Understanding climate change caused by different external forcings is an urgent need for crisis management and sustainable economic development. It remains unclear how differently global precipitation changes in response to global temperature variations induced by the change of individual solar, volcanic, or greenhouse gas (GHG) forcings. We address this issue by performing three last millennium simulations under each of these individual forcings with the Community Earth System Model version 1.0. The results show that all three forcings can excite strong low‐frequency variations that are longer than one decade, that is, global warming under strong solar radiation or high GHG concentration and global cooling under frequent volcanic eruptions. For a given global temperature change, the global precipitation change under volcanic forcing is larger than that under solar and GHG forcings. The reason is that the volcanic forcing induces the strongest solar irradiance change in the wet tropics. Among the three forcings we examined, the GHG forcing excites the strongest high‐latitude warming, especially the Arctic amplification of global warming. There is no Arctic amplification of temperature decrease under the volcanic forcing‐induced global cooling. The volcanic forcing weakens the Intertropical Convergence Zone and reduces precipitation. The results suggest that while volcanic eruptions can reduce precipitation, they do not mitigate the Arctic amplification of temperature increase under the GHG‐induced warming. The underlying mechanisms for these different climate responses are also discussed.

Lightning plays a major role in tropospheric oxidation chemistry (Murray et al., 2013). It can produce nitrogen oxides, hydrogen oxides, and ozone through electrical discharges in the atmosphere. Electric discharges in gases have three regions, the dark discharge, glow discharge, and arc discharge, with voltage building up during the first two regions before suddenly dropping during the last region because of neutralization (National Research Council, 1986). We refer these three regions respectively as the subvisible discharge, corona discharge, and flash discharge for atmospheric lightning in this work.

Abstract. Here we use satellite observations of formaldehyde (HCHO) vertical column densities (VCD) from the TROPOspheric Monitoring Instrument (TROPOMI), aircraft measurements, combined with a nested regional chemical transport model (GEOS-Chem at 0.5×0.625∘ resolution), to better understand the variability and sources of summertime HCHO in Alaska. We first evaluate GEOS-Chem with in-situ airborne measurements during the Atmospheric Tomography Mission 1 (ATom-1) aircraft campaign. We show reasonable agreement between observed and modeled HCHO, isoprene, monoterpenes and the sum of methyl vinyl ketone and methacrolein (MVK+MACR) in the continental boundary layer. In particular, HCHO profiles show spatial homogeneity in Alaska, suggesting a minor contribution of biogenic emissions to HCHO VCD. We further examine the TROPOMI HCHO product in Alaska in summer, reprocessed by GEOS-Chem model output for a priori profiles and shape factors. For years with low wildfire activity (e.g., 2018), we find that HCHO VCDs are largely dominated by background HCHO (58 %–71 %), with minor contributions from wildfires (20 %–32 %) and biogenic VOC emissions (8 %–10 %). For years with intense wildfires (e.g., 2019), summertime HCHO VCD is dominated by wildfire emissions (50 %–72 %), with minor contributions from background (22 %–41 %) and biogenic VOCs (6 %–10 %). In particular, the model indicates a major contribution of wildfires from direct emissions of HCHO, instead of secondary production of HCHO from oxidation of larger VOCs. We find that the column contributed by biogenic VOC is often small and below the TROPOMI detection limit, in part due to the slow HCHO production from isoprene oxidation under low NOx conditions. This work highlights challenges for quantifying HCHO and its precursors in remote pristine regions.

Lightning plays a major role in tropospheric oxidation, and its role on modulating tropospheric chemistry was thought to be emissions of nitrogen oxides (NOx). Recent field and laboratory measurements demonstrate that lightning generates extremely large amounts of oxidants, including hydrogen oxides (HOx) and O 3 . We here implement the lightning-produced oxidants in a global chemical transport model to examine its global impact on tropospheric composition. We find that lightning-produced oxidants can increase global mass weighted OH by 0.3-10%, and affect CO, O 3 , and reactive nitrogen substantially, depending on the emission strength of oxidants from lightning. Our work highlights the importance and uncertainties of lightning-produced oxidants, as well as the need for rethinking the role of lightning in tropospheric oxidation chemistry.

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