Biogenic Emissions and Nocturnal Ozone Depletion Events at the Amphitrite Point Observatory on Vancouver Island

Abstract

Routine monitoring stations on the west coast of North America serve to monitor baseline levels of criteria pollutants such as ozone (O3) arriving from the Pacific Ocean. In Canada, the Amphitrite Point Observatory (APO) on Vancouver Island has been added to this network to provide regional baseline measurements. In 2014, McKendry and co-workers reported frequent nocturnal O3 depletion events (ODEs) at APO that generally correlated with alongshore winds, elevated concentrations of carbon dioxide (CO2) and stable boundary layer conditions, but whose cause (or causes) has (have) remained unclear.This manuscript presents results from the Ozone-depleting Reactions in a Coastal Atmosphere (ORCA) campaign, which took place in July, 2015 to further investigate ODEs at APO. In addition to the long-term measurements at the site (e.g., of CO2 and O3 mixing ratios), abundances of biogenic volatile organic compounds (BVOC) and aerosol size distributions were quantified. ODEs were observed on the majority of measurement nights and were characterized by a simultaneous increase of CO2 and BVOC abundances, in particular of limonene, a terpene 2.5 more reactive with respect to oxidation of O3 than other monoterpenes.Back trajectory calculations showed that ODEs occurred mainly in air masses that originated from the WNW where the air would have travelled parallel to the coastline and above kelp forests. Head space analyses of sea weed samples showed that bull kelp is a source of gas-phase limonene, consistent with its high relative abundance in air masses from the WNW sector. However, the enhanced terpene and CO2 content showed that the air likely also came in contact with terrestrial vegetation via mesoscale transport phenomena (such as slope flows and land-sea breeze circulations) that were generally poorly captured by the back trajectories. This absence of aerosol growth during ODEs indicates that dry deposition is likely the primary O3 loss mechanism.

Responses to the three referees' comments

“…Observations indicate that aerosol particle formation and growth events occur frequently in the summertime Canadian Arctic Archipelago region between 60-100 • W and 66-85 • N (Chang et al, 2011b;Croft et al, 2016b;Burkart et al, 2017a, b;Collins et al, 2017;Tremblay et al, 2019). These events contribute towards shaping a summertime aerosol number size distribution that is characterized by a dominant Aitken mode (particles with diameters between 10 and 100 nm) in this region (Croft et al, 2016a), similar to observations at other pan-Arctic sites (Tunved et al, 2013;Asmi et al, 2016;Nguyen et al, 2016;Freud et al, 2017;Gunsch et al, 2017;Heintzenberg et al, 2017;Kolesar et al, 2017).…”

A broadband cavity-enhanced spectrometer for atmospheric trace gas measurements and Rayleigh scattering cross sections in the cyan region (470–540 nm)

“…The output spectrum is integrated yielding extinction spectra from which mixing ratios are retrieved using known absorption spectra and knowledge of the mirror reflectivity and effective optical absorption path (Meinen et al, 2010). To date, CEAS instruments have been used to quantify mixing ratios of many atmospherically important trace gases, including nitrogen dioxide (NO 2 ) (Langridge et al, 2008;Gherman et al, 2008;Triki et al, 2008;Thalman and Volkamer, 2010;Wu et al, 2014;Min et al, 2016;, the nitrate radical (NO 3 ) (Langridge et al, 2008;Schuster et al, 2009;Venables et al, 2006;Varma et al, 2009), iodine (I 2 ) (Ball et al, 2010;Dixneuf et al, 2009;Bahrini et al, 2018), the iodine oxides IO and OIO (Vaughan et al, 2008), glyoxal (HCOHCO) (Washenfelder et al, 2008;Thalman and Volkamer, 2010;Coburn et al, 2014;Min et al, 2016), methyl glyoxal (CH 3 COCHO) (Thalman and Volkamer, 2010;Thalman et al, 2015;Min et al, 2016), molecular bromine (Br 2 ), bromine monoxide (BrO) (Chen and Venables, 2011;Hoch et al, 2014), formaldehyde (Washenfelder et al, 2016), and nitrous acid (HONO) (Gherman et al, 2008;Wu et al, 2014;Min et al, 2016).…”

Stable sulfur isotope measurements to trace the fate of SO2 in the Athabasca oil sands region

“…They also have the ability to form cloud condensation nuclei (CCN; Kulmala et al, 2004Kulmala et al, , 2007Benson et al, 2008). After forming CCN they can increase the albedo and lifetime of clouds (Twomey, 1991;Boucher and Lohmann, 1995). Homogeneous oxidation of SO 2 in the gas phase by OH is as follows (Burkholder et al, 2015):…”