HYDROCARBON RATIOS DURING PEM-WEST-A - A MODEL PERSPECTIVE

A useful application of the hydrocarbon measurements collected during the Pacific Exploratory Mission (PEM-West A) is as markers or indices of atmospheric processing. Traditionally, ratios of particular hydroca rbons have been interpreted as photochemical indices, since much of th e effect due to atmospheric transport is assumed to cancel by using ra tios. However, an ever increasing body of observatonial and theoretica l evidence suggests that turbulent mixing associated with atmospheric transport influences certain hydrocarbon ratios significantly. In this study a three-dimensional mesoscale photochemical model is used to st udy the interaction of photochemistry and atmospheric mixing on select hydrocarbons. In terms of correlations and functional relationships b etween various alkanes the model results and PEM-West A hydrocarbon ob servations share many similar characteristics as well as explainable d ifferences. When the three-dimensional model is applied to inert trace rs, hydrocarbon ratios and other relationships exactly follow those ex pected by simple dilution with model-imposed ''background air,'' and t he three-dimensional results for reactive hydrocarbons are quite consi stent with a combined influence of photochemistry and simple dilution. Analogous to these model results, relationships between various hydro carbons collected during the PEM-West A experiment appear to be consis tent with this simplified picture of photochemistry and dilution affec ting individual air masses, When hydrocarbons are chosen that have neg ligeble contributions to clean background air, unambiguous determinati ons of the relative contributions to photochemistry and dilution can b e estimated from the hydrocarbon samples, Both the three-dimensional m odel results and the observations imply an average characteristic life time for dilution with background air roughly equivalent to the photoc hemical lifetime of butane for the western Pacific lower troposphere. Moreover, the dominance of OR as the primary photochemical oxidant dow nwind of anthropogenic source regions can be inferred from correlation s between the highly reactive alkane ratios, By incorporating back-tra jectory information within the three-dimensional model analysis, a cor respondence between time and a particular hydrocarbon or hydrocarbon r atio can be determined, and the influence of atmospheric mixing or pho tochemistry can be quantified, Results of the three-dimensional model study are compared and applied to the PEM-West A hydrocarbon dataset, yielding a practical methodology for determining average OH concentrat ions and atmospheric mixing rates from the hydrocarbon measurements. A ircraft data taken below 2 km during wall flights east of Japan imply a diurnal average OH concentration of similar to 3 x 10(6) cm(-3). The characteristic time for dilution with background air is estimated to be similar to 2.5 days for the two study areas examined in this work.