Particularly and in particular are used in English when we want to highlight something important, or individual. Both uses of particular have the same meaning, but are used in different ways. We could also say in specific and specifically (which is more formal but has the same meaning).

In particular…

In particular is an idiomatic phrase. It is used to indentify nouns specifically. These are nouns that are singled out (shown as individual) by the verb. It can be placed in the beginning, middle or end of a sentence, but is most common either at the start of the end of a phrase. When used at the beginning of a sentence, it should be separated by a comma.

For example:

• I like the blue shirt in particular.
• There are many beautiful towns in Sussex. In particular, tourists should visit Lewes and Arundel.

Particularly

Particularly is an adverb that can be used in much the same way, but it has a few different meanings. Particularly modifies a verb, adjective, adverb or preposition – essentially anything but the noun. It is used to say to an unusual degree. It works best placed before the word it modifies, but for verbs it can also come at the end of a phrase or clause.

• I particularly like the blue shirt. (I like the blue shirt more than the others.)
• We particularly want to see the pier. (We want to see it more than other locations.)
• They were particularly quiet this morning. (They were quieter than usual.)

Examples of uses to modify words:

• The ceiling is particularly high.
• It is particularly cold here in Winter.
• He runs particularly fast.

Often, particularly can also be used as an adverb in place of in particular to specify something individually. But in this case it is used relative to earlier details.

• The weather was bad this week, particularly on Wednesday.

…has the same meaning as:

• The weather was bad this week, on Wednesday in particular.

Here, used in a relative clause (one that works relating to the main clause), particularly means specifically or especially, and is essentially the same as in particular. But if we used it in the main clause, the meanings of the two phrases would differ:

• The weather was particularly bad this week. (The weather was worse than usual.)
• In particular, the weather was bad this week. (The weather was one thing that was specifically bad, amongst other things.)

Note, though, that in particular can never be used in place of particularly to modify verbs.