Search for: All records

Abstract Isoprene (C₅H₈) is the non-methane hydrocarbon with the highest emissions to the atmosphere. It is mainly produced by vegetation, especially broad-leaved trees, and efficiently transported to the upper troposphere in deep convective clouds, where it is mixed with lightning NO_x. Isoprene oxidation products drive rapid formation and growth of new particles in the tropical upper troposphere. However, isoprene oxidation pathways at low temperatures are not well understood. Here, in experiments at the CERN CLOUD chamber at 223 K and 243 K, we find that isoprene oxygenated organic molecules (IP-OOM) all involve two successive$${{{\rm{OH}}}}^{\bullet}$$ ${\mathrm{OH}}^{\bullet }$oxidations. However, depending on the ambient concentrations of the termination radicals ($${{{{\rm{HO}}}}_{2}}^{\bullet},\,{{{\rm{NO}}}}^{\bullet}$$ ${{\mathrm{HO}}_2}^{\bullet },{\mathrm{NO}}^{\bullet }$, and$${{{\rm{NO}}}}_{2}^{\bullet}$$ ${\mathrm{NO}}_2^{\bullet }$), vastly-different IP-OOM emerge, comprising compounds with zero, one or two nitrogen atoms. Our findings indicate high IP-OOM production rates for the tropical upper troposphere, mainly resulting in nitrate IP-OOM but with an increasing non-nitrate fraction around midday, in close agreement with aircraft observations. 

Anthropogenic ammonia (NH₃) emissions have significantly increased in recent decades due to enhanced agricultural activities, contributing to global air pollution. While the effects of NH₃on surface air quality are well documented, its influence on particle dynamics in the upper troposphere-lower stratosphere (UTLS) and related aerosol impacts remain unquantified. NH₃reaches the UTLS through convective transport and can enhance new particle formation (NPF). This modeling study evaluates the global impact of anthropogenic NH₃on UTLS particle formation and quantifies its effects on aerosol loading and cloud condensation nuclei (CCN) abundance. We use the EMAC Earth system model, incorporating multicomponent NPF parameterizations from the CERN CLOUD experiment. Our simulations reveal that convective transport increases NH₃-driven NPF in the UTLS by one to three orders of magnitude compared to a baseline scenario without anthropogenic NH₃, causing a doubling of aerosol numbers over high-emission regions. These aerosol changes induce a 2.5-fold increase in upper tropospheric CCN concentrations. Anthropogenic NH₃emissions increase the relative contribution of water-soluble inorganic ions to the UTLS aerosol optical depth (AOD) by 20% and increase total column AOD by up to 80%. In simulations without anthropogenic NH₃, UTLS aerosol composition is dominated by sulfate and organic species, with a marked reduction in ammonium nitrate and aerosol water content. This results in a decline of aerosol mass concentration by up to 50%. These findings underscore the profound global influence of anthropogenic NH₃emissions on UTLS particle formation, AOD, and CCN production, with important implications for cloud formation and climate. 

We present a “diagonal” Volatility Basis Set (dVBS) comparing gas-phase concentrations of oxygenated organic molecules (OOM) to their condensed-phase mass fractions. 

Abstract. Highly oxygenated organic molecules (HOMs) contributesubstantially to the formation and growth of atmospheric aerosol particles,which affect air quality, human health and Earth's climate. HOMs are formedby rapid, gas-phase autoxidation of volatile organic compounds (VOCs) suchas α-pinene, the most abundant monoterpene in the atmosphere. Due totheir abundance and low volatility, HOMs can play an important role innew-particle formation (NPF) and the early growth of atmospheric aerosols,even without any further assistance of other low-volatility compounds suchas sulfuric acid. Both the autoxidation reaction forming HOMs and theirNPF rates are expected to be strongly dependent ontemperature. However, experimental data on both effects are limited.Dedicated experiments were performed at the CLOUD (Cosmics Leaving OUtdoorDroplets) chamber at CERN to address this question. In this study, we showthat a decrease in temperature (from +25 to −50 ∘C) results ina reduced HOM yield and reduced oxidation state of the products, whereas theNPF rates (J1.7 nm) increase substantially.Measurements with two different chemical ionization mass spectrometers(using nitrate and protonated water as reagent ion, respectively) providethe molecular composition of the gaseous oxidation products, and atwo-dimensional volatility basis set (2D VBS) model provides their volatilitydistribution. The HOM yield decreases with temperature from 6.2 % at 25 ∘C to 0.7 % at −50 ∘C. However, there is a strongreduction of the saturation vapor pressure of each oxidation state as thetemperature is reduced. Overall, the reduction in volatility withtemperature leads to an increase in the nucleation rates by up to 3orders of magnitude at −50 ∘C compared with 25 ∘C. Inaddition, the enhancement of the nucleation rates by ions decreases withdecreasing temperature, since the neutral molecular clusters have increasedstability against evaporation. The resulting data quantify how the interplaybetween the temperature-dependent oxidation pathways and the associatedvapor pressures affect biogenic NPF at the molecularlevel. Our measurements, therefore, improve our understanding of purebiogenic NPF for a wide range of tropospherictemperatures and precursor concentrations.