Abstract. Continuous hourly measurements of gas-phase ammonia (NH3(g)) were taken from 13 July to 7 August 2014 on a research cruise throughout Baffin Bay and the eastern Canadian Arctic Archipelago. Concentrations ranged from 30 to 650 ng m−3 (40–870 pptv) with the highest values recorded in Lancaster Sound (74°13′ N, 84°00′ W). Simultaneous measurements of total ammonium ([NHx]), pH and temperature in the ocean and in melt ponds were used to compute the compensation point (χ), which is the ambient NH3(g) concentration at which surface–air fluxes change direction. Ambient NH3(g) was usually several orders of magnitude larger than both χocean and χMP (< 0.4–10 ng m3) indicating these surface pools are net sinks of NH3. Flux calculations estimate average net downward fluxes of 1.4 and 1.1 ng m−2 s−1 for the open ocean and melt ponds, respectively. Sufficient NH3(g) was present to neutralize non-sea-salt sulfate (nss-SO42−) in the boundary layer during most of the study. This finding was corroborated with a historical data set of PM2.5 composition from Alert, Nunavut (82°30′ N, 62°20′ W) wherein the median ratio of NH4+/nss-SO42− equivalents was greater than 0.75 in June, July and August. The GEOS-Chem chemical transport model was employed to examine the impact of NH3(g) emissions from seabird guano on boundary-layer composition and nss-SO42− neutralization. A GEOS-Chem simulation without seabird emissions underestimated boundary layer NH3(g) by several orders of magnitude and yielded highly acidic aerosol. A simulation that included seabird NH3 emissions was in better agreement with observations for both NH3(g) concentrations and nss-SO42− neutralization. This is strong evidence that seabird colonies are significant sources of NH3 in the summertime Arctic, and are ubiquitous enough to impact atmospheric composition across the entire Baffin Bay region. Large wildfires in the Northwest Territories were likely an important source of NH3, but their influence was probably limited to the Central Canadian Arctic. Implications of seabird-derived N-deposition to terrestrial and aquatic ecosystems are also discussed.

In many parts of the world, the implementation of air quality regulations has led to significant decreases in SO emissions with minimal impact on NH emissions. In Canada and the United States, the molar ratio of NH : SO emissions has increased dramatically between 1990 and 2014. In many regions of North America, this will lead the molar ratio of NH : SO, where NH is the sum of particle phase NH and gas phase NH, and SO is the sum of particle phase HSO and SO, to exceed 2. A thermodynamic model (E-AIM model II) is used to investigate the sensitivity of particle pH, and the gas-particle partitioning of NH and inorganic nitrate, to the atmospheric NH : SO ratio. Steep increases in pH and the gas fraction of NH are found as NH : SO varies from below 1 to above 2. The sensitivity of the gas fraction of nitrate also depends strongly on temperature. The results show that if NH : SO exceeds 2, and the gas and particle phase NH are in equilibrium, the particle pH will be above 2. Observations of the composition of particulate matter and gas phase NH from two field campaigns in southern Canada in 2007 and 2012 have median NH : SO ratios of 3.8 and 25, respectively. These campaigns exhibited similar amounts of NH, but very different particle phase loadings. Under these conditions, the pH values calculated using the observations as input to the E-AIM model were in the range of 1-4. The pH values were typically higher at night because the higher relative humidity increased the particle water content, diluting the acidity. The assumption of equilibration between the gas and particle phase NH was evaluated by comparing the observed and modelled gas fraction of NH. In general, E-AIM was able to reproduce the partitioning well, suggesting that the dominant constituents contributing to particle acidity were measured, and that the estimated pH values were realistic. These results suggest that regions of the world where the ratio of NH : SO emissions is beginning to exceed 2 on a molar basis may be experiencing rapid increases in aerosol pH of 1-3 pH units. This could have important consequences for the rates of condensed phase reactions that are acid-catalyzed.

