How rainfall events modify trace gas mixing ratios in central Amazonia

This study investigates the rain-initiated mixing and variability in the mixing ratio of selected trace gases in the atmosphere over the central Amazon rain forest. It builds on comprehensive data from the Amazon Tall Tower Observatory (ATTO), spanning from 2013 to 2020 and comprising the greenhouse gases (GHGs) carbon dioxide (CO.sub.2) and methane (CH4); the reactive trace gases carbon monoxide (CO), ozone (O.sub.3 ), nitric oxide (NO), and nitrogen dioxide (NO.sub.2 ); and selected volatile organic compounds (VOCs). Based on more than 1000 analyzed rainfall events, the study resolves the trace gas mixing ratio patterns before, during, and after the rain events, along with vertical mixing ratio gradients across the forest canopy. The assessment of the rainfall events was conducted independently for daytime and nighttime periods, which allows us to elucidate the influence of solar radiation. The mixing ratios of CO.sub.2, CO, and CH.sub.4 clearly declined during rainfall, which can be attributed to the downdraft-related entrainment of pristine air from higher altitudes into the boundary layer, a reduction of the photosynthetic activity under increased cloud cover, and changes in the surface fluxes. Notably, CO showed a faster reduction than CO.sub.2, and the vertical gradient of CO.sub.2 and CO is steeper than for CH.sub.4 . Conversely, the O.sub.3 mixing ratio increased across all measurement heights in the course of the rain-related downdrafts. Following the O.sub.3 enhancement by up to a factor of 2, NO, NO.sub.2, and isoprene mixing ratios decreased. The temporal and vertical variability of the trace gases is intricately linked to the diverse sink and source processes, surface fluxes, and free-troposphere transport. Within the canopy, several interactions unfold among soil, atmosphere, and plants, shaping the overall dynamics. Also, the mixing ratio of biogenic VOCs (BVOCs) clearly varied with rainfall, driven by factors such as light, temperature, physical transport, and soil processes.