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Abstract. Aerosol interactions with clouds represent a significant uncertainty in our understanding of the Earth system. Deep convective clouds may respond to aerosol perturbations in several ways that have proven difficult to elucidate with observations. Here, we leverage the two busiest maritime shipping lanes in the world, which emit aerosol particles and their precursors into an otherwise relatively clean tropical marine boundary layer, to make headway on the influence of aerosol on deep convective clouds. The recent 7-fold change in allowable fuel sulfur by the International Maritime Organization allows us to test the sensitivity of the lightning to changes in ship plume aerosol number-size distributions. We find that, across a range of atmospheric thermodynamic conditions, the previously documented enhancement of lightning over the shipping lanes has fallen by over 40 %. The enhancement is therefore at least partially aerosol-mediated, a conclusion that is supported by observations of droplet number at cloud base, which show a similar decline over the shipping lane. These results have fundamental implications for our understanding of aerosol–cloud interactions, suggesting that deep convective clouds are impacted by the aerosol number distribution in the remote marine environment. 

Abstract Soils are a major source of nitrogen oxides, which in the atmosphere help govern its oxidative capacity. Thus the response of soil nitric oxide (NO) emissions to forcings such as warming or forest loss has a meaningful impact on global atmospheric chemistry. We find that the soil emission rate of NO in Amazonia from a common inventory is biased low by at least an order of magnitude in comparison to tower‐based observations. Accounting for this regional bias decreases the modeled global methane lifetime by 1.4%–2.6%. In comparison, a fully deforested Amazonia, representing a 37% decrease in global emissions of isoprene, decreases methane lifetime by at most 4.6%, highlighting the sensitive response of oxidation rates to changes in emissions of NO compared to those of terpenes. Our results demonstrate that improving our understanding of soil NO emissions will yield a more accurate representation of atmospheric oxidative capacity. 

Abstract Exceptionally high‐energy lightning strokes >10⁶ J (X1000 stronger than average) in the very low‐frequency band between 5 and 18 kHz, also known as superbolts (SB), occur mostly during winter over the North‐East Atlantic, the Mediterranean Sea, and over the Altiplano in South America. Here we compare the World‐Wide Lightning Location Network database with meteorological and aerosol data to examine the causes of lightning stroke high energies. Our results show that the energy per stroke increases sharply as the distance between the cloud'scharging zone(where the cloud electrification occurs) and the surface decreases. Since thecharging zoneoccurs above the 0°C isotherm, this distance is shorter when the 0°C isotherm is closer to the surface. This occurs either due to cold air mass over the ocean during winter or high surface altitude in the Altiplano during summer thunderstorms. Stroke energy decreases with the warm phase of the cloud, as proxied by the cloud base temperature, and increases with a more developed cloud, as proxied by the cloud top temperature, but to a much lesser extent than the distance between the surface and 0°C isotherm. Aerosols play no significant role. It is hypothesized that a shorter distance between thecharging zoneand the ground represents less electrical resistance that allows stronger discharge currents. 

Atmospheric oxidation of monoterpenes (C₁₀H₁₆) contributes to ambient particle number and mass concentrations due, in part, to the resulting peroxy radicals undergoing auto-oxidation to low-volatility highly oxygenated molecules (HOMs). 

Rapid production of formic acid in biomass burning smoke is not captured by the Master Chemical Mechanism (MCM) nor simplified GEOS-Chem chemistry, likely due to missing secondary chemical production. 

Abstract The known effects of thermodynamics and aerosols can well explain the thunderstorm activity over land, but fail over oceans. Here, tracking the full lifecycle of tropical deep convective cloud clusters shows that adding fine aerosols significantly increases the lightning density for a given rainfall amount over both ocean and land. In contrast, adding coarse sea salt (dry radius > 1 μm), known as sea spray, weakens the cloud vigor and lightning by producing fewer but larger cloud drops, which accelerate warm rain at the expense of mixed-phase precipitation. Adding coarse sea spray can reduce the lightning by 90% regardless of fine aerosol loading. These findings reconcile long outstanding questions about the differences between continental and marine thunderstorms, and help to understand lightning and underlying aerosol-cloud-precipitation interaction mechanisms and their climatic effects. 

Wildfire smoke contains numerous different reactive organic gases, many of which have only recently been identified and quantified. Consequently, their relative importance as an oxidant sink is poorly constrained, resulting in incomplete representation in both global chemical transport models (CTMs) and explicit chemical mechanisms. Leveraging 160 gas-phase measurements made during the Western Wildfire Experiment for Cloud Chemistry, Aerosol Absorption, and Nitrogen (WE-CAN) aircraft campaign, we calculate OH reactivities (OHRs) for western U.S. wildfire emissions, smoke aged >3 days, smoke-impacted and low/no smoke-impacted urban atmospheres, and the clean free troposphere. VOCs were found to account for ∼80% of the total calculated OHR in wildfire emissions, with at least half of the field VOC OHR not currently implemented for biomass burning (BB) emissions in the commonly used GEOS-Chem CTM. To improve the representation of OHR, we recommend CTMs implement furan-containing species, butadienes, and monoterpenes for BB. The Master Chemical Mechanism (MCM) was found to account for 88% of VOC OHR in wildfire emissions and captures its observed decay in the first few hours of aging, indicating that most known VOC OH sinks are included in the explicit mechanisms. We find BB smoke enhanced the average total OHR by 53% relative to the low/no smoke urban background, mainly due to the increase in VOCs and CO thus promoting urban ozone production. This work highlights the most important VOC species for daytime BB plume oxidation and provides a roadmap for which species should be prioritized in next-generation CTMs to better predict the downwind air quality and health impacts of BB smoke.