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The reductions in aerosols often exacerbate climate warming. It remains unclear how to effectively alleviate PM2.5 pollution while minimizing the penalty on climate warming. Here we identify the clean air measures in China that are associated with low aerosol climate penalty efficiency (ACPE), which is defined as aerosol radiative forcing per unit PM2.5 concentration reduction. The measures in transportation, residential combustion, and open burning sectors generally caused lower ACPE [0.07, 0.24, and 0.10 (W m−2)/(μg m−3)] than those from other sectors [0.34–0.46 (W m−2)/(μg m−3)]. This is ascribed to relatively small decreases in cloud concentration nuclei per unit PM2.5 reduction in these sectors, which is further attributed to either relatively low aerosol hygroscopicity or relatively small decrease in aerosol number. Most measures in the former three sectors have low ACPE of <0.15 [(W m−2)/(μg m−3)] and thus may be prioritized for synergistically controlling PM2.5 pollution and climate warming. Plain Language Summary: Aerosols not only cause serious air pollution but also alter the Earth's climate. Previous studies reported that PM2.5 improvement since 2013 has led to considerable positive aerosol radiative forcing over China, exacerbating climate warming. However, it remains unclear how to effectively alleviate PM2.5 pollution while minimizing the penalty on climate warming. Here we identify clean air measures in China that are associated with low aerosol climate penalty efficiency (ACPE), which is defined as aerosol radiative forcing per unit PM2.5 concentration reduction. The measures taken in transportation, residential combustion, and open burning sectors generally caused lower ACPE [0.07, 0.24, and 0.10 (W m−2)/(μg m−3)] than those from other sectors [0.34–0.46 (W m−2)/(μg m−3)]. This is ascribed to relatively small decreases in cloud concentration nuclei per unit PM2.5 reduction in these sectors, which is further attributed to either relatively low aerosol hygroscopicity or relatively small decreases in aerosol number. Furthermore, we found that most clean air measures in the former three sectors have low ACPE [<0.15 (W m−2)/(μg m−3)] and possess high emission reduction potential. These measures may be prioritized for promotion to reduce PM2.5 concentrations and minimize climate penalties. Key Points: The emission reductions in transportation, residential and open burning caused relatively small radiative forcing per unit PM2.5 reductionThe smaller radiative forcing is primarily ascribed to weaker decreases in cloud condensation nuclei per unit PM2.5 reductionThe measures with the smallest radiative forcing per unit PM2.5 reduction are identified and may be implemented in priority [ABSTRACT FROM AUTHOR]

The processes of new particle formation (NPF) and growth are important contributors to cloud condensation nuclei (CCN) concentrations, and CCN are important for climate from their impact on planetary radiative forcing. While the general ubiquity and importance of NPF is understood, the vertical extent and governing mechanisms of NPF and growth in the lower troposphere are uncertain. We present an analysis of four NPF events and two non‐NPF events during the HI‐SCALE field campaign at the Southern Great Plains observatory in Oklahoma, USA. First, we analyzed airborne and ground‐based observations of aerosol and gas‐phase properties. Second, we used a column aerosol chemistry and microphysics model to probe factors that influence the vertical profile of NPF. During HI‐SCALE, we found several instances of enhanced NPF occurring several hundred meters above the surface; however, the spatio‐temporal characteristics of the observed NPF made comparisons between airborne‐ and ground‐based observations difficult. The model represented the observed NPF (or lack of NPF) and particle growth at the surface to final diameters within 10 nm. The model predicted enhanced NPF rates in the upper mixed layer, and this enhancement is primarily due to the temperature dependence in the NPF schemes, but this was also dependent on the vertical profile of gas‐phase precursors measured during HI‐SCALE. We found vertical mixing in the model either enhanced or suppressed NPF rates, aerosol number concentrations, and particle growth rates at the surface. Finally, our analysis provides insights for future field campaigns and modeling efforts investigating the vertical profile of NPF. Plain Language Summary: Aerosol particles in the atmosphere are important for climate, weather, and human health. In our work, we are trying to understand some of the processes that create new aerosol particles in the atmosphere. To accomplish this, we looked at measurements of these aerosol particles taken from a measurement site located on the ground and in an airplane. We also looked at some of the different gases in the atmosphere that might contribute to creating, and growing, these new particles. Finally, we took the measurements and put them into a complex aerosol‐chemistry model in order to understand some of the processes "under the hood" that measurements alone cannot tell us. We had to make some educated guesses and assumptions about certain processes in the model; however, we found that the new particles often form several hundred to thousands of meters above Earth's surface over central Oklahoma, USA. Unfortunately, many of the assumptions we had to make limit the robustness of our findings. Key Points: New particle formation (NPF) was observed at the surface on many days of the HI‐SCALE field campaignAirborne observations provide evidence of NPF initiating aloft on one of these daysModel results suggest that the vertical profiles of temperature and precursor gases are strong predictors of the vertical profile of NPF [ABSTRACT FROM AUTHOR]