Ann Louise Sumner

Snowpack production of formaldehyde and its effect on the Arctic troposphere

The oxidative capacity of the atmosphere determines the lifetime and ultimate fate of atmospheric trace species. It is controlled by the presence of highly reactive radicals, particularly OH· formed as a result of ozone photolysis. The dramatic depletion of ozone in Arctic surface air during polar sunrise, therefore offers an opportunity to improve our understanding of the processes controlling ozone abundance and hence the oxidative capacity of the atmosphere. Ozone destruction is catalysed by bromine atoms and is terminated once bromine reacts with formaldehyde to form relatively inert hydrogen bromide, but neither the activation of bromine nor the contribution of formaldehyde are fully understood. Particularly troubling is the failure of current models to simulate the high formaldehyde concentrations in Arctic surface air. Here we report measurements in Arctic snow and near-surface air, which suggest that photochemical production at the air–snow interface accounts for the discrepancy between observed and predicted formaldehyde concentrations. The strength of this source is comparable to that of the dominant formaldehyde source in the free troposphere (the reaction between OH· and methane) and implies that formaldehyde photolysis canbe a dominant source of oxidizing free radicals in the lower polar troposphere. We expect that formaldehyde will also affect photochemistry at the snow surface to facilitate the release of bromine into the lower troposphere—the initial step in Arctic tropospheric ozone depletion.

HO x budgets in a deciduous forest: Results from the PROPHET summer 1998 campaign

Results from a tightly constrained photochemical point model for OH and HO2 are compared to OH and HO2 data collected during the Program for Research on Oxidants: Photochemistry, Emissions, and Transport (PROPHET) summer 1998 intensive campaign held in northern Michigan. The PROPHET campaign was located in a deciduous forest marked by relatively low NOx levels and high isoprene emissions. Detailed HOx budgets are presented. The model is generally unable to match the measured OH, with the observations 2.7 times greater than the model on average. The model HO2, however, is in good agreement with the measured HO2. Even with an additional postulated OH source from the ozonolysis of unmeasured terpenes, the measured OH is 1.5 times greater than the model; the model HO2 with this added source is 15% to 30% higher than the measured HO2. Moreover, the HO2/OH ratios as modeled are 2.5 to 4 times higher than the measured ratios, indicating that the cycling between OH and HO2 is poorly described by the model. We discuss possible reasons for the discrepancies.

Impacts of snowpack emissions on deduced levels of OH and peroxy radicals at Summit, Greenland

Abstract Levels of OH and peroxy radicals in the atmospheric boundary layer at Summit, Greenland, a location surrounded by snow from which HOx radical precursors are known to be emitted, were deduced using steady-state analyses applied to (OH+HO2+CH3O2), (OH+HO2), and OH–HO2 cycling. The results indicate that HOx levels at Summit are significantly increased over those that would result from O3 photolysis alone, as a result of elevated concentrations of HONO, HCHO, H2O2, and other compounds. Estimated midday levels of (HO2+CH3O2) reached 30– 40 pptv during two summer seasons. Calculated OH concentrations averaged between 05:00 and 20:00 (or 21:00) exceeded 4×106 molecules cm−3, comparable to (or higher than) levels expected in the tropical marine boundary layer. These findings imply rapid photochemical cycling within the boundary layer at Summit, as well as in the upper pore spaces of the surface snowpack. The photolysis rate constants and OH levels calculated here imply that gas-phase photochemistry plays a significant role in the budgets of NOx, HCHO, H2O2, HONO, and O3, compounds that are also directly affected by processes within the snowpack.

Atmospheric chemistry of formaldehyde in the Arctic troposphere at Polar Sunrise, and the influence of the snowpack

The role of formaldehyde in the atmospheric chemistry of the Arctic marine boundary layer has been studied during both polar day and night at Alert, Nunavut, Canada. Formaldehyde concentrations were determined during two separate field campaigns (PSE 1998 and ALERT2000) from polar night to the light period. The large differences in the predominant chemistry and transport issues in the dark and light periods are examined here. Formaldehyde concentrations during the dark period were found to be dependent on the transport of air masses to the Alert site. Three regimes were identified during the dark period, including background (free-tropospheric) air, transported polluted air from Eurasia, and halogen-processed air transported across the dark Arctic Ocean. In the light period, background formaldehyde levels were compared to a calculation of the steady-state formaldehyde concentrations under background and low-ozone conditions. We found that, for sunlit conditions, the ambient formaldehyde concentrations cannot be reproduced by known gas-phase chemistry. We suggest that snowpack photochemistry contributes to production and emission of formaldehyde in the light period, which could account for the high concentrations observed at Alert.

An investigation of the interaction of carbonyl compounds with the snowpack

Measurements of formaldehyde, acetaldehyde, and acetone in ambient and snowpack air were conducted as a part of the SNOW99 study in northern Michigan. Vertical profiles of ambient and snowpack air illustrate large concentration gradients through the top ∼10 cm of the snowpack, implying a positive flux of these species from the surface. Snow chamber experiments that involved flushing a snow-filled 34L Teflon-lined chamber with zero air at 20 slpm indicated that release from the snow followed first order kinetics, with decay constants of 0.19, 0.44, and 0.34 hr−1 for formaldehyde, acetaldehyde, and acetone, respectively. Although it is likely that temperature dependent adsorption/desorption processes play a role, the data are not inconsistent with loss from the snowpack via snow grain metamorphism. The data also imply that formaldehyde is not hydrated in the snow grain surface layer.

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.

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)