Troy Thornberry

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.

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.

Loss of isoprene and sources of nighttime OH radicals at a rural site in the United States: Results from photochemical models

[1] A one-dimensional Lagrangian model for atmospheric transport and photochemistry has been developed and used to interpret measurements made at Pellston, Michigan, during the summer of 1998. The model represents a moving vertical column of air with vertical resolution of 25 m near the ground. Calculations have been performed for a series of trajectories, with representation of emissions, vertical mixing, and photochemistry for a 3-day period ending with the arrival of the air column at Pellston. Results have been used to identify causes of the observed decrease in isoprene at night, to investigate causes of high nighttime OH. Significant OH can be generated at night by terpenes if it is assumed that some fast-reacting monoterpenes are emitted at rates comparable to inventory emissions for terpenes. However, this nighttime OH is confined to a shallow surface layer (0–25 m) and has little impact on nighttime chemistry. The observed decrease in isoprene at night can be reproduced in models with low OH, and is attributed primarily to vertical dilution. There is also evidence that transport from Lake Michigan contributes to low nighttime isoprene at Pellston. Model results compare well with measured isoprene, NOx, and with isoprene vertical profiles. Significant model-measurement discrepancies are found for OH, HO2, methylvinylketone, and formaldehyde. INDEX TERMS: 0365 Atmospheric Composition and Structure: Troposphere— composition and chemistry; 0315 Atmospheric Composition and Structure: Biosphere/atmosphere interactions; 0345 Atmospheric Composition and Structure: Pollution—urban and regional (0305)

Fast‐turnaround alkyl nitrate measurements during the PROPHET 1998 summer intensive

An automated gas chromatographic system for making fast-turnaround measurements of alkyl nitrates was developed and tested. Every 30 min, samples were automatically acquired on a 1/16-inch OD Tenax trap made from fused silica-lined stainless steel tubing and injected by thermal desorption into the gas chromatograph. The system exhibited good chromatographic resolution, minimal breakthrough, and on-trap stability for C3-C5 nitrates. The detection limit of the instrument was <0.1 parts per trillion by volume for C3-C5 nitrates with an overall uncertainty of 30% for isopropyl nitrate and 50% for all other quantified compounds. Overall uncertainty of the method was limited by calibration precision rather than by sampling method or chromatography. The system was deployed in the field as part of the Program for Research on Oxidants: Photochemistry, Emissions, and Transport intensive held at the University of Michigan Biological Station in northern Michigan during the summer of 1998 from July 15 to August 22. Ambient results show a significant dynamic range with absolute levels affected by air mass history and pollution sources. The sum of measured C3-C5 alkyl nitrates obtained during this intensive ranged from 3.45 to 65.8 pptv, with a mean of 18.6 pptv and contributed 1% on average to total NOy, with a range of 0.5% to 3%. Differences in the contribution to NOy are found when the data are filtered according to air mass origin. A positive relationship between n-propyl nitrate and peroxypropionyl nitrate was observed over the course of the intensive.