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Trans-pacific transport of reactive nitrogen and ozone to Canada during spring

by T. W. Walker, R. V. Martin, A. Van Donkelaar, W. R. Leaitch, A. M. MacDonald, K. G. Anlauf, R. C. Cohen, T. H. Bertram, L. G. Huey, M. A. Avery, A. J. Weinheimer, F. M. Flocke, D. W. Tarasick, A. M. Thompson, D. G. Streets, X. Liu show all authors
Atmospheric Chemistry and Physics ()
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Abstract

We interpret observations from the Intercontinental Chemical Transport\nExperiment, Phase B (INTEX-B) in spring 2006 using a global chemical\ntransport model (GEOS-Chem) to evaluate sensitivities of the free\ntroposphere above the North Pacific Ocean and North America to Asian\nanthropogenic emissions. We develop a method to use satellite\nobservations of tropospheric NO2 columns to provide timely estimates of\ntrends in NOx emissions. NOx emissions increased by 33% for China and\n29% for East Asia from 2003 to 2006. We examine measurements from three\naircraft platforms from the INTEX-B campaign, including a Canadian\nCessna taking vertical profiles of ozone near Whistler Peak. The\ncontribution to the mean simulated ozone profiles over Whistler below\n5.5 km is at least 7.2 ppbv for Asian anthropogenic emissions and at\nleast 3.5 ppbv for global lightning NOx emissions. Tropospheric ozone\ncolumns from OMI exhibit a broad Asian outflow plume across the Pacific,\nwhich is reproduced by simulation. Mean modelled sensitivities of\nPacific (30 degrees N-60 degrees N) tropospheric ozone columns are at\nleast 4.6 DU for Asian anthropogenic emissions and at least 3.3 DU for\nlightning, as determined by simulations excluding either source.\nEnhancements of ozone over Canada from Asian anthropogenic emissions\nreflect a combination of trans-Pacific transport of ozone produced over\nAsia, and ozone produced in the eastern Pacific through decomposition of\nperoxyacetyl nitrates (PANs). A sensitivity study decoupling PANs\nglobally from the model's chemical mechanism establishes that PANs\nincrease ozone production by removing NOx from regions of low ozone\nproduction efficiency (OPE) and injecting it into regions with higher\nOPE, resulting in a global increase in ozone production by 2% in spring\n2006. PANs contribute up to 4 ppbv to surface springtime ozone\nconcentrations in western Canada. Ozone production due to PAN transport\nis greatest in the eastern Pacific; commonly occurring transport\npatterns advect this ozone northeastward into Canada. Transport events\nobserved by the aircraft confirm that polluted airmasses were advected\nin this way.

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