Associate Editor: Paul Palmer; The University of Edinburgh, Edinburgh, United Kingdom
The central North Atlantic Ocean is a remote region where transport of pollutants from North America, Europe, and Africa has been observed. The meteorology in this region is usually controlled by a persistent mid-latitude anticyclone, known as the Azores-Bermuda High. Its strength and location change seasonally and interannually, causing synoptic transport patterns to vary, bringing air masses from different continents. There has been much effort to study the tropospheric chemistry and long-range transport over the North Atlantic. In the 1990s, the North Atlantic Regional Experiment (NARE, (Fehsenfeld et al., 1996)) was conducted to investigate transport of CO and O3 from North America. It was found that transport of air pollutants has a significant influence on air composition over the North Atlantic Ocean. During the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT, Fehsenfeld et al., 2006) experiment, observations including ground and aircraft measurements were collected over North America, the Azores and Europe to study the impacts of long-range transport crossing the North Atlantic. The more recent BORTAS campaign focused on remarkable long-range transport of wildfire emissions to the North Atlantic (Parrington et al., 2012). The majority of the studies for the North Atlantic Ocean focused on transport events or short-term observations. The chemical climatology and the roles of different transport patterns have not been discussed based on long-term observations.
Pico Mountain Observatory (PMO), in the Azores, Portugal, is an island site established in 2001 for the purpose of studying North American outflow (Honrath et al., 2004). The elevation of PMO is 2225 m asl, so the site allows monitoring long-range transport in the free troposphere for most of the time. Tracer gases have been observed at PMO since 2001. Data collected until now provide precious long records that are very useful to investigate different transport patterns and the impacts on air composition over the central North Atlantic.
Studies of chemical transformation during transport can rely on relationships of simultaneously-observed chemicals in downwind regions. d[O3]/d[CO] (the linear regression slope between O3 and CO concentrations) has been used as a measure of O3 enhancement in downwind regions of continental sources (e.g., Parrish et al., 1993). Variations in the slope observed in previous studies reflected differences in air composition and differing O3 chemistry. A d[O3]/d[CO] value of 0.3 was found to be a signature value for rural sites in eastern North America, and was concluded to be a result of mixing of fresh pollution emissions and aged air mass (Chin et al., 1994). Cooper et al. (2001) investigated d[O3]/d[CO] in different airstreams associated with mid-latitude cyclones, and suggested that the slope could be different depending on air composition and photochemistry. Honrath et al. (2004) found a higher summertime d[O3]/d[CO] (~1.0) in the free troposphere over the central North Atlantic and suggested potential O3 production during transport from North America. A previous study (Zhang et al., 2014) investigated the evolution of d[O3]/d[CO] during selected transport events of pollution plumes from North America to PMO in a semi-Lagrangian view, using data from PMO and a chemical transport model. It was found that photochemical CO loss can also contribute to increases in d[O3]/d[CO] in long-range transport, so the higher d[O3]/d[CO] at PMO compared to places closer to emission sources (i.e., over the U.S.) were due to not only photochemical ozone production but also CO loss. For the comparisons of d[O3]/d[CO] at a given location, such as PMO, the differences in CO loss for pollution impacted transport should not be significant. Zhang et al. (2014) estimated 10% and 12% CO loss due to photochemistry for two transport events of polluted plume having significant different OH concentrations and transport pathways. Thus, differences in observed d[O3]/d[CO] at PMO should mainly reflect varying emission strength and ozone chemistry for the pollution impacted transport.
Non-methane hydrocarbons (NMHC) mainly undergo reactions with OH radicals in the troposphere, and their relative reaction ratios are useful tools to determine the photochemical age of transported air masses. Lighter NMHC usually react more slowly than heavier ones. Consequently, concentrations of lighter NMHC are still measurable after long-range transport. The differences in NMHC decay rates produce specific patterns in the observed mixing ratios, which have been used to reflect photochemical ages of air masses (Mckeen and Liu, 1993). The natural logarithm of [n-butane]/[ethane] versus that of [propane]/[ethane] has been used as a photochemical clock of air masses (Parrish et al., 1992). The butane isomer ratio, [i-butane]/[n-butane], has been used to investigate oxidation pathways by OH, ocean-emitted chlorine atom, and other oxidants (Hopkins et al., 2002).
In this work, a ten-year record of trace gas observations at PMO was used to study relationships between air composition and transport patterns. The objectives of this work are to provide a classification of major transport patterns to PMO that are characterized by different pollution sources and transport pathways, and to investigate the associated chemical signatures. We investigated O3 and O3 precursors in spring (April and May), summer (June, July and August) and fall (September and October). We only focused our analyses on months from April to September, because data from PMO were most available during the periods.