Abstract. Motivated by the need to predict how the Arctic atmosphere will change in a warming world, this article summarizes recent advances made by the research consortium NETCARE (Network on Climate and Aerosols: Addressing Key Uncertainties in Remote Canadian Environments) that contribute to our fundamental understanding of Arctic aerosol particles as they relate to climate forcing. The overall goal of NETCARE research has been to use an interdisciplinary approach encompassing extensive field observations and a range of chemical transport, earth system, and biogeochemical models. Several major findings and advances have emerged from NETCARE since its formation in 2013. (1) Unexpectedly high summertime dimethyl sulfide (DMS) levels were identified in ocean water (up to 75 nM) and the overlying atmosphere (up to 1 ppbv) in the Canadian Arctic Archipelago (CAA). Furthermore, melt ponds, which are widely prevalent, were identified as an important DMS source (with DMS concentrations of up to 6 nM and a potential contribution to atmospheric DMS of 20 % in the study area). (2) Evidence of widespread particle nucleation and growth in the marine boundary layer was found in the CAA in the summertime, with these events observed on 41 % of days in a 2016 cruise. As well, at Alert, Nunavut, particles that are newly formed and grown under conditions of minimal anthropogenic influence during the months of July and August are estimated to contribute 20 % to 80 % of the 30–50 nm particle number density. DMS-oxidation-driven nucleation is facilitated by the presence of atmospheric ammonia arising from seabird-colony emissions, and potentially also from coastal regions, tundra, and biomass burning. Via accumulation of secondary organic aerosol (SOA), a significant fraction of the new particles grow to sizes that are active in cloud droplet formation. Although the gaseous precursors to Arctic marine SOA remain poorly defined, the measured levels of common continental SOA precursors (isoprene and monoterpenes) were low, whereas elevated mixing ratios of oxygenated volatile organic compounds (OVOCs) were inferred to arise via processes involving the sea surface microlayer. (3) The variability in the vertical distribution of black carbon (BC) under both springtime Arctic haze and more pristine summertime aerosol conditions was observed. Measured particle size distributions and mixing states were used to constrain, for the first time, calculations of aerosol–climate interactions under Arctic conditions. Aircraft- and ground-based measurements were used to better establish the BC source regions that supply the Arctic via long-range transport mechanisms, with evidence for a dominant springtime contribution from eastern and southern Asia to the middle troposphere, and a major contribution from northern Asia to the surface. (4) Measurements of ice nucleating particles (INPs) in the Arctic indicate that a major source of these particles is mineral dust, likely derived from local sources in the summer and long-range transport in the spring. In addition, INPs are abundant in the sea surface microlayer in the Arctic, and possibly play a role in ice nucleation in the atmosphere when mineral dust concentrations are low. (5) Amongst multiple aerosol components, BC was observed to have the smallest effective deposition velocities to high Arctic snow (0.03 cm s−1).

Abstract. Several field studies have proposed that the volatilization of NH3 from evaporating dew is responsible for an early morning pulse of ammonia frequently observed in the atmospheric boundary layer. Laboratory studies conducted on synthetic dew showed that the fraction of ammonium (NH4+) released as gas-phase ammonia (NH3) during evaporation is dependent on the relative abundances of anions and cations in the dew. Hence, the fraction of NH3 released during dew evaporation (Frac(NH3)) can be predicted given dew composition and pH. Twelve separate ambient dew samples were collected at a remote high-elevation grassland site in Colorado from 28 May to 11 August 2015. Average [NH4+] and pH were 26 µM and 5.2 respectively and were on the lower end of dew [NH4+] and pH observations reported in the literature. Ambient dew mass (in g m−2) was monitored with a dewmeter, which continuously measured the mass of a tray containing artificial turf representative of the grass canopy to track the accumulation and evaporation of dew. Simultaneous measurements of ambient NH3 indicated that a morning increase in NH3 was coincident in time with dew evaporation and that either a plateau or decrease in NH3 occurred once the dew had completely evaporated. This morning increase in NH3 was never observed on mornings without surface wetness (neither dew nor rain, representing one-quarter of mornings during the study period). Dew composition was used to determine an average Frac(NH3) of 0.94, suggesting that nearly all NH4+ is released back to the boundary layer as NH3 during evaporation at this site. An average NH3 emission of 6.2 ng m−2 s−1 during dew evaporation was calculated using total dew volume (Vdew) and evaporation time (tevap) and represents a significant morning flux in a non-fertilized grassland. Assuming a boundary layer height of 150 m, the average mole ratio of NH4+ in dew to NH3 in the boundary layer at sunrise is roughly 1.6 ± 0.7. Furthermore, the observed loss of NH3 during nights with dew is approximately equal to the observed amount of NH4+ sequestered in dew at the onset of evaporation. Hence, there is strong evidence that dew is both a significant night-time reservoir and strong morning source of NH3. The possibility of rain evaporation as a source of NH3, as well as dew evaporation influencing species of similar water solubility (acetic acid, formic acid, and HONO), is also discussed. If release of NH3 from dew and rain evaporation is pervasive in many environments, then estimates of NH3 dry deposition and NHx ( ≡  NH3 + NH4+) wet deposition may be overestimated by models that assume that all NHx deposited in rain and dew remains at the surface.

