Aircraft measurements of vertical profiles of aerosol mixing states

Abstract

To examine the overall impact of aerosols on climate, direct measurements of the size‐resolved mixing states of atmospheric particles are needed as a function of altitude. During the Ice in Clouds Experiment‐Layer Clouds, the recently developed aircraft aerosol time‐of‐flight mass spectrometer directly measured the vertical profiles of size‐resolved single‐particle chemistry in cloud‐free air over Wyoming and northern Colorado. These represent the first aircraft‐based, dual polarity mass spectrometry measurements, allowing a detailed examination of in situ single‐particle mixing state as a function of altitude. Measurement of both positive and negative ions for each individual particle provides the ability to identify the primary particle type/source, such as biomass burning, organic carbon, or soot, and examine the extent of mixing with secondary species, such as ammonium, nitrate, sulfate, and sulfuric acid. For the primary particle cores, biomass burning represented the largest source of submicron particles: ∼33–39% by number from 1 to 7 km. Organic carbon particles were the second most abundant type (maximum of ∼33% by number from 1.2 to 2.0 km) with elemental carbon (soot) particles comprising 14–22% by number from 1 to 7 km. In general, biomass burning, organic carbon, and soot particles were frequently internally mixed with ammonium, nitrate, and sulfate at lower altitudes, switching to sulfate and sulfuric acid mixtures at higher altitudes. Further, the number fraction of externally mixed sulfuric acid particles increased with altitude from 1 to 9%, likely because of cloud processing of SO2. The variance of particle mixing state with altitude significantly changes absorption and hygroscopic properties, and must be taken into account in models calculating aerosol direct and indirect radiative forcings.