2.1. PMO measurements and research
PMO was established in the summer of 2001 on top of Pico Mountain (38.47° N, 28.40° W, 2225 m a.s.l.) in the Azores Islands, Portugal. The elevation of PMO in the central North Atlantic ocean makes it an excellent site for conducting research on gaseous and particle species transported in the free troposphere. Since the establishment of PMO, studies have been conducted to understand the impacts of pollution transport, photochemistry, and meteorology on the air composition at this baseline site. Honrath et al. (2004) and Owen et al. (2006) focused on CO and O3 observations in the earlier years, and specifically investigated the enhancements due to transport from North America. Both Lapina et al. (2006) and Val Martín et al. (2006) studied impacts of fire emissions on enhancement of air pollutants at PMO. Val Martín et al. (2008b) focused on a three-year period of nitrogen oxides observation and studied the seasonal variations and sources of nitrogen oxides. Helmig et al. (2008) and Honrath et al. (2008) assessed NMHC levels observed at PMO, and used NMHC data to investigate oxidation chemistry and plume aging over the North Atlantic. Aerosol properties and composition have also been measured to investigate aerosol radiative effect and aging during long-range transport (Fialho et al., 2005; Dzepina et al., 2015; China et al., 2015). A recent work by Helmig et al. (2015) discussed observed seasonal cycles and the degree of photochemical aging of NMHC at PMO using data collected during 2004–2014. The study here uses ten years of measurements (2001–2010) of CO, O3, nitrogen oxides (NOx= NO + NO2), total reactive nitrogen (NOy), and NMHC to study the chemical signatures of different transport patterns (data coverage is shown in Figure 1). Data used in this work are archived at http://instaar.colorado.edu/pico/pico_archive/default.html. Details about the instrumentation of individual species are summarized in Honrath et al. (2004) (CO and O3), Val Martín et al. (2006) (NOx and NOy), Kleissl et al. (2007) (meteorology), Tanner et al. (2006) (NMHC), and Helmig et al. (2008) (NMHC).
2.2. Transport and CO tracer simulations
The Lagrangian particle dispersion model FLEXPART (version 8.2; (Stohl et al., 1998)) was used to build a ten-year archive of transport trajectories for PMO. Backward simulations were launched every three hours from PMO. Meteorology fields from the European Center for Medium-Range Weather Forecasting (ECMWF), featured with 3-hourly temporal resolution, 1° horizontal resolution, and 60 vertical levels, were used to drive simulations for 2001–2004. Meteorology fields of the Global Forecast System (GFS) and its Final Analysis (FNL), featured with the same temporal and horizontal resolutions, but 26 vertical levels, were used to drive FLEXPART for the years 2005–2010. The output was saved in a grid with a horizontal resolution of 1° latitude by 1° longitude, and eleven vertical levels from the surface to 15,000 m a.s.l. A “retroplume” obtained in FLEXPART backward simulations describes upwind distributions of residence time (introduced with details in supplemental information (SI)), which is a useful tool to study transport pathways. Retroplumes are multiplied with CO emission inventories from the Emissions Database for Global Atmospheric Research (EDGAR version 3.2 (Olivier and Berdowski, 2001)) and the Global Fire Emissions Database (GFED v3.1, daily averaged fire emissions (Mu et al., 2011)) to estimate influence from anthropogenic and wildfire sources, respectively. The product is hereafter called “FLEXPART_CO” (examples are given in Figure S1). It has been found that GFED v3.1 misses biomass burning emissions from small fires, but the bias due to such misses was estimated to be only 9% in annual total emissions in the temperate to boreal North America (Randerson et al., 2012). Not all of the missing emissions can be transported to PMO, so the chance of missing “Fire” events should be even lower and is believed to have minor impacts on the analyses in this work.
