On the competition among aerosol number, size and composition in predicting CCN variability: a multi-annual field study in an urbanized desert

Abstract

A 2-year data set of measured CCN (cloud condensation nuclei) concentrations at 0.2 % supersaturation is combined with aerosol size distribution and aerosol composition data to probe the effects of aerosol number concentrations, size distribution and composition on CCN patterns. Data were collected over a period of 2 years (2012–2014) in central Tucson, Arizona: a significant urban area surrounded by a sparsely populated desert. Average CCN concentrations are typically lowest in spring (233 cm−3), highest in winter (430 cm−3) and have a secondary peak during the North American monsoon season (July to September; 372 cm−3). There is significant variability outside of seasonal patterns, with extreme concentrations (1 and 99 % levels) ranging from 56 to 1945 cm−3 as measured during the winter, the season with highest variability. Modeled CCN concentrations based on fixed chemical composition achieve better closure in winter, with size and number alone able to predict 82% of the variance in CCN concentration. Changes in aerosol chemical composition are typically aligned with changes in size and aerosol number, such that hygroscopicity can be parameterized even though it is still variable. In summer, models based on fixed chemical composition explain at best only 41% (pre-monsoon) and 36% (monsoon) of the variance. This is attributed to the effects of secondary organic aerosol (SOA) production, the competition between new particle formation and condensational growth, the complex interaction of meteorology, regional and local emissions and multi-phase chemistry during the North American monsoon. Chemical composition is found to be an important factor for improving predictability in spring and on longer timescales in winter. Parameterized models typically exhibit improved predictive skill when there are strong relationships between CCN concentrations and the prevailing meteorology and dominant aerosol physicochemical processes, suggesting that similar findings could be possible in other locations with comparable climates and geography.

Dimethylamine as a major alkyl amine species in particles and cloud water: Observations in semi-arid and coastal regions

“…3) with peaks between June–September. The trend of these parameters is in sharp contrast to anthropogenic tracers in the gas (CO, NO 2 ) and aerosol (EC) phases that are highest in winter months and lowest between May and August (Youn et al, 2013; Crosbie et al, 2015). These results suggest that DMA is more influenced by biogenic sources than anthropogenic sources in Tucson during months with higher temperatures, humidity, and plant growth.…”

Ambient observations of hygroscopic growth factor and f(RH) below 1: Case studies from surface and airborne measurements

“…Winter in the Tucson metropolitan area is characterized by enhanced residential burning for heat leading to increased PM 2.5 levels nearing exceedance of National Ambient Air Quality Standards [ Kramer et al ., ]. As the area is surrounded by mountains with strong boundary layer inversions during cold nights, residential burning emissions are trapped in a shallow layer and often are not ventilated in periods of prolonged cooler temperatures [ Crosbie et al ., ], leading to the highest year‐round PM 2.5 mass concentrations of species linked to biomass burning, which include elemental carbon, organic carbon, and water‐soluble organic carbon [ Youn et al ., ]. While it cannot be proven unambiguously that GFs less than 1 in Tucson are due to biomass burning, certainly, the overlap in time of burning during periods with these data points supports the case for a potential link.…”

Effects of chemical composition and mixing state on size‐resolved hygroscopicity and cloud condensation nuclei activity of submicron aerosols at a suburban site in northern Japan in summer

“…Similarly, Crosbie et al . [] reported that the lowest hygroscopicity values correspond with the maximum concentrations of EC and OC, which they found in the early morning in Central Tucson. On the other hand, during nighttime, internally mixed aerosols dominated by sulfate increased the hygroscopicity of particles during P1 in this study (Figure c).…”

“…Many of the previous studies showed diurnal cycles of particle hygroscopicity, with peaks typically appearing in the afternoon in urban [ Crosbie et al , ], rural [ Cerully et al , ], marine [ Kalivitis et al , ], boreal forest, and grassland areas [ Paramonov et al , ]. This has been explained by atmospheric aging of aerosols through condensational growth of photochemically oxidized organics and sulfate.…”