David R. Fitzjarrald

Summertime photochemistry of the troposphere at high northern latitudes

The budgets of O3, NOx (NO+NO2), reactive nitrogen (NOy), and acetic acid in the 0–6 km column over western Alaska in summer are examined by photochemical modeling of aircraft and ground-based measurements from the Arctic Boundary Layer Expedition (ABLE 3A). It is found that concentrations of O3 in the region are regulated mainly by input from the stratosphere, and losses of comparable magnitude from photochemistry and deposition. The concentrations of NOx (10–50 ppt) are sufficiently high to slow down O3 photochemical loss appreciably relative to a NOx-free atmosphere; if no NOx were present, the lifetime of O3 in the 0–6 km column would decrease from 46 to 26 days because of faster photochemical loss. The small amounts of NOx present in the Arctic troposphere have thus a major impact on the regional O3 budget. Decomposition of peroxyacetyl nitrate (PAN) can account for most of the NOx below 4-km altitude, but for only 20% at 6-km altitude. Decomposition of other organic nitrates might supply the missing source of NOx. The lifetime of NOy, in the ABLE 3A flight region is estimated at 29 days, implying that organic nitrate precursors of NOx could be supplied from distant sources including fossil fuel combustion at northern mid-latitudes. Biomass fire plumes sampled during ABLE 3A were only marginally enriched in O3; this observation is attributed in part to low NOx emissions in the fires, and in part to rapid conversion of NOx to PAN promoted by low atmospheric temperatures. It appears that fires make little contribution to the regional O3 budget. Only 30% of the acetic acid concentrations measured during ABLE 3A can be accounted for by reactions of CH3CO3 with HO2 and CH3O2. There remains a major unidentified source of acetic acid in the atmosphere.

Deposition of ozone to tundra

Vertical turbulent fluxes of O3 were measured by eddy correlation from a 12-m high tower erected over mixed tundra terrain (dry upland tundra, wet meadow tundra, and small lakes) in western Alaska during the Arctic Boundary Layer Expedition (ABLE 3A). The measurements were made continuously for 30 days in July-August 1988. The mean O3 deposition flux was 1.3 × 1011 molecules cm−2 s−1. The mean O3 deposition velocity was 0.24 cm s−1 in the daytime and 0.12 cm s−1 at night. The day-to-night difference in deposition velocity was driven by both atmospheric stability and surface reactivity. The mean surface resistance to O3 deposition was 2.6 s cm−1 in the daytime and 3.4 s cm−1 at night. The relatively low surface resistance at night is attributed to light-insensitive uptake of O3 at dry upland tundra surfaces (mosses, lichens). The small day-to-night difference in surface resistance is attributed to additional stomatal uptake by wet meadow tundra plants in the daytime. Flux measurements from the ABLE 3A aircraft flying over the tower are in agreement with the tower data. The mean O3 deposition flux to the world north of 60°N in July–August is estimated at 8.2 × 1010 molecules cm−2 s−1, comparable in magnitude to the O3 photochemical loss rate in the region derived from the ABLE 3A aircraft data. Suppression of photochemical loss by small anthropogenic inputs of nitrogen oxides could have a major effect on O3 concentrations in the summertime Arctic troposphere.

BOREAS TF-8 NSA-OJP Tower Flux, Meteorological, and Soil Temperature Data

The BOReal Ecosystem-Atmosphere Study Tower Flux (BOREAS TF-3) team collected tower flux, surface meteorological, and soil temperature data at the BOREAS Northern Study Area-Old Black Spruce (NSA-OBS) site continuously from the March 1994 through October 1996. The data are available in tabular ASCII files.

BOREAS TF-8 NSA-OJP and SSA-OBS Ceilometer Data

The BOREAS TF-8 team used ceilometers to collect data on the fraction of the sky covered with clouds and the cloud height. Included with these data is the surface-based lifting condensation level, derived from temperature and humidity values acquired at the flux tower at the NSA-OJP site. Ceilo-meter data were collected at the NSA-OJP site in 1994 and at the NSA-OJP and SSA-OBS sites in 1996. The data are available in tabular ASCII files. The data files are available on a CD-ROM (see document number 20010000884).