Steven B. Bertman

Regional ozone from biogenic hydrocarbons deduced from airborne measurements of PAN, PPN, and MPAN

NOx-catalyzed production of ozone over large regions of North America and Europe is a serious air quality problem that often involves biogenic hydrocarbons, mainly isoprene. Peroxy-methacrylic nitric anhydride (MPAN, CH2C(CH3)C(O)OONO2) is formed uniquely from isoprene-NOx photochemistry hence is an indicator of recent ozone production from isoprene. Presented here are the first airborne measurements of MPAN along with PAN (peroxyacetic nitric anhydride, CH3C(O)OONO2), PPN (peroxypropionic nitric anhydride, CH3CH2C(O)OONO2) and ozone measurements. Relationships between these species are used to estimate the contributions of anthropogenic and biogenic hydrocarbons (BHC) to regional tropospheric ozone production, providing direct evidence of ozone production from BHC-NOx photochemistry.

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.

Measurements of PAN, PPN, and MPAN made during the 1994 and 1995 Nashville Intensives of the Southern Oxidant Study: Implications for regional ozone production from biogenic hydrocarbons

Isoprene and a variety of other reactive hydrocarbons are released in large quantities by vegetation in forested regions and are thought to participate in the NOx-catalyzed production of ozone, a serious air quality problem in North America and Europe [National Research Council, 1991]. The determination of the fraction of O3 formed from anthropogenic NOx and biogenic hydrocarbons (BHC) is a crucial step in the formulation of effective control strategies. Peroxymethacrylic nitric anhydride (MPAN, CH2C(CH3)C(O)OONO2) is formed almost entirely from the atmospheric oxidation of isoprene in the presence of NOx and is an excellent indicator of recent ozone production from isoprene and therefore biogenic hydrocarbons. Measurements are presented here of MPAN, peroxyacetic nitric anhydride (PAN, CH3C(O)OONO2), peroxypropionic nitric anhydride (PPN, CH3CH2C(O)OONO2) and ozone from separate data sets acquired during the 1994 and 1995 Nashville intensive studies of the Southern Oxidant Study. It was found that PAN, a general product of HC-NOx photochemistry, could be well represented as a simple linear combination of contributions from BHC and anthropogenic hydrocarbon (AHC) chemistries as indicated by MPAN and PPN, respectively. The PAN:MPAN ratios found to be characteristic of BHC-dominated chemistry ranged from 6 to 10. The PAN:PPN ratios found to be characteristic of AHC-dominated chemistry ranged from 5.8 to 7.4. These BHC and AHC attributions were used to estimate the contributions of anthropogenic and biogenic hydrocarbons to regional tropospheric ozone production, and substantial BHC-O3 (50–60 ppbv) was estimated in cases where high NOx from power plants was present in areas of high BHC emission. This estimation method provides direct evidence of significant photochemical ozone production from the oxidation of biogenic hydrocarbons in the presence of NOx.

Overview of the Program for Research on Oxidants: PHotochemistry, Emissions, and Transport (PROPHET) summer 1998 measurements intensive

In this paper we introduce results obtained from the Program for Research on Oxidants: PHotochemistry, Emissions, and Transport (PROPHET) program that is being conducted at the University of Michigan Biological Station in northern Michigan. PROPHET is an independent consortium of individually funded scientists whose mutual interests and varied experiences have created a synergistic collaboration focused on studies of atmospheric chemical and meteorological processes linked to tropospheric ozone. Since 1997, the PROPHET science team has combined expertise to characterize the important atmospheric issues in this region and to begin to push the limits of our knowledge of the links between the biosphere and the atmosphere. The opportunity to conduct research in the physical context of the Biological Station enables this team to interact with a tremendous range of activities related to forest and ecosystem health and uniquely positions PROPHET to make contributions to the emerging field of biosphere-atmosphere interactions.

Observations of reactive oxidized nitrogen and speciation of NO y during the PROPHET summer 1998 intensive

Measurements of NO y , NO x , PAN, PPN, MPAN, C 3 -C 5 alkyl nitrates, total isoprene nitrates, HNO 3 , HONO, and aerosol NO 3 - and NO 2 - were made during the summer 1998 intensive of the Program for Research on Oxidants: Photochemistry, Emissions, and Transport (PROPHET). Mixing ratios of NO y , NO x , PAN, PPN, and alkyl nitrates were observed to have a strong dependence on the direction of transport to the site as was expected from the distribution of major urban and industrial sources. A peak in NO x and NO x /NO y during the morning in southerly flow provides evidence for the transport of relatively unprocessed emissions in layers above the nocturnal boundary layer. The difference in PAN and PPN levels between north and south flow directions indicates a net transport of reactive nitrogen to regions farther north. Isoprene nitrates were observed to typically comprise less than 1% of NO y in spite of the dominance of isoprene in local VOC chemistry, although due to the rapid losses of isoprene nitrates through reaction, vertical mixing, and deposition, their role in the processing of NO y may be significant. General agreement was observed between NO y and the sum of the individually measured constituents during ten 24 hour periods, although a linear regression indicates the potential for an interference in the individual measurements and a possible shortfall in NO y . Measurements indicate that HONO may play a larger role in the reactive nitrogen budget than previously expected for a rural site. HONO/NO 2 for a 24 hour period was observed to be 0.09-0.25 and suggests the likelihood of a significant heterogeneous production pathway or pathways.

