Lyric Opera’s fourth annual gift to the city, an end-of-summer free concert in Grant Park showcasing marquee singers from its upcoming productions, is, of course, also a high-profile advertisement for the company–an especially appropriate tactic after last season’s alarming drop in attendance. As usual, it’s gone all out, corralling three of the four principals from this fall’s opener, a revival of The Marriage of Figaro: lyric soprano Ruth Ann Swenson (who’ll be singing Countess Almaviva for the first time), soprano Isabel Bayrakdarian (Susanna), and bass-baritone Ildebrando D’Arcangelo (Figaro), all repeat performers at Lyric and in the prime of their careers. Bayrakdarian and D’Arcangelo, both celebrated for their insight into Mozart roles, make such a fetching pair that they’ve already been engaged for next season’s Don Giovanni, and Swenson, a well-traveled diva, rendered the death of Violetta an exquisite swoon in a recent Traviata. They’ll join tenor Ben Heppner, who’s not singing here this season but is flying in as a favor. In a departure from the nothing-but-hits format of previous outings, the program contains a number of obscure gems–among familiar works from Rossini, Donizetti, and Bellini are the aria “Rachel, quand du Seigneur” from Halevy’s La juive (sung by Heppner) and the cavatina “Nobles seigneurs, salut!” from Les Huguenots by Meyerbeer (performed by mezzo-soprano Lauren McNeese, from Lyric’s Center for American Artists). The second half of the concert is devoted to music of Berlioz, on the bicentennial of his birth–maybe Lyric feels this is one way to make up for having dropped from its slate a potentially expensive premiere of his Benvenuto Cellini in favor of a new production of The Pirates of Penzance. Selections from Cellini, the Shakespearean adaptation Beatrice et Benedict, and Berlioz’s greatest opera, Les Troyens, are near-certain highlights, to be sung by Heppner, Bayrakdarian, McNeese, tenor Michael Schade, and Guang Yang, a Chinese mezzo with an astonishing voice. Quinn Kelsey and Wayne Tigges round out the roster. Lyric music director Andrew Davis conducts members of its orchestra. Saturday, September 6, 7:30 PM, Petrillo Music Shell, Grant Park, Columbus and Jackson; 312-742-4763 or 312-827-5925.