[i] Harry looked down at his empty gold plate. He had only just realized how hungry he was. The pumpkin pasties seemed ages ago. (Harry Potter and the Sorcerer's Stone)
[ii] Albus Dumbledore had gotten to his feet. He was beaming at the students, his arms opened wide, as if nothing could have pleased him more than to see them all there. (Harry Potter and the Sorcerer's Stone)
[iii] The plane crashed killing all 157 passengers aboard. (OALD)
[iv] The black men have long since hidden themselves aboard the Flying Squadron, and I'm not at all convinced that the mob won't content themselves with a redheaded dwarf instead. (Sara Gruen, Water for Elephants)
The examples have an adverb that works as the peripheral modifier. or postmodifier, or postnoun modifier (source: CGEL,p436; English Grammar,Angela Downing, p509; Understanding English Grammar, Koll and Funk, p160), I think.
From the examples, I got the feeling that the adverbs, the modifiers are very similar to adjectives in semantic and syntactic aspects – they seem to have even their own complements like in [iv]. From said aspects, I don’t find any difference between adverbial and adjective. But there seem to be some differences, for they don’t have adjective examples in dictionaries. How do I have to understand that adverbs modify noun phrase and license their own complement.
Best Answer
In the example [iv], you could say the "aboard" is a preposition. Some dictionaries define ago and aboard as adjective; therefore the interpretation could depend on linguists.
http://www.collinsdictionary.com/dictionary/english/aboard?showCookiePolicy=true
In this dictionary, the part of speech for "aboard" is adverb, adjective, or preposition.
http://dictionary.reference.com/browse/ago?s=t
Here, it defines "ago" as adjective.