A study of formaldehyde chemistry above a forest canopy

Gas-phase formaldehyde (HCHO) was measured at a mixed deciduous/coniferous forest site as a part of the PROPHET 1998 summer field intensive. For the measurement period of July 11 through August 20, 1998, formaldehyde mixing ratios ranged from 0.5 to 12 ppb at a height ∼10 m above the forest canopy, with the highest concentrations observed in southeasterly air masses. Concentrations varied on average from a mid-afternoon maximum influenced by photochemical production of 4.0 ppb, to a late night minimum of 2.2 ppb, probably resulting from dry depositional loss. An analysis of local HCHO sources revealed that isoprene was the most important of the measured formaldehyde precursors, contributing, on average, 82% of the calculated midday HCHO production rate. We calculate that the nighttime HCHO dry deposition velocity is 2.6 times that of ozone, or approximately 0.65 cm/s. In the daytime, photolysis, dry deposition, and reaction with hydroxyl radical (OH) are roughly equally important as loss processes. Explicit calculations of HCHO chemical behavior highlighted the probable importance of transport and surface deposition to understanding the diel behavior of formaldehyde.

Measurement of alkyl nitrates at Chebogue Point, Nova Scotia during the 1993 North Atlantic Regional Experiment (NARE) intensive

Measurements of six alkyl nitrates, methyl-, ethyl-, 1-propyl-, 2-propyl-, 2-butyl, and 3-pentyl nitrates, and two peroxycarboxylic nitric anhydrides, peroxyacetic nitric anhydride (PAN) and peroxypropionic nitric anhydride (PPN) were made at Chebogue Point, Nova Scotia, during the 1993 North Atlantic Regional Experiment campaign. The correlation of alkyl nitrates (RONO2) with carbon monoxide makes it clear that the chemistry at this site is being driven by continental emissions, since RONO2 compounds are known to be by-products of HC-NOx photochemistry. Comparison of the ratios of alkyl nitrates to their parent hydrocarbons to that of 2-butyl nitrate/butane showed significant deviations from trends predicted from rate constants, branching ratios, and loss rates. For ethyl nitrate this deviation is likely due to a combination of many additional pathways to the intermediate peroxy radical from the decomposition of larger alkoxy radicals. The propyl nitrates showed trends that differed from one another; 2-propyl nitrate was a factor of 2 to 3 higher than predicted, while 1-propyl nitrate was a factor of 8 to 10 higher than predicted. The trend of 3-pentyl nitrate/pentane was relatively close (about 50% lower) to the predicted trend. The relationship between PAN and PPN showed unexpectedly high ratios of PPN to PAN in air masses coming from the south and west of the site, implying a slight local source of PPN.

Variations in NOy composition at Idaho Hill, Colorado

Measurements of NO y and the principal constituent NO y species (NO, NO 2 , PAN, HNO 3 , PPN, NO 3 -) were taken during the fall of 1993 at a high-elevation site in the Colorado mountains west of Boulder. The meteorology provided two principal flow regimes: strong westerly flow, bringing drier and cleaner air over the Continental Divide from the west where emission sources are sparse, and weaker easterly upslope flow which brought moist and more polluted air from the Denver-Boulder urban corridor and perhaps from smaller more local sources such as the mountain town of Nederland. The upslope flow data generally indicate a balance between measured NO y and the separately measured NO y species with NO x , PAN, and HNO 3 accounting for over 90%. Under these conditions, NO x was the dominant fraction of NO y , and occasionally fresh emissions were indicated as shown by NO x /NO y close to unity. For downslope flow there was generally a shortfall in the NO y balance where measured NO y was higher than the constituent species by about 20%, on average. Known potential interferences in the measurements do not appear to account for the NO y deficit. Missing NO y correlated directly with NO x /NO y , O 3 , and aerosols and inversely with temperature. These relationships suggest the presence of one or more NO y species that were not measured by the individual techniques.

Episodic removal of NOy species from the marine boundary layer over the North Atlantic

Measurements of ozone, carbon monoxide, NOy, NOx, HNO3, particulate NO3−, PAN, PPN, and alkyl nitrates were made at Chebogue Point, Nova Scotia, during July, August, and September 1993, and measurements of ozone, carbon monoxide, NOx and N0y were made at Sable Island, Nova Scotia, during August and September 1993 as part of the North Atlantic Regional Experiment (NARE). Periods of photochemical pollution were observed during which concentrations of the above species were elevated relative to average conditions. The best indicator of the extent of impact was found to be the sum of the C1-C4 alkyl nitrates. The removal of PAN and PPN by fog droplets through thermal decomposition and subsequent uptake of the peroxyacetyl and peroxypropionyl radicals (PA and PP) was suggested by loss of PAN and PPN during fog. The effect of this chemistry on NOy species in the marine environment will need to be considered. The loss of inorganic nitrate (HNO3 and particulate NO3−), PAN (and to a minor extent PPN) resulted in deviation of the O3 versus NOy-NOx correlations from that observed in eastern North America.

Measurements of isoprene nitrates above a forest canopy

Measurements of atmospheric organic nitrates derived from isoprene, i.e., “isoprene nitrates”, were conducted from July 14 to August 19, 1998, as part of the 1998 summer intensive measurement campaign of the Program for Research on Oxidants: PHotochemistry, Emissions, and Transport (PROPHET) at the University of Michigan Biological Station in Pellston, Michigan. The measurements were conducted using on-line chromatography in conjunction with a nitrate-selective detection scheme. Measured concentrations of isoprene nitrates ranged from 0.5 parts per trillion (ppt), the detection limit of the method employed, to 35 ppt. In this paper we discuss the contribution of the isoprene nitrates to NOy, which was typically 0.5–1.5% of total odd nitrogen, but up to ∼4% for well-aged air. Concentrations of isoprene nitrates exhibited a strong diurnal variation consistent with their expected chemical and physical removal rates. In this work we also discuss the chemistry of the precursor peroxy radicals and the NOx dependence of isoprene nitrate formation.