Abstract
Abstract. We interpret observations from the Intercontinental Chemical Transport Experiment, Phase B (INTEX-B) in spring 2006 using a global chemical transport model (GEOS-Chem) to evaluate sensitivities of the free troposphere above the North Pacific Ocean and North America to Asian anthropogenic emissions. We develop a method to use satellite observations of tropospheric NO2 columns to provide timely estimates of trends in NOx emissions. NOx emissions increased by 33% for China and 29% for East Asia from 2003 to 2006. We examine measurements from three aircraft platforms from the INTEX-B campaign, including a Canadian Cessna taking vertical profiles of ozone near Whistler Peak. The contribution to the mean simulated ozone profiles over Whistler below 5.5 km is at least 7.2 ppbv for Asian anthropogenic emissions and at least 3.5 ppbv for global lightning NOx emissions. Tropospheric ozone columns from OMI exhibit a broad Asian outflow plume across the Pacific, which is reproduced by simulation. Mean modelled sensitivities of Pacific (30° N–60° N) tropospheric ozone columns are at least 4.6 DU for Asian anthropogenic emissions and at least 3.3 DU for lightning, as determined by simulations excluding either source. Enhancements of ozone over Canada from Asian anthropogenic emissions reflect a combination of trans-Pacific transport of ozone produced over Asia, and ozone produced in the eastern Pacific through decomposition of peroxyacetyl nitrates (PANs). A sensitivity study decoupling PANs globally from the model's chemical mechanism establishes that PANs increase ozone production by removing NOx from regions of low ozone production efficiency (OPE) and injecting it into regions with higher OPE, resulting in a global increase in ozone production by 2% in spring 2006. PANs contribute up to 4 ppbv to surface springtime ozone concentrations in western Canada. Ozone production due to PAN transport is greatest in the eastern Pacific; commonly occurring transport patterns advect this ozone northeastward into Canada. Transport events observed by the aircraft confirm that polluted airmasses were advected in this way.
Highlights
Long-range transport of reactive nitrogen and ozone pollution from Asia is a growing concern in light of rapid economic and industrial growth in Asia (Parrish et al, 1992; Jacob et al, 1999; Richter et al, 2005)
The short chemical lifetime of NOx inhibits its direct transport from Asian sources to the remote Pacific, NOx can be redistributed through the injection of peroxyacyl nitrates (PAN = peroxyacetyl nitrate and its structural analogues) from polluted areas into the upper troposphere where cold temperatures allow peroxyacetyl nitrates (PANs) to persist for weeks
We examine the contribution of lightning to ozone and reactive nitrogen observations over the North Pacific Ocean during INTEX-B
Summary
Long-range transport of reactive nitrogen and ozone pollution from Asia is a growing concern in light of rapid economic and industrial growth in Asia (Parrish et al, 1992; Jacob et al, 1999; Richter et al, 2005). The Intercontinental Transport Experiment, Phase B (INTEX-B) campaign characterizes this transport with measurement of gases and aerosols from aircraft and ozonesondes over the North Pacific Ocean and western North America in April–May 2006, along with concurrent observations from satellite instruments (Singh et al, 2009) Interpretation of such measurements requires an understanding of the chemical and physical mechanisms involved in long-range transport. The short chemical lifetime of NOx inhibits its direct transport from Asian sources to the remote Pacific, NOx can be redistributed through the injection of peroxyacyl nitrates (PAN = peroxyacetyl nitrate and its structural analogues) from polluted areas into the upper troposphere where cold temperatures allow PANs to persist for weeks This process removes reactive nitrogen from high emission regions and redistributes it to remote areas (Moxim et al, 1996). A transport event observed by the aircraft on 4–5 May 2006 is examined in detail as evidence of a transport pathway for ozone pollution due to Asia sources entering western Canada
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