanisms or missing information regarding the kinetics of isoprene oxidation within the model. In a prognostic model application, additional hydrocarbons are added to the model in order to force calculated OH to observed levels. Although the products and oxidation steps related to pinenes and other biogenic hydrocarbons are somewhat uncertain, the addition of a species with an oxidation mechanism similar to that expected from C10 pinenes would be consistent with the complete set of observations, as opposed to naturally emitted isoprene or any of the anthropogenic hydrocarbons examined in the model. Further constraints on the abundance of peroxy radicals are necessary in order to fill the gaps in our understanding of OH photochemistry for the clean continental conditions typical of Idaho Hill.

Intercomparison of ground-based NOy measurement techniques

An informal intercomparison of NOy measurement techniques was conducted from June 13 to July 22, 1994, at a site in Hendersonville, Tennessee, near Nashville. The intercomparison involved five research institutions: Brookhaven National Laboratory, Environmental Science and Engineering, Georgia Institute of Technology, NOAA/Aeronomy Laboratory, and Tennessee Valley Authority. The NOy measurement techniques relied on the reduction of NOy species to NO followed by detection of NO using O3-chemiluminescence. The NOy methods used either the Au-catalyzed conversion of NOy to NO in the presence of CO or H2 or the reduction of NOy to NO on a heated molybdenum oxide surface. Other measurements included O3, NOx, PAN and other organic peroxycarboxylic nitric anhydrides, HNO3 and particulate nitrate, and meteorological parameters. The intercomparison consisted of six weeks of ambient air sampling with instruments and inlet systems normally used by the groups for field measurements. In addition, periodic challenges to the instruments (spike tests) were conducted with known levels of NO, NO2, NPN, HNO3 and NH3. The NOy levels were typically large and highly variable, ranging from 2 ppbv to about 100 ppbv, and for much of the time was composed mostly of NOx from nearby sources. The spike tests results and ambient air results were consistent only when NOx was a substantial fraction of NOy. Inconsistency with ambient air data and the other spike test results is largely attributed to imprecision in the spike results due to the high and variable NOy background. For the ambient air data, a high degree of correlation was found with the different data sets. Of the seven NOy instrument/converters deployed at the site, two (one Au and one Mo) showed evidence of some loss of conversion efficiency. This occurred when the more oxidized NOy species (e.g., HNO3) were in relatively high abundance, as shown by analysis of one period of intense photochemical activity. For five of the instruments, no significant differences were found in the effectiveness of NOy conversion at these levels of NOy with either Au or Mo converters. Within the estimated uncertainty limits there was agreement between the sum of the separately measured NOy species and the NOy measured by the five of the seven techniques. These results indicate that NOy can be measured reliably in urban and suburban environments with existing instrumentation.

Decadal change in carbon monoxide to nitrogen oxide ratio in U.S. vehicular emissions

[1] Accurate emission inventories and their temporal trends must be incorporated into pollutant inventories to allow for reliable modeling of the country’s past, present, and future air quality. Measured carbon monoxide (CO) and nitrogen oxide (NOx) concentrations from two urban areas show that the CO/NOx vehicular emission ratio has decreased at an average rate of 7–9% per year from 1987 to 1999. This amounts to a factor of nearly 3 over the 12 years. The current U.S. Environmental Protection Agency tabulations of estimated pollutant emission trends indicate a rate of decrease smaller by a factor of 2–3. The trend in maximum ambient CO levels in U.S. cities suggests a 5.2 ± 0.8% per year average annual decrease in CO vehicular emissions, which implies a 2–3% annual increase in NOx emissions from vehicles. Thus over the decade of the 1990s, annual U.S. CO emissions from vehicles have decreased from 65 to 38 Tg, representing approximate decreases of 6 and 3% in the annual global fuel-use CO emissions and in total global anthropogenic CO emissions, respectively. It is expected that the volatile organic compound (VOC)/NOx vehicular exhaust emission ratio has decreased similarly, implying that the character of atmospheric photochemistry in U.S. urban areas has changed significantly over the decade. INDEX TERMS: 0345 Atmospheric Composition and Structure: Pollution—urban and regional (0305); 0322 Atmospheric Composition and Structure: Constituent sources and sinks; 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; KEYWORDS: vehicle emissions, carbon monoxide, nitrogen oxides, emission inventories, temporal change

Peroxy radicals in the ROSE experiment: Measurement and theory

The concentrations of the HO2-RO2 species measured during July 11, 1990, in the ROSE (Rural Oxidants in the Southern Environment) study in Alabama are compared to those expected in theory from calculations based upon detailed hourly measurements of a variety of trace gases including the hydrocarbons, NO, NO2, carbonyl compounds, CO, PAN (peroxyacetylnitrate) and calculated jO3 values. The measurements are also compared with the [HO2] + [RO2] as estimated from deviations from the NO2 + hv (+O2) ⇄ NO + O3 photostationary state. Within the error of the measurements all of the data appear to be in reasonable accord.

