Chemical compounds in the remote Pacific troposphere: Comparison between MLOPEX measurements and chemical transport model calculations

A global three-dimensional chemical transport model, called MOZART (Model of OZone And Related species in the Troposphere), is used to compare calculated abundances of chemical species and their seasonal evolution in the remote Pacific troposphere near Hawaii with values observed during the Mauna Loa Observatory Photochemistry Experiments (MLOPEX 1 and 2). MOZART is a fully diurnal model which calculates the time evolution of about 30 chemical species from the surface to the upper stratosphere. It accounts for surface emissions of source gases, wet and dry depositions, photochemical transformations and transport processes. The dynamical variables are provided by the National Center for Atmospheric Research (NCAR) Community Climate Model (CCM2) at T42 resolution (2.8 degrees x2.8 degrees) and 18 levels in the vertical. Simulated abundances of Rn-222 reveal an underestimate of the transport of continental emissions to the remote Pacific troposphere, more particularly during winter and summer. Calculated concentrations of chemical species are generally in fair agreement with observations. However, the abundances of soluble species are overestimated, leading, for example, to concentrations of nitric acid (HNO3) and hydrogen peroxide (H2O2) which are overpredicted by a factor of 3-8, depending on the season. This feature is attributed to insufficient washout by clouds and precipitation in the model. MOZART succesfully reproduces the development of high-NOx episodes at Mauna Loa Observatory (MLO) associated with anticyclonic conditions to the north of Hawaii and breakdown of the polar jet which tends to deflect to the central Pacific the how of NOx transported from eastern Asia (China, Japan). During high NOx episodes, the calculated NOx mixing ratio in the vicinity of the MLO increases by about a factor of 3 over its background level (reaching 90-100 pptv) within 3-5 days.