The UA group studies particle formation in the atmosphere, such as the event shown here from Santa Ana, Mexico, a small town that lies near the southeastern border of Mexico City. The image, captured by a scanning mobility particle sizer, shows a 24-hour plot of particle diameter (vertical axis) versus particle concentration (with warmer colors represent higher concentrations). The plot is dominated by a new particle formation event that began at 9:00 a.m. The new particles grew over time through the uptake of gases such as sulfuric acid, reaching diameters of 40 nanometers and above by 2:00 p.m.