'particularly grave' is correct and can be used in written English.
You can use it to emphasize the severity of a situation. For example, "This situation is particularly grave and requires immediate action."
The current situation in Tibet is particularly grave.
Please define your position in respect to this particularly grave accusation.
Russia's interior ministry described the charges against him as "not of a particularly grave nature".
But even against that backdrop, the situation in South Los Angeles is particularly grave.
She said a "particularly grave" feature was that they had systematically starved him.
The potential for abuse, of course, is particularly grave in the inherently political context of public-corruption prosecutions.
The situation is particularly grave in Latin America and south-east Asia, but it affects every region of the world.
Likewise, many people who deliberately or inadvertently disclose their private information will never suffer any particularly grave consequences.
"Unregulated issue advertisements and independent expenditures by special interests present a particularly grave and immediate threat," the justices said.
Sheer weight of numbers makes the marketing of false hope to Aids victims in South Africa a particularly grave business.