Uncertainties in transport pathways simulated by FLEXPART can be due to the parameterizations representing temporally and spatially unresolved transport processes (Stohl et al., 2011). In terms of vertical transport processes, boundary layer mixing and updrafts in convection are both considered in FLEXPART. Time-varying PBL height determines the vertical mixing of air parcels. Failure to capture the daily PBL peak can lead to overestimation of surface concentrations. In FLEXPART, PBL height is calculated using the Richardson number concept based on the wind and temperature fields given in the meteorology (Vogelezang and Holtslag, 1996). The meteorology fields used in this work had a temporal resolution of 3 hours, and at least one local afternoon time was included. Sub-grid variables, including topography and land use, can also affect vertical mixing. Oversimplified topography may overlook shear stress and sensible heat, creating biases in vertical distributions of tracers. Another highly parameterized sub-grid process is cloud convection. FLEXPART redistributes air parcels vertically in convection-activated grids using the approach of Emanuel and Emanuel and Zivkovic-Rothman (1999), which determines air parcel displacement in up- and down-drafts based on temperature and humidity fields. The convection scheme is not necessarily the same scheme used in driving meteorology. These parameterizations have been fully tested and validated using surface and in situ measurements (Stohl et al., 1998; Brioude et al., 2013); therefore we do not believe that these associated uncertainties are significant enough to change the conjectures and conclusions of this paper.
2.3. Classification of transport patterns
In this section, we list the constraints used to determine different transport patterns. Helmig et al. (2015) also studied long-term transport patterns to Pico in different seasons by using HYSPLIT modeling. They found that fast transport was associated with shorter photochemical ages as indicated by observed ln[propane]/[ethane]. Here, we intend to further explore the transport types. By analyzing FLEXPART products and meteorological data, we identified several major transport patterns in the ten-year record of trace gas observations. The constraints are discussed in the following and summarized in Table 1.
|Transport Patterns||Abbrev.||Constraint*||Occurrence Frequency ** %
|North American flow||NA||RT over North America in 0–5 km a.s.l. greater than 50% for at least 1 day||40||30||32|
|NA affected by anthropogenic emissions||NA-anthro||North America anthropogenic FLEXPART_CO contribution greater than 15 ppbv, fire <15 ppbv||16||15||13|
|NA-anthro lifted during transport||NA-anthro-lifted||1. Same as NA-anthro||4.0||2.7||2.5|
|2. RT in a vertical range 0–2.5 km reduced 30% within 24 hour during 0–15 days upwind|
|NA-anthro within or close to MBL||NA-anthro-low||1. Same as NA-anthro||3.1||1.7||<1.0|
|2. RT within 0–2.5 km greater than 40% from 0–5 days upwind|
|NA-anthro quick transport||NA-anthro-young||1. Same as NA-anthro||6.7||4.3||4.0|
|2. FLEXPART_CO age from anthropogenic emissions less than 7 days|
|NA-anthro aged transport||NA-anthro-aged||1. Same as NA-anthro||6.2||5.3||4.5|
|2. FLEXPART_CO age from anthropogenic emissions greater than 10 days|
|Wildfire affected||Fire||Wildfire FLEXPART_CO contribution greater than 15 ppbv, Anthropogenic <15 ppbv||<1.0||7.3||2.6|
|Fire quick transport||Fire-young||1. Same as Fire||<1.0||2.5||1.0|
|2. FLEXPART_CO age from wildfire emissions less than 7 days|
|Fire slow transport||Fire-aged||1. Same as Fire||<1.0||3.0||1.3|
|2. FLEXPART_CO age from wildfire emissions greater than 10 days|
|European flow||EU||RT over Europe greater than 50% for at least 1 day||1.8||<1.0||<1.0|
|African flow||AF||RT over Afreica greater than 50% for at least 1 day||<1.0||<1.0||2.6|
|North Atlantic free troposphere background||NATL||RT over North Atlantic in 0–5 km a.s.l. greater than 70% through 0–10 days upwind||17||19||8.3|
|Downward mixing from UTLS||Upper||RT less than 40% in 0–5 km through 2–12 days upwind||2.1||2.3||4.1|
|Upslope flow||Upslope||Simulated PBL height greater than altitude of Pico or DSL less than PBL height||24||13||15|
2.3.1. Air mass origin
The primary consideration in categorizing the transport is where it originates. We considered transports originating from North America, Europe and Africa, downward mixing from the upper troposphere and lower stratosphere (UTLS), and North Atlantic background air. Transport origin was determined based on the distributions of upwind residence time (RT) over the North Atlantic Ocean and the surrounding continents. Figure 2a provides the regional fraction of upwind RT averaged for all the retroplumes simulated for the ten years, which shows the averaged RT fractions in different regions at upwind days. During 0–3 days upwind, retroplume RT mainly distribute over the North Atlantic. The North American RT contribution started to appear between 1–2 days upwind, indicating the extreme shortest transport time from the North American continent to PMO. The average North American RT reached its maximum 7–8 days upwind at 22%. We defined the retroplumes that had greater than 50% RT distributed in the low-middle troposphere (0–5 km a.s.l.) over North America for a period longer than one day as “North American transport”, with an assigned abbreviation of “NA”. The altitude limitation was applied for targeting ground emissions because air originating from the upper troposphere would certainly have a different chemical composition. The upper limit of 5 km was set much higher than the typical boundary layer height to take into account surface air masses lofted by convection over continents. Using even higher levels (i.e., 7.5 km or 10 km) would have certainly captured deep convection but would also have increased the probability of including free troposphere air that was decoupled from surface emissions. The North Atlantic free troposphere background, “NATL”, was constrained by a higher percentage (70%) of RT and a longer time limit (10 days) within 0–5 km a.s.l. over the North Atlantic in order to reduce the probability of overlapping with air masses originating from the continents. It is worth mentioning that we did not exhaustively classify all observations into a specific transport pattern. Instead, we determined the observations with clear transport characteristics and paid less attention to transport that had multiple origins and complicated pathways.