An unprecedented wildfire impacted the northern Alberta city of Fort McMurray in May 2016 causing a mandatory city wide evacuation and the loss of 2,400 homes and commercial structures. A two-hectare wildfire was discovered on May 1, grew to ~157,000 ha by May 5, and continued to burn an estimated ~590,000 ha by June 13. A comprehensive air monitoring network operated by the Wood Buffalo Environmental Association (WBEA) in and around Fort McMurray provided essential health-related real-time air quality data to firefighters during the emergency, and provided a rare opportunity to elucidate the impact of gaseous and particulate matter emissions on near-field communities and regional air pollution concentrations. The WBEA network recorded 188 fire-related exceedances of 1-hr and 24-hr Alberta Ambient Air Quality Objectives. Two air monitoring sites within Fort McMurray recorded mean/maximum 1-hr PM2.5 concentrations of 291/5229 μg m−3 (AMS-6) and 293/3259 μg m−3 (AMS-7) during fire impact periods. High correlations (r2 = 0.83–0.97) between biomass combustion related gases (carbon monoxide (CO), non-methane hydrocarbons (NMHC), total hydrocarbons (THC), total reduced sulfur (TRS), ammonia) and PM2.5 were observed at the sites. Filter-based 24-hr integrated PM2.5 samples collected every 6 days showed maximum concentrations of 267 μg m−3 (AMS-6) and 394 μg m−3 (AMS-7). Normalized excess emission ratios relative to CO were 149.87 ± 3.37 μg m−3 ppm−1 (PM2.5), 0.274 ± 0.002 ppm ppm−1 (THC), 0.169 ± 0.001 ppm ppm−1 (NMHC), 0.104 ± 0.001 ppm ppm−1 (CH4), 0.694 ± 0.007 ppb ppm−1 (TRS), 0.519 ± 0.040 ppb ppm−1 (SO2), 0.412 ± 0.045 ppb ppm−1 (NO), 1.968 ± 0.053 ppb ppm−1 (NO2), and 2.337 ± 0.077 ppb ppm−1 (NOX). A subset of PM2.5 filter samples was analyzed for trace elements, major ions, organic carbon, elemental carbon, and carbohydrates. Sample mass reconstruction and fire specific emission profiles are presented and discussed. Potential fire-related photometric ozone instrument positive interferences were observed and were positively correlated with NO and NMHC.

Abstract. Summertime Arctic aerosol size distributions are strongly controlled by natural regional emissions. Within this context, we use a chemical transport model with size-resolved aerosol microphysics (GEOS-Chem-TOMAS) to interpret measurements of aerosol size distributions from the Canadian Arctic Archipelago during the summer of 2016, as part of the “NETwork on Climate and Aerosols: Addressing key uncertainties in Remote Canadian Environments” (NETCARE) project. Our simulations suggest that condensation of secondary organic aerosol (SOA) from precursor vapors emitted in the Arctic and near Arctic marine (ice-free seawater) regions plays a key role in particle growth events that shape the aerosol size distributions observed at Alert (82.5∘ N, 62.3∘ W), Eureka (80.1∘ N, 86.4∘ W), and along a NETCARE ship track within the Archipelago. We refer to this SOA as Arctic marine SOA (AMSOA) to reflect the Arctic marine-based and likely biogenic sources for the precursors of the condensing organic vapors. AMSOA from a simulated flux (500 µgm-2day-1, north of 50∘ N) of precursor vapors (with an assumed yield of unity) reduces the summertime particle size distribution model–observation mean fractional error 2- to 4-fold, relative to a simulation without this AMSOA. Particle growth due to the condensable organic vapor flux contributes strongly (30 %–50 %) to the simulated summertime-mean number of particles with diameters larger than 20 nm in the study region. This growth couples with ternary particle nucleation (sulfuric acid, ammonia, and water vapor) and biogenic sulfate condensation to account for more than 90 % of this simulated particle number, which represents a strong biogenic influence. The simulated fit to summertime size-distribution observations is further improved at Eureka and for the ship track by scaling up the nucleation rate by a factor of 100 to account for other particle precursors such as gas-phase iodine and/or amines and/or fragmenting primary particles that could be missing from our simulations. Additionally, the fits to the observed size distributions and total aerosol number concentrations for particles larger than 4 nm improve with the assumption that the AMSOA contains semi-volatile species: the model–observation mean fractional error is reduced 2- to 3-fold for the Alert and ship track size distributions. AMSOA accounts for about half of the simulated particle surface area and volume distributions in the summertime Canadian Arctic Archipelago, with climate-relevant simulated summertime pan-Arctic-mean top-of-the-atmosphere aerosol direct (−0.04 W m−2) and cloud-albedo indirect (−0.4 W m−2) radiative effects, which due to uncertainties are viewed as an order of magnitude estimate. Future work should focus on further understanding summertime Arctic sources of AMSOA.

