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

Abstract

Abstract. A two-year dataset of measured CCN concentrations at 0.2% supersaturation is combined with aerosol size distribution and aerosol chemistry data to probe the effects of aerosol number concentrations, size distribution and composition on CCN patterns. Data have been collected over a period of two 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 chemistry are typically aligned with changes in size and aerosol number, such that composition 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, and 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. Regimes where parameterized models exhibit improved predictive skill are typically explained by strong relationships between CCN concentrations and the prevailing meteorology and dominant aerosol chemistry mechanisms suggesting that similar findings could be possible in other locations with comparable climates and geography.

CCN characteristics during the Indian Summer Monsoon over a rain-shadow region

“…14a) This study underlines the inability to make use of either aerosol size distribution or uniform (internally mixed) composition to explain the CCN activation during the monsoon (wet) conditions. This finding is similar to the weak CCN closure reported by Crosbie et al, (2015) for North-American monsoon conditions. The complex meteorological pattern including the monsoon showers and regional aerosol production (both primary and secondary) causes large variability in the aerosol NSD as seen in Figure 11.…”

“…Since the CCN activity depends mainly on the aerosol size and chemical composition (Dusek et al, 2006;McFiggans et al, 2006), d cri estimated from concurrent aerosol NSD and CCN measurements can be considered as a proxy for the variations in the chemical composition of the aerosol system. As the aerosol size distribution and chemical composition are intrinsically associated with each other, any shift in the physical size distribution is mostly associated with the change in the aerosol composition, arising mainly due to the change in the sources or due to different processes such as ageing, coating or scavenging, except for externally mixed systems (Crosbie et al, 2015). Quinn et al, (2008) have correlated the d cri with the HOA mass, and found that HOA can explain about 40 % of the variance in the d cri .…”

“…2015; Crosbie et al, 2015;Jayachandran et al, 2017) CCN are generally approximated as AP above 100 nm (accumulation mode) in many studies when there are no concurrent CCN measurements (eg. Andreae, 2005).…”

Size‐resolved measurements of mixing state and cloud‐nucleating ability of aerosols in Nanjing, China

“…Petters and Kreidenweis [] have proposed a new κ ‐Köhler theory to connect particle size and CCN activity with a single parameter κ , where all chemical composition data are merged into κ representing the hygroscopicity of particles. CCN closure studies to compare modeled and measured CCN concentrations have been shown to be generally satisfactory at background and remote sites [e.g., Jurányi et al ., ] but less so in urban areas [e.g., Burkart et al ., ; Leng et al ., ], which was attributed to the complexity of the aerosol composition and variability in the aerosol mixing state [e.g., Crosbie et al ., ; Ervens et al ., ]. For instance, when internally mixed with other aerosol constituents, such as sulfate and organics, freshly emitted BC particles may exhibit stronger direct and indirect radiative forcing [ Johnson et al ., ; Khalizov et al ., ; Moffet and Prather , ; Pagels et al ., ; Riemer et al ., ; Zhang and Zhang , ].…”

Cloud droplet activation through oxidation of organic aerosol influenced by temperature and particle phase state

“…Global model studies of CCN concentration [ Spracklen et al ., ] and aerosol hygroscopicity [ Pringle et al ., ] have reported low biases compared to observations and large variability, particularly over land and in regions with high OA mass loadings. Some studies achieved closure with measured aerosol hygroscopicity by incorporating the effects of chemical aging [ Asa‐Awuku et al ., ; Crosbie et al ., ] while others found that CCN concentrations are not particularly sensitive to such effects [ Lee et al ., ]. Overall, the available studies suggest potentially important but not yet well understood impacts of OA chemical aging, meriting further investigation.…”