Measurement report: Long-range transport patterns into the tropical northwest Pacific during the CAMP<sup>2</sup>Ex aircraft campaign: chemical composition, size distributions, and the impact of convection

Abstract

Abstract. The tropical Northwest Pacific (TNWP) is a receptor for pollution sources throughout Asia and is highly susceptible to climate change, making it imperative to understand long-range transport in this complex aerosol-meteorological environment. Measurements from the NASA Cloud, Aerosol, and Monsoon Processes Philippines Experiment (CAMP2Ex; 24 August to 5 October 2019) and back trajectories from the National Oceanic and Atmospheric Administration Hybrid Single Particle Lagrangian Integrated Trajectory Model (HYSPLIT) were used to examine transport into the TNWP from the Maritime Continent (MC), peninsular Southeast Asia (PSEA), East Asia (EA), and the West Pacific (WP). A mid-campaign monsoon shift on 20 September 2019 led to distinct transport patterns between the southwest monsoon (SWM; before 20 September) and monsoon transition (MT; after 20 September). During the SWM, long-range transport was a function of southwesterly winds and cyclones over the South China Sea. Low- (high-) altitude air generally came from MC (PSEA), implying distinct aerosol processing related to convection and perhaps wind shear. The MT saw transport from EA and WP, driven by Pacific northeasterly winds, continental anticyclones, and cyclones over the East China Sea. Composition of transported air differed by emission source and accumulated precipitation along trajectories (APT). MC air was characterized by biomass burning tracers while major components of EA air pointed to Asian outflow and secondary formation. Convective scavenging of PSEA air was evidenced by considerable vertical differences between aerosol species but not trace gases, as well as notably higher APT and smaller particles than other regions. Finally, we observed a possible wet scavenging mechanism acting on MC air aloft that was not strictly linked to precipitation. These results are important for understanding the transport and processing of air masses with further implications for modeling aerosol lifecycles and guiding international policymaking to public health and climate, particularly during the SWM and MT.

Aerosol responses to precipitation along North American air trajectories arriving at Bermuda

“…CO exhibits three important traits qualifying it as a species to normalize PM 2.5 by: (i) being a reliable marker of anthropogenic pollution stemming from North America ( Corral et al, 2021 ), (ii) being relatively insensitive to wet scavenging processes, and (iii) having a long lifetime in the atmosphere (~ 1 month; Weinstock, 1969 ) compared to aerosol particles. Consequently, we normalize PM 2.5 by ΔCO to quantify transport efficiency and to reveal the potential effects of wet scavenging as has been done in past studies for other regions ( Park et al, 2005 ; Garrett et al, 2010 ; Hilario et al, 2021 ; Matsui et al, 2011 ; Moteki et al, 2012 ; Oshima et al, 2012 ). We first determine the 5th percentile value of surface CO at Bermuda for each season for Cluster 1 trajectories and assume those are the seasonal background values as also done by Matsui et al (2011) .…”

“…Wet scavenging rates are very difficult to constrain over open-ocean areas such as the WNAO ( Kadko and Prospero, 2011 ) not only because of the complexity of physical mechanisms in play but also scarce necessary field measurements. Overall, more work is warranted to better constrain wet scavenging of aerosol particles along trajectories as such studies are sparse not only for the WNAO but also for other regions ( Tunved et al, 2013 ; Hilario et al, 2021 ). Arimoto et al (1999) used aerosol radionuclide data in relation to airflow pattern information to conclude that pollutant transport to Bermuda is common from the northwest and that precipitation scavenging can be influential; their analysis of rain effects on nuclide activities were based on rain data collected at Bermuda without knowledge of precipitation transport history prior to arrival.…”

Total organic carbon and the contribution from speciated organics in cloud water: airborne data analysis from the CAMP<sup>2</sup>Ex field campaign

“…The Navy Aerosol Analysis and Prediction System (NAAPS) global aerosol model was implemented to assist in identifying biomass burning cases (Lynch et al, 2016) (https:// www.nrlmry.navy.mil/aerosol/, last access: 10 August 2021). NAAPS relies on global meteorological fields from the Navy Global Environmental Model (NAVGEM) (Hogan and Brody, 1993;Hogan and Rosmond, 1991) that analyzes and forecasts a 1 • × 1 • grid with 6 h intervals and 24 vertical lev-els. In terms of identifying biomass burning cases, surface smoke concentrations were examined.…”

Total organic carbon and contribution from speciated organics in cloud water: Airborne data analysis from the CAMP<sup>2</sup>Ex field campaign

“…To complement the cloud water composition results, we use aerosol composition results from the High-Resolution Time-of-Flight Aerosol Mass Spectrometer (AMS; Aerodyne, Inc.), which reports non-refractory composition for the submicrometer range (DeCarlo et al, 2006). As summarized by Hilario et al (2021), the AMS deployed in CAMP 2 Ex functioned in 1 Hz Fast-MS mode with data averaged to 30 s time resolution with the lower limit of detection (units of µg m -3 ) as follows for the measured species: organic (0.169), NH4 + (0.169), SO4 2-(0.039), NO3 -(0.035), Cl -(0.036). Negative mass concentrations were recorded owing to the difference method used with the limits of detection.…”