Observations of an atmospheric chemical equator and its implications for the tropical warm pool region

Abstract

This paper reports observations of a tropospheric chemical equator in the Western Pacific region during the Austral monsoon season, separating the polluted Northern Hemisphere from the cleaner Southern Hemisphere. Measurements of carbon monoxide, ozone, aerosol size/composition, and non‐methane hydrocarbons were made from aircraft, flying north from Darwin, Australia as part of the Aerosol and Chemical Transport In tropical conVEction (ACTIVE) campaign. A chemical equator, defined as a sharp gradient in the chemical background, was found not to be coincident with the Intertropical Convergence Zone during this period. A pronounced interfacial region was identified between 8.5 and 10°S, where tracer mixing ratios increased rapidly within the boundary layer, e.g. CO from 40 ppbv to 160 ppbv within 0.5° latitude (50 km), with inhibited inter‐hemispheric mixing. These measurements are discussed in context using a combination of meteorological and Earth‐observing satellite imagery, back trajectory analysis and chemical model data with the conclusion that air flowing into and subsequently uplifted by the active convection of the Tropical Warm Pool (TWP) region in the Western Pacific is likely to be highly polluted, and will perturb the composition of the Tropical Tropopause Layer. The main source of CO and other pollutants within the TWP region is expected to be biomass burning, with extensive fires in North Sumatra and Thailand during this period. The sharp gradient in composition at the chemical equator seen here results from extensive burning to the north, contrasting with pristine maritime air advected from the Southern Indian Ocean by a strong land‐based cyclone over the Northern Territory of Australia.

Atmospheric mercury in the Southern Hemisphere tropics: seasonal and diurnal variations and influence of inter-hemispheric transport

“…With seasonal changes in the latitudinal position of the ITCZ, ATARS is periodically located north of the atmospheric equator (Hamilton et al, 2008) and so the possibility of interhemispheric transport to the site was also of interest. Figure 2 shows the GEOS-Chem output for NH-released tracer concentrations at ATARS.…”

“…Towards the equator, the existence of the Intertropical Convergence Zone (ITCZ) and the associated upward/poleward movement of the Hadley circulation leads to reduced tropospheric mixing across the atmospheric or chemical equator (Bowman and Cohen, 1997;Hamilton et al, 2008;Holmes and Prather, 2017) and hence a broad, hemispheric gradient of GEM concentrations (Slemr et al, 1985;Sprovieri et al, 2016). Stationary observations of GEM within the tropics are rare but those that are available report significant changes in concentration as source regions shift across hemispheres with the drift of the atmospheric equator (Müller et al, 2012;Wang et al, 2014).…”

A global model study of the impact of land-use change in Borneo on atmospheric composition

“…It has been used extensively for tropospheric studies and is described in more detail in Cook et al (2007) and Hamilton et al (2008). For this study, the model was run at a relatively high horizontal resolution (1 • ×1…”

Evaluation of ACCMIP ozone simulations and ozonesonde sampling biases using a satellite-based multi-constituent chemical reanalysis

“…Figure 7 compares the inter-hemispheric gradient (NH / SH ratio) of the annual mean ozone concentration. We calculated the gradient of area-weighted ozone concentrations across the Equator; however, we recognize a more careful definition of the boundary between the two hemispheres would be required to isolate air masses originated from each hemisphere (e.g., Hamilton et al, 2008). For the estimation of the gradient using the ozonesonde observations, we made a gridded dataset from the ozonesonde observations based on the completion by Tilmes et al (2012) at 2 • × 2.5 • spatial resolution and then calculated area-weighted hemispheric mean concentrations using the gridded dataset.…”