Citric Acid can be substituted – it will adjust the acidity (lower the pH) of the fruit, which is what you want to do. ¼ teaspoon powdered Citric Acid is equivalent to 1 Tablespoon lemon juice. 1 teaspoon powdered Citric Acid is equivalent to ¼ cup lemon juice (4 Tablespoons). Citric Acid lowers pH and imparts tartness to the fruit mixture, but it doesn’t add a particular flavor.
Ascorbic Acid cannot be substituted for lemon or lime juice or vinegar in a Pomona’s recipe. It is simply Vitamin C powder. It will not lower the pH of the fruit. It is commonly used to prevent browning in cut fresh fruit or fruit that will be canned.
Citric Acid and Ascorbic Acid are two different acids, with different chemical compositions. Both are present in lemon juice.