Model study of tropospheric trace species distributions during PEM‐West A

S C Liu,A R Bandy,J D Bradshaw,B G Heikes,D R Blake,S T Sandholm,D C Thornton,R Newell,G W Sachse,F S Rowland,K K Kelly,E V Browell,S A Mckeen,G L Gregory,X Lin,E‐Y Hsie,R W Talbot,H Singh

doi:10.1029/95jd02277

Abstract

A three‐dimensional mesoscale transport/photochemical model is used to study the transport and photochemical transformation of trace species over eastern Asia and western Pacific for the period from September 20 to October 6, 1991, of the Pacific Exploratory Mission‐West A experiment. The influence of emissions from the continental boundary layer that was evident in the observed trace species distributions in the lower troposphere over the ocean is well simulated by the model. In the upper troposphere, species such as O3, NOy (total reactive nitrogen species), and SO2 which have a significant source in the stratosphere are also simulated well in the model, suggesting that the upper tropospheric abundances of these species are strongly influenced by stratospheric fluxes and upper tropospheric sources. In the case of SO2 the stratospheric flux is identified to be mostly from the Mount Pinatubo eruption. Concentrations in the upper troposphere for species such as CO and hydrocarbons, which are emitted in the continental boundary layer and have a sink in the troposphere, are significantly underestimated by the model. Two factors have been identified to contribute significantly to the underestimate: one is emissions upwind of the model domain (eastern Asia and western Pacific); the other is that vertical transport is underestimated in the model. Model results are also grouped by back trajectories to study the contrast between compositions of marine and continental air masses. The model‐calculated altitude profiles of trace species in continental and marine air masses are found to be qualitatively consistent with observations. However, the difference in the median values of trace species between continental air and marine air is about twice as large for the observed values as for model results. This suggests that the model underestimates the outflow fluxes of trace species from the Asian continent and the Pacific rim countries to the ocean. Observed altitude profiles for species like CO and hydrocarbons show a negative gradient in continental air and a positive gradient in marine air. A mechanism which may be responsible for the altitude gradients is proposed.

Highlights

The 1985NAPAP modeler'semissioninventory[EPA, 1989]to Initial and boundaryconditionsare applied differentlyfor deriveaverageemissionratiosrelativeto NOx for eachof the the bottom 10 levels(0-5 km) and the top 6 levels(5-20 km) hydrocarbonclassesusedin the oxidationmechanismH. ydro- of the modeldomain.To evaluatethe impactof anthropogenic carbonand CO emissionsfor eachmodelgrid are calcu- emissionson the distributionsof trace species,the initial and lated basedon the spatialdistributionof the NOx emissions boundary conditionsfor the lower portion of the model are and the fuel use/sourcecategoryfor eachcountryor province. chosento representclean marine air
A straightforwarddisplayof the contrastbetweencontinen- becauseof their short and/orvariable lifetimes.For example, tal and marine air massescan be done by plotting the latitude- the photochemicalifetime of 03 variesfrom a few daysin the longitudedistributionsat variousaltitudesfor the key species. continentalboundarylayer to a few monthsin the upper tro
The continentalboundarylayer and the lightningsource?The answermustbe yes massesfrequently reachbeyondthe boundariesof the model for the boundarylayer sources.Apparently, wet and dry re- domain.To have enoughdata points,the time requirementof movalofNOr isettScienetnoughtopreventhesesourcefsrom the marine air classificationis reduced to 6 daysthe model making a significantcontributionin the upper troposphere. results.This reductionin time requirementisnot a problem, as

Summary

Introduction

As statedin the overviewpaperby Hoell et at. [thisissue],a this issue;Talbotet at., this issue].In this paper we will study this subjectfrom the model perspective.We use a three-dimensional mesoscale model that covers major objective of the Pacific Exploratory Mission (PEMWest)campaignisto evaluatethe anthropogenicimpactonthe distributionsof trace gasesand aerosolsover the westernPacific basin,particularlythose associatedwith 03, 03 precursors,and sulfur species.A number of papersin this special issuehave addressedthis subjectby analyzingdata obtained duringthe campaign[e.g.,Gregoryetal., thisissue;Smythetal., easternAsia and the westernPacific(Figure la) to simulate the transportand photochemicaltransformationof trace species for the period from September 20 to October 6, 1991, when the DC-8 made intensive observations over the westernPacificfrom Yokota, Japan,Hong Kong, and Guam [Hoell et al, this issue].A brief discussionof the model, its initial and boundaryconditions,and emissioninventoriesis givenin section 1.To get a general idea of how well the model simulatestheUniversityof Colorado,Boulder. 3GeorgiaInstituteof TechnologyA,tlanta. 4NASALangleyResearchCenter,HamptonV, irginia. 5DrexeUl niversityP, hiladelphiaP,ennsylvania. 6Universitoyf Californiai,rvine.the model are first compared directly to observations.Latitude-longitudedistributionsfor somekeyspeciesin the boundary layer and in the upper tropospherefrom model resultsare plotted and compared to the aircraft measurements.Model resultsare alsogroupedby back trajectoriesto studythe con-SUniversitoyf Rhodeisland,Narragansett. 9NASAAmesResearchCenter,MoffettField, California.løUniversitoyf NewHampshireD, urham.trast betweenthe compositionof marine and continentalair and to compare with similar analysesthat were applied to observationsby Gregoryet at. [thisissue]and Talbotet al. [this issue]. Ydro- of the modeldomain.To evaluatethe impactof anthropogenic carbonand CO emissionsfor eachmodelgrid are calcu- emissionson the distributionsof trace species,the initial and lated basedon the spatialdistributionof the NOx emissions boundary conditionsfor the lower portion of the model are and the fuel use/sourcecategoryfor eachcountryor province.

Results

Conclusion