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Understanding the exchange processes between the atmospheric boundary layerand the free troposphere is crucial for estimating hemispheric transport ofair pollution. Most studies of hemispheric air pollution transport havetaken a large-scale perspective using global chemical transport models withfairly coarse spatial and temporal resolutions. In support of United NationsTask Force on Hemispheric Transport of Air Pollution (TF HTAP;<a href="www.htap.org"target="_blank">www.htap.org</a>), this study employs two high-resolution atmospheric chemistrymodels (WRF-Chem and CMAQ; 36×36 km) driven with chemical boundaryconditions from a global model (MOZART; 1.9×1.9°) to examine the role offine-scale transport and chemistry processes in controlling pollution exportand import over the Asian continent in spring (March 2001). Our analysisindicates the importance of rapid venting through deep convection thatdevelops along the leading edge of frontal system convergence bands, whichare not adequately resolved in either of two global models compared withTRACE-P aircraft observations during a frontal event. Both regional modelsimulations and observations show that frontal outflows of CO, O3 andPAN can extend to the upper troposphere (6–9 km). Pollution plumes in theglobal MOZART model are typically diluted and insufficiently lofted tohigher altitudes where they can undergo more efficient transport in strongerwinds. We use sensitivity simulations that perturb chemical boundaryconditions in the CMAQ regional model to estimate that the O3production over East Asia (EA) driven by PAN decomposition contributes20% of the spatial averaged total O3 response to European (EU)emission perturbations in March, and occasionally contributes approximately50% of the total O3 response in subsiding plumes at mountainobservatories (at approximately 2 km altitude). The response to decomposingPAN of EU origin is strongly affected by the O3 formation chemicalregimes, which vary with the model chemical mechanism and NOx/VOCemissions. Our high-resolution models demonstrate a large spatialvariability (by up to a factor of 6) in the response of local O3 to 20% reductions in EUanthropogenic O3 precursor emissions. The response in the highly populatedAsian megacities is 40–50% lower in our high-resolution models than the global model,suggesting that the source-receptor relationships inferred fromthe global coarse-resolution models likely overestimate health impacts associated withintercontinental O3 transport. Our results highlight the importantroles of rapid convective transport, orographic forcing, urbanphotochemistry and heterogeneous boundary layer processes in controllingintercontinental transport; these processes may not be well resolved in thelarge-scale models

Quantifying pollution inflow and outflow over East Asia in spring with regional and global models