Peroxy radicals as measured in ROSE and estimated from photostationary state deviations

Ambient measurements of peroxy radical concentrations were made using the chemical amplifier (CA) and determined independently from photostationary state deviations (PSSD), NO2 + hv (+O2) ⇌ NO + O3, derived from simultaneous measurements of O3, NO, NO2 and jNO2. The data were collected in the Rural Oxidants in the Southern Environment experiment during 19 days of July 1990. A reasonably good correspondence between the two methods is observed for many of the days, although estimates from the PSSD method for some of the days are higher by as much as a factor of 2. Scatter observed between estimates probably results from several sources: uncertainties in the calibration of the RO2-HO2 instrument, rapid changes in the ozone background, rapid alterations in the solar flux induced by intermittent cloud cover, and imprecisions in making simultaneous measurements of [NO], [NO2], [O3], and jNO2 required for the PSSD method. Possible origins of bias in the two measurement techniques are discussed. Theoretical estimates of the peroxy radical concentrations were made using the measured suite of trace gas concentrations for 2 days, one for which the CA and PSSD estimates of peroxy radical concentrations differed significantly (July 14) and one for which they showed good agreement (July 11). Theoretical estimates for July 11 checked well with the results from both methods. Those for July 14 fell between the GA and PSSD estimates. These results suggest that the PSSD method may have a bias toward higher estimates on some days and/or that the CA method may have a bias for the lower estimates for reasons which are discussed.

HO2/OH and RO2/HO2 ratios during the Tropospheric OH Photochemistry Experiment: Measurement and theory

Ambient concentrations of the hydroxyl (OH), hydroperoxyl (HO2), and total peroxy (ΣRO2) radicals were measured as part of the Tropospheric OH Photochemistry Experiment at Idaho Hill, Colorado, during August and September of 1993. OH radicals were measured using ion-assisted mass spectroscopy and low-pressure laser-induced fluorescence (LIF) detection techniques. HO2 was measured using chemical conversion and LIF detection of OH. ΣRO2 radicals were measured using a chemical amplifier technique. The simultaneous measurements of these key species provide an opportunity to test our present understanding of the fast photochemistry of the troposphere. Measured HO2/OH ratios were typically between 15 and 80, and agreed well with predictions under conditions where NO mixing ratios were greater than 100 pptv. However, under clean conditions the measured ratio was a factor of 3–4 lower than predicted. The RO2/HO2 ratio was typically a factor of 4–15 larger than predicted by present theories of tropospheric chemistry. A steady state model was used in an attempt to analyze the discrepancies between the measured HO2/OH and RO2/HO2 ratios with present theories of hydrocarbon oxidation in the troposphere.

Isoprene and its oxidation products, methacrolein and methylvinyl ketone, at an urban forested site during the 1999 Southern Oxidants Study

Isoprene (ISOP) and its oxidation products, methacrolein (MACR) and methyl vinyl ketone (MVK), were measured at an urban forested site in Nashville, Tennessee, as part of the 1999 Southern Oxidants Study (SOS). Hourly observations were performed at Cornelia Fort Airpark for a 4 week period between June 13 and July 14. At the midday photochemical peak (1200 local standard time, LST), average mixing ratios of isoprene, MACR, and MVK were 410 parts per trillion by volume (pptv), 240 pptv, and 430 pptv, respectively. Median isoprene, MACR, and MVK mixing ratios were 400 pptv, 200 pptv, and 360 pptv, respectively, at 1200 LST. An emissions inventory calculation for Davidson County, encompassing Nashville, suggests that MACR and MVK were produced predominately from isoprene oxidation rather than direct combustion emissions. The observations are compared with results from two chemical models: a simple sequential reaction scheme and a one-dimensional (1-D) numerical box model. The daytime ratios of MVK/ISOP and MACR/ISOP varied in a systematic manner and can be reproduced by the analytical solution of the sequential reaction scheme. Air masses with more photochemically aged isoprene were observed during SOS 1999 at Cornelia Fort (0.3-1.6 hours) compared to the SOS 1990 canopy study at Kinterbish (0.1-0.6 hours). This is consistent with the proximity of the tower inlets to the forest canopies during both campaigns. Isoprene had a chemical lifetime of 20 min at the average observed midday HO mixing ratio of 8 x 10 6 molecules/cm 3 . As a result, significant conversion of isoprene to its oxidation products was observed on the timescale of transport from the dense forest canopies surrounding Nashville. The systematic diurnal behavior in the MVK/MACR ratio can also be simulated with a 1-D photochemical box model. General agreement between the observations of MACR and MVK during SOS 1999 with the two chemical models suggests we have a comprehensive understanding of the first few stages of isoprene oxidation in this urban forested environment.

Photochemical ozone production in the rural southeastern United States during the 1990 Rural Oxidants in the Southern Environment (ROSE) program

Extensive measurements of ozone and its photochemical precursors and coproducts were made in the 1990 Rural Oxidants in the Southern Environment (ROSE) program. Peroxy radical production, loss, and partitioning are described at a rural site in Alabama, showing the important role that biogenic organic compounds play in ozone production. Estimates of the peroxy radical concentration obtained by four methods along with the measured nitric oxide level are used to predict the instantaneous rate of photochemical ozone production at the site. The four methods agree on the diurnal behavior of peroxy radicals and ozone production rates, while consistent discrepancies between the methods generally are within their combined uncertainties. Selected aircraft measurements are used to derive ozone production rates above the ground site, with the highest rates occurring in the boundary layer and in industrial plumes. The dependences of peroxy radical concentration and ozone production rate on the level of nitrogen oxides exhibit good agreement between the various methods and are consistent throughout the lower troposphere. Surface deposition and entrainment are shown to be as important as photochemical production in determining the diurnal evolution of ozone at this site.