Kaisa has taken advantage of the recent warm and sunny days to carry out some tests for her upcoming field season: taking VOC (volatile organic compound) measurements from urban tree foliage. To capture the VOCs – gaseous compounds related to plant stress and defences – she wraps a tree shoot in a bag and collects air samples from both the ambient air being pumped into the bag and from the sample air leaving the bag. For optimal results, the sampling time and air flow rate through the bag must be carefully tested to avoid overheating or large changes in the conditions that the leaves in the bag experience.