At left, the composite annual mean organic aerosol surface air concentrations of the models, defined as the median of the model fields on a 5 ° × 5 ° grid. (giss.nasa.gov)
Free - air concentration enrichment demonstrates enhanced yield of approximately half of what was projected by enclosure studies — which have as of yet (2006) to be incorporated into the models. (realclimate.org)
They found thousands of places with air concentrations significantly above the 2 parts per million background level typically found in cities, with some areas as high as 100 ppm. (sciencemag.org)