On the www.airchina.us site, if you try a booking through the flight selection page, the following rather ominous terminology appears at the bottom:

Customer Advisory
• Please be informed that Air China is a global website and that you may be charged an international transaction fee. Please also note that the final payment amount may be made at the conversion rate charged by your credit card company at the actual time of payment. Please check with your credit card company for further details.

You can try calling Air China to see if you can get an answer to your question--trusting the answer you get is another issue entirely. I'd personally tend to buy elsewhere, as I can't think of any advantage buying online from Air China really gives you.