Atmospheric CO2 and Particulates

The Keeling curve, measured atop Mauna Loa since 1958, is the clearest record of rising CO2: a steady climb from about 315 ppm to over 420 ppm today. Zoom in, though, and the line isn't smooth. It saws up and down by about 5-6 ppm every single year. That's because the Northern Hemisphere holds most of Earth's land and forest. Each spring and summer, leaves burst out and photosynthesis pulls CO2 out of the air faster than it goes in, so the curve dips. Each autumn and winter, those leaves die and decompose, respiration and decay release carbon back, and the curve climbs again. The Southern Hemisphere, mostly ocean, can't counteract this because it has far less land biomass cycling with the seasons.

Particulate matter follows a different logic: size decides where it lands in you and how much damage it does. PM10, particles under 10 micrometers, get trapped by nose hairs and mucus in the upper airway. PM2.5, under 2.5 micrometers, slip past those defenses, lodge deep in the alveoli, and can even cross into the bloodstream, reaching the heart and other organs. That's why PM2.5 draws more health concern even when it's the smaller share of total particulate mass.

Not all particulates come from tailpipes or smokestacks. Volcanoes, sea spray, pollen, and windblown dust are natural sources, and a big eruption can inject sulfate aerosols into the stratosphere that reflect sunlight and cool a region for a year or more, Mount Pinatubo in 1991 being the classic case. This topic sits alongside the rest of Unit 7 as the longer-timescale, global counterpart to local smog and inversions: same smokestacks and tailpipes, but now tracked in parts per million over decades and in lung tissue over a lifetime.