During May 2016 a very large boreal wildfire burned throughout the Athabasca Oil Sands Region (AOSR) in central Canada, and in close proximity to an extensive air quality monitoring network. This study examines speciated 24-h integrated polycyclic aromatic hydrocarbon (PAH) and volatile organic compound (VOC) measurements collected every sixth day at four and seven sites, respectively, from May to August 2016. The sum of PAHs (ΣPAH) was on average 17 times higher in fire-influenced samples (852 ng m, n = 8), relative to non-fire influenced samples (50 ng m, n = 64). Diagnostic PAH ratios in fire-influenced samples were indicative of a biomass burning source, whereas ratios in June to August samples showed additional influence from petrogenic and fossil fuel combustion. The average increase in the sum of VOCs (ΣVOC) was minor by comparison: 63 ppbv for fire-influenced samples (n = 16) versus 46 ppbv for non-fire samples (n = 90). The samples collected on August 16th and 22nd had large ΣVOC concentrations at all sites (average of 123 ppbv) that were unrelated to wildfire emissions, and composed primarily of acetaldehyde and methanol suggesting a photochemically aged air mass. Normalized excess enhancement ratios (ERs) were calculated for 20 VOCs and 23 PAHs for three fire influenced samples, and the former were generally consistent with previous observations. To our knowledge, this is the first study to report ER measurements for a number of VOCs and PAHs in fresh North American boreal wildfire plumes. During May the aged wildfire plume intercepted the cities of Edmonton (∼380 km south) or Lethbridge (∼790 km south) on four separate occasions. No enhancement in ground-level ozone (O) was observed in these aged plumes despite an assumed increase in O precursors. In the AOSR, the only daily-averaged VOCs which approached or exceeded the hourly Alberta Ambient Air Quality Objectives (AAAQOs) were benzene (during the fire) and acetaldehyde (on August 16th and 22nd). Implications for local and regional air quality as well as suggestions for supplemental air monitoring during future boreal fires, are also discussed.

Abstract.A growing number of ambient measurements of isocyanic acid (HNCO) are being made, yet little is known about its fate in the atmosphere. To better understand HNCO's loss processes and particularly its atmospheric partitioning behaviour, we measure its effective Henry's Law coefficient K eff H with a bubbler experiment using chemical ionization mass spectrometry as the gas phase analytical technique. By conducting experiments at different pH values and temperature, a Henry's Law coefficient K H of 26 ± 2 M atm −1 is obtained, with an enthalpy of dissolution of −34 ± 2 kJ mol −1 , which translates to a K eff H of 31 M atm −1 at 298 K and at pH 3. Our approach also allows for the determination of HNCO's acid dissociation constant, which we determine to be K a = 2.1 ± 0.2 × 10 −4 M at 298 K. Furthermore, by using ion chromatography to analyze aqueous solution composition, we revisit the hydrolysis kinetics of HNCO at different pH and temperature conditions. Three pH-dependent hydrolysis mechanisms are in play and we determine the Arrhenius expressions for each rate to be k 1 = (4.4 ± 0.2) × 10 7 exp(−6000 ± 240/T ) M s −1 , k 2 = (8.9 ± 0.9) × 10 6 exp(−6770 ± 450/T ) s −1 and k 3 = (7.2 ± 1.5) × 10 8 exp(−10 900 ± 1400/T ) s −1 , where k 1 is for HNCO + H + + H 2 O → NH + 4 + CO 2 , k 2 is for HNCO + H 2 O → NH 3 + CO 2 and k 3 is for NCO − + 2 H 2 O → NH 3 + HCO − 3 . HNCO's lifetime against hydrolysis is therefore estimated to be 10 days to 28 years at pH values, liquid water contents, and temperatures relevant to tropospheric clouds, years in oceans and months in human blood. In all, a better parameterized Henry's Law coefficient and hydrolysis rates of HNCO allow for more accurate predictions of its concentration in the atmosphere and consequently help define exposure of this toxic molecule.