Marine productivity and synoptic meteorology drive summer-time variability in Southern Ocean aerosols

Abstract

<jats:p>Abstract. Cloud-radiation interactions over the Southern Ocean are not well constrained in climate models, in part due to uncertainties in the sources, concentrations and cloud-forming potential of aerosol in this region. To date, most studies in this region have reported measurements from fixed terrestrial stations or a limited set of instrumentation, and often present findings as broad seasonal or latitudinal trends. Here, we present an extensive set of aerosol and meteorological observations obtained during an austral summer cruise across the full width of the Southern Ocean south of Australia. Three episodes of continental-influenced air masses were identified, including an apparent transition between the Ferrel atmospheric cell and the polar cell at approximately 64° S. During the other two episodes, synoptic-scale weather patterns diverted air masses across distances greater than 1000 km from the Australian and Antarctic coastlines, respectively, indicating that a large proportion of the Southern Ocean may be periodically influenced by continental air masses. In all three cases, a highly cloud-active accumulation mode dominated the size distribution, with up to 93 % of the total number concentration activating as cloud condensation nuclei. In contrast, sampling periods influenced by marine air masses frequently demonstrated a correlation between air mass trajectories over regions of high biological productivity and subsequent enhancement of an Aitken mode centred at approximately 30 nm and contributing an average of 71 % of the total aerosol number concentration. Although these small diameters limited their contribution to cloud condensation nuclei concentrations, Aitken number concentrations and diameters were highly variable. A detailed investigation of the marine air masses revealed that this variability may be attributed to the availability of biogenic precursors, the competing influence of condensation sinks (such as sea spray aerosol) and vertical transport between the marine boundary layer and the free troposphere. This variability of the marine Aitken mode as well as the instances of long-range transport were governed by synoptic-scale weather systems, through their influence on air mass trajectories and both generation and depletion of condensation sinks. These results demonstrate the highly dynamic nature of Southern Ocean aerosol and their complex dependence on both biological productivity and synoptic-scale weather systems. </jats:p>

Southern Ocean Cloud Properties Derived From CAPRICORN and MARCUS Data

“…We further note that this step‐change in aerosol chemistry and CCN is similar to events described by Humphries et al. (2016) at similar latitudes (see also Alroe et al., 2020). Up to this time during CAPRICORN II, sulfate concentrations had remained mostly below 0.2 μg m −3 , and the step change on 29 January to values above 0.4 μg m −3 marked the beginning of elevated surface CCN and sulfate that persisted until 4 February while the ship operated south of 65°S.…”

“…While additional study is needed, this variability is consistent with distinct air mass changes along the East‐Antarctic marginal ice zone associated with air that contains significantly different aerosol characteristics. Such variability in aerosol has been documented in previous field data by Humphries et al., (2016) and more recently by Alroe et al., (2020).…”

“…Because the Southern Ocean's physical oceanography and seasonal biology influences cloud properties (i.e., Arloe et al., 2020; Armour et al., 2016; Deppeler & Davidson, 2017; Krüger & Graßl, 2011; D. T. McCoy et al., 2015), we take a closer look at the combined data set's latitudinal variability. We impose a convenient set of boundaries based on Deppler and Davidson (2017; Figure 2) that shows apparent variations in Chlorophyll‐a in the longitudinal domain we consider.…”

Summer aerosol measurements over the East Antarctic seasonal ice zone

“…This finding has been observed previously in the east Antarctic seasonal ice zone. In addition to the spring 2012 campaign SIPEXII (Humphries et al, 2016) and identification of katabatic influence on local CCN concentration (Chambers et al, 2018) referred to previously, there is evidence for a change in aerosol population across the polar front during a summer latitudinal transect of the Southern Ocean in 2016 (Alroe et al, 2019), along with variability in the Aitken mode dependant on marine biological precursors and synoptic scale systems. Modal size bins in median distributions are retrieved directly from the median distributions: that is the bin with highest median concentration in the Aitken and accumulation size range is reported as the mode.…”

Morphological and optical properties of carbonaceous aerosol particles from ship emissions and biomass burning during a summer cruise measurement in the South China Sea

“…The own ship emissions can be identified using various measures, for example, high CO, NOx concentrations (Sun et al, 2020b), high BC concentrations (Alroe et al, 2019;Shank et al, 2012), regular cooking emissions (Cai et al, 2020), and wind speeds/directions between the ship stop and start operation (Ausmeel et al, 2020;Kwak et al, 2022). The contribution of ship Jiang et al, 2018;Karjalainen et al, 2022;Lack and Corbett, 2012;Wu et al, 2021;.…”