Phototoxic reaction. UV makes any sap/skin reactions far worse, I had a crappy job celery cutting once and didn’t react at all until the weather got sunny. Then it was savage fluid blisters on everyone’s hands and arms, looking exactly like yours after a couple of days. I can pull bindweed all day if the sun’s not out, but need sleeves and gloves if it is, same with hedgecutting Ivy back and getting mashed leaves up my arms. Only happens on sunny days, and some plants are far worse than others.Its fairly sure to be parsnip. Apparently strong sun makes it worse?