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.
Abstract. An incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS)
instrument for quantification of atmospheric trace gases that absorb in the
cyan region of the electromagnetic spectrum (470 to 540 nm), including
NO2 and I2, is described. The instrument uses a
light-emitting diode coupled to a 1 m optical cavity consisting of a pair of
mirrors in stable resonator configuration. Transmitted light is monitored
using a grating spectrometer and charge-coupled device array detector. The
average mirror reflectivity was determined from the N2∕He and
Ar∕He
ratios of scattering coefficients and was ∼99.98 % at its maximum,
yielding an effective optical path length of 6.3 km. Cross sections of
N2, O2, air, Ar, CO2, and CH4 scattering and
of O4 absorption were measured and agree with literature values within
the measurement uncertainty. Trace gas mixing ratios were retrieved using the
spectral fitting software DOASIS (DOAS intelligent system) from 480 to 535 nm. Under laboratory
conditions, the 60 s, 1σ measurement precisions were ±124 and
±44 pptv for NO2 and I2, respectively. The IBBCEAS
instrument sampled ambient air in Ucluelet, BC, Canada, in July 2015. IBBCEAS
retrievals agreed with independent measurements of NO2 by blue
diode laser cavity ring-down spectroscopy (r2=0.975), but ambient
I2 concentrations were below the detection limit.
Abstract. This work describes an incoherent broadband cavity-enhanced absorption
spectroscopy (IBBCEAS) instrument for quantification of HONO and NO2 mixing ratios in ambient air. The instrument is operated in the
near-ultraviolet spectral region between 361 and 388 nm. The mirror
reflectivity and optical cavity transmission function were determined from
the optical extinction observed when sampling air and helium. To verify the
accuracy of this approach, Rayleigh scattering cross sections of nitrogen
and argon were measured and found to be in quantitative agreement with literature
values. The mirror reflectivity exceeded 99.98 %, at its maximum near 373 nm, resulting in an absorption path length of 6 km from a 1 m long optical cavity. The instrument precision was assessed through Allan variance analyses and showed minimum deviations of ±58 and ±210 pptv (1σ) for HONO and NO2, respectively, at an optimum acquisition time of 5 min. Measurements of HONO and NO2 mixing ratios in laboratory-generated mixtures by IBBCEAS were compared to thermal dissociation cavity ring-down spectroscopy (TD-CRDS) data and agreed within combined experimental uncertainties. Sample ambient air data collected in Calgary are presented.
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