Asian Pollution at the California Coast, and African Plumes in the Mid-Pacific: High Resolution CO Simulations Reveal Unexpectedly Long Transports

 

Research Staff: Robert B. Chatfield

One facet of our work highlights the origins of pollution flowing into the American West Coast. Figure 1 shows a feature discovered for the first time in the interpretation of NASA’s broad airborne survey, Pacific Exploratory Mission, Tropic, Phase B, ("PEM-T B"). The map shows a general description of the pollution of the global troposphere by a pollutant normally associated with automobile exhaust and wintertime, namely carbon monoxide (CO). CO actually has a variety of pollution sources, including biomass burning, and also natural sources. It is a chemical reaction product in the atmosphere of the oxidation of methane and isoprene, which have biological sources.

Both pollutant and natural sources of CO are concentrated over continents. In the figure, the southern part of the yellow area off the coast of the Americas, southwest from Mexico, comes from pollution processes in nearby Mexico and even the southeastern USA. Consequently, CO levels of ~90 parts per billion, about twice the clean-atmosphere background off the coast from California, were thought to originate from such nearby land areas. These levels were sampled by NASA’s DC-8 and P3B aircraft in an intensive sampling mission.

In truth, this near-coastal pollution instead appears to follow a much longer path leading over the coast of East Asia, and typically arrived by sinking from a high altitude (~6 km). Meteorological trajectories tracing individual "parcels" indicated this, but could not tell origin of the air or describe quantitatively what sources produced the observed CO. This higher level of understanding is required as scientists attempt to predict our global future. Our simulations with the chemistry model nicknamed GRACES and the weather simulation system called MM5 confirmed the general origins: Air had passed over Asia. In addition, our techniques quantified mixing by fine-scale motions in the lowest kilometers near the sampling region, and also lofting of pollution through clouds and storms over Asia.

Is Asia partly to blame for polluting the air entering the West Coast, exacerbating slightly the native smog problems of the United States? Our simulations indicate that this is often the case. Also, they suggest that Asian pollution emissions may be underestimated in the published data, since our model produces credible peaks, but at too low a level.

Even longer-distance transport is the result shown in Figure 2. Occasionally, the air reaching our western shores has not been influenced by pollution over Asia. Instead, trajectories and analysis of maps such as Figure 2 suggest origins even further upwind. Exact attributions over these long distances are still difficult to make. Our analyses indicate origins from pollution, mostly tropical biomass burning, over northern subtropical Africa and South America. This is a surprise, and is indicated only by models with highly detailed spatial resolution. Both MM5 and GRACES have simulation grid points at ~100 km (60 mi) resolution

More theoretical studies that are referenced by a large variety of airborne sampling studies have also aided Ames’ research. Work with Dalhousie University, Nova Scotia, has helped unravel the processes of ozone in the upper troposphere. Not only NOx (reactive nitrogen oxides, NO + NO2) but also a supply of active photochemical species, radicals like OH and HO2, are required to describe the ozone and oxidation chemistry of the upper troposphere.

We have done an analysis of the chemical effects of compounds like acetone, hydrogen peroxide, and other peroxides on the chemistry of the upper troposphere. These compounds are active radical sources, and tend to ignite the fuse of a photochemical reaction chemistry that determines ozone levels in the upper troposphere. The analyses have shown that acetone sources should do this but also decrease the available NOx to much lower quantities. The peroxide compounds were found to have a particularly important role in determining ozone. This has helped interpret the situational modeling and global analysis simulations. Such studies have broader implications. Ozone in this region has its strongest role as a greenhouse gas, but responds slowly to many influences.

More information may be found at the WWW site: http://sky.arc.nasa.gov/chatfield/

Collaborator: I. Folking, Dalhousie University

Point of Contact: Robert B. Chatfield, 650/604-5490, chatfield@clio.arc.nasa.gov

 

[N Hemisphere CO Pollution]

Fig 1. Carbon monoxide pollution of the Northern hemisphere and its flow into the clean Pacific as sampled during the PEM-T B intensive airborne investigations of the Global Tropospheric Experiment. Note especially the large yellow region off the coast of Western North America in this map of simulated CO at 1 km. The air in a larger region off the coast of Southern California has moved rapidly from Asia, or points upwind, and then subsided. Red figures indicate number of days’ journey along a back trajectory. Meanwhile, a partially separated region of CO pollution off the Mexican Coast has moved slowly westward from sources in Mexico, the southern United States, and possibly mixed with burning pollution from tropical countries to the east.

 

[N Pacific CO Pollution]

Fig 2. Carbon monoxide pollution of the North Pacific originating from a "subtropical global plume" originating in Africa. (a) A map of CO at 8 km on March 16, 1999, showing several northern subtropical plumes extending from South America, Africa, and perhaps South Asia at a high point in the Northern Hemisphere spring burning season. Comparisons were made between data and simulation, with good results. (b) Back trajectory calculations made from the 8.2 km level at the approximate DC-8 position. No cloud lofting was apparent along these paths over Asia, though some trajectories encountered light convection as they passed through the meridian ~160 E in the Pacific. The trajectories and maps suggest the possibility that the southern portions of the northern hemispheric CO maximum (light green, >75 ppb CO) could be influenced by northern subtropical plume stranding together with broader patterns of Northern Hemisphere pollution. Strands of the subtropical pollution plume are clearly visible over Asia.