Abstract
Abstract. The Sulfur Transport and dEposition Model (STEM) is applied to the analysis of observations obtained during the Intercontinental Chemical Transport Experiment-Phase B (INTEX-B), conducted over the eastern Pacific Ocean during spring 2006. Predicted trace gas and aerosol distributions over the Pacific are presented and discussed in terms of transport and source region contributions. Trace species distributions show a strong west (high) to east (low) gradient, with the bulk of the pollutant transport over the central Pacific occurring between ~20° N and 50° N in the 2–6 km altitude range. These distributions are evaluated in the eastern Pacific by comparison with the NASA DC-8 and NSF/NCAR C-130 airborne measurements along with observations from the Mt. Bachelor (MBO) surface site. Thirty different meteorological, trace gas and aerosol parameters are compared. In general the meteorological fields are better predicted than gas phase species, which in turn are better predicted than aerosol quantities. PAN is found to be significantly overpredicted over the eastern Pacific, which is attributed to uncertainties in the chemical reaction mechanisms used in current atmospheric chemistry models in general and to the specifically high PAN production in the SAPRC-99 mechanism used in the regional model. A systematic underprediction of the elevated sulfate layer in the eastern Pacific observed by the C-130 is another issue that is identified and discussed. Results from source region tagged CO simulations are used to estimate how the different source regions around the Pacific contribute to the trace gas species distributions. During this period the largest contributions were from China and from fires in South/Southeast and North Asia. For the C-130 flights, which operated off the coast of the Northwest US, the regional CO contributions range as follows: China (35%), South/Southeast Asia fires (35%), North America anthropogenic (20%), and North Asia fires (10%). The transport of pollution into the western US is studied at MBO and a variety of events with elevated Asian dust, and periods with contributions from China and fires from both Asia and North America are discussed. The role of heterogeneous chemistry on the composition over the eastern Pacific is also studied. The impacts of heterogeneous reactions at specific times can be significant, increasing sulfate and nitrate aerosol production and reducing gas phase nitric acid levels appreciably (~50%).
Highlights
Economic development in the last few decades throughout much of Asia has led to rapid increase in anthropogenic emissions of aerosols and trace gases (Streets et al, 2003; Garg et al, 2006; Zhang et al, 2009)
The DC-8 flights span the region of the central Pacific from southern Alaska to Hawaii, while the C-130 airborne observations cover a large portion of the eastern Pacific off the coast of the Northwest USA
The CO distribution (Fig. 2a) reveals emission hotspots associated with anthropogenic sources, and those associated with open biomass burning
Summary
Economic development in the last few decades throughout much of Asia has led to rapid increase in anthropogenic emissions of aerosols and trace gases (Streets et al, 2003; Garg et al, 2006; Zhang et al, 2009). In the recent decade several airborne field campaigns have been conducted to study the rising impact of Asian pollution over the Pacific and North America. The TRACE-P (Jacob et al, 2003) and ACE-Asia (Seinfeld et al, 2004) field campaigns studied the outflow of Asian emissions to the western Pacific during the spring of 2001. During the INTEX-B field campaign, STEM was used in the flight planning and to interpret observations made from the airborne platforms We present the impact of heterogeneous chemistry on dust aerosol surfaces and the contribution of aerosols in influencing photochemistry over the Pacific (Sect. 3.5) followed by conclusions (Sect. 4)
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