2.3.2. Impact of emission type and air mass age
FLEXPART_CO was used in this study to determine the periods when PMO was affected by two types of pollution emissions, i.e., anthropogenic and wildfire emissions (see details in SI). We used a FLEXPART_CO mixing ratio enhancement of 15 ppbv (compared to 80–100 ppbv as a typical background CO at PMO) as the cutoff in determining the periods affected by fresh emissions. For example, if FLEXPART_CO from biomass burning emission was greater than 15 ppbv for a particular time, then the period was considered as a fire event. The 15 ppbv used here equals the standard deviation of observed half-hourly CO for the ten-year period. We used the same anthropogenic emission inventory in FLEXPART for all the years in order to keep a consistent threshold for identification of affected periods, although the actual emissions of CO and reactive nitrogen oxides over North America have decreased in recent decades. The simulated FLEXPART_CO shows a wide range from 3 ppbv (small emission influence) to more than 150 ppbv (large emission influence). An example of FLEXPART_CO originating from different continents and types of emission is given in Figure SI1a.
FLEXPART_CO was also used to estimate the age of continental emissions after long-range transport by averaging the time elapsed between CO emission pick-up and the arrival at PMO in the model (see SI for details). Honrath et al. (2004) found that transport from North America that causes significant changes in CO at PMO usually takes 5 to 7 days. Thus, we defined the transport retroplumes that had average FLEXPART_CO ages less than 7 days as short transport and those that were older than 10 days as aged transport. A one-month FLEXPART_CO age spectrum time series is shown in Figure SI1b for demonstration purpose. This constraint can be combined with the determined air mass origin to further distinguish transport patterns. For example, aged transport of North American anthropogenic emissions is referred to as “NA-anthro-aged”. In previous work, this method was used to assess the influence of wildfire emission and transport pathway (Val Martín et al., 2008a; Dzepina et al., 2015; China et al., 2015).
2.3.3. Transport height
We also defined a few subcategories determined by transport at different heights or that experienced RT redistribution in height. Kleissl et al. (2007) reported that the MBL height at Pico was typically 800–1500 m simulated by the Global Data Assimilation System (GDAS, 2014). We studied the GDAS data again for the ten- year period and found the MBL occasionally reduced to 500 m or reached up to 2000 m. For such estimates, we defined a “low” transport pattern which had over 40% of its RT below 2.5 km during the transport period. This height was chosen because it was the first vertical level in the configured FLEXPART that reaches above 2 km. In order to capture the transport scenario, in which pollution emission was lifted by deep convection or by warm conveyor belts, we defined the “lifted” scenario when the RT was reduced by at least 30% in the 0–2.5 km within 24 hours during the simulated upwind transport. The fraction of RT change during lifting refers to the findings in a climatology study of warm conveyor belts over 15 years (Eckhardt et al., 2004). The last transport scenario defined by height was downward mixing, which brought air from the UTLS. Retroplumes having less than 40% of their RT within 0–5 km during 2–12 upwind days were defined as downward mixing having the abbreviation “Upper”. In this definition, most of the air mass subsides from above 5 km to the level of PMO in the last few days of transport. The last two days were not considered because 100% of the RT should be at the level of PMO (2–2.5 km) when the simulated air parcels approached PMO as shown in Figure 2b.
2.3.4. Potential upslope flow
Local emissions from human activities and vegetation on Pico Island can be carried to PMO by upslope flow, which may lead to different chemical signatures. Upslope flow occurs through mechanically-forced lifting, in which strong synoptic winds blowing against the mountain are deflected upwards by the mountain slope, or through buoyant forcing, in which the surface air mass is lifted as a result of solar heating and release of latent heat. The frequency of