Atmospheric Chemistry and Composition of Air Over the North Atlantic Ocean

Abstract

The North Atlantic Regional Experiment (NARE) Activity of IGAC seeks to study chemical processing and transport over the North Atlantic Ocean of pollutants emitted from the surrounding continents. It has various components with research activities based on measurements of aerosol composition, and of ozone and its precursors.

The results obtained on the chemical composition of aerosols over the past several years clearly show that the entire North Atlantic atmosphere is heavily impacted by anthropogenic sources. For example, mean aerosol NO₃- concentrations are 5 to 17 times higher than those measured in the remote South Pacific; non-seasalt-SC₄ ^2- values are 2 to 7 times higher. High concentrations of aerosols (and some gases) can be clearly related to the transport of pollutants from continental sources. The measured concentrations of sulfate aerosols equal or exceed those used in recent calculations that suggest that anthropogenic aerosols could have a significant cooling effect on climate in the northern hemisphere (Langner and Rodhe, 1991; Charlson et al. , 1992). Nonetheless, at some locations at various times of year, natural sources do play an important role in the cycles of a number of important aerosols, gases and precipitation constituents.

Ozone is produced photochemically from anthropogenic pollution in North America and is transported to the North Atlantic. To evaluate the significance of this transport, ozone and carbon monoxide (CO) levels were measured during 1991 and 1992 at sites on the Atlantic coast of Canada. A strong positive summertime correlation between carbon monoxide and ozone was attributed to photochemical production of ozone from anthropogenic emissions. In contrast, during the winter when ozone is titrated by NO and other anthropogenic emissions, carbon monoxide and ozone were anti-correlated. The measured relations between the concentrations of ozone and carbon monoxide provide the basis for an estimate of ozone exported from North America: approximately 100 billion moles of ozone per summer. This amount is greater than that reaching the lower troposphere in this region from the stratosphere, the primary natural source of ozone. This conclusion supports the contention that ozone derived from anthropogenic pollution has a hemisphere-wide effect at northern temperature latitudes.

Observations of ozone at two locations in the free troposphere at an altitude of ∼3 km over the North Atlantic (Izania [∼25°N] and off the west coast of Scotland [50°N]) show a marked seasonal oscillation, with a maximum of about 55 ppbv in late spring (May-June) and a minimum of about 35 ppbv in winter (December-January). The same seasonal pattern is exhibited by the hydrocarbon chemical precursors of ozone. These results suggest that the increase in ozone between January and May is in part due to increased production from photochemical processes within the troposphere. A further feature of interest, which comes from close examination of the Atlantic free tropospheric hydrocarbon data, is that the distribution of hydrocarbons, particularly with respect to the ratios of the “normal” to the “iso” isomers of pentane, hexane, heptane and octane, can be interpreted in terms of an efficient NO₃ chemistry preferentially removing the “iso” isomer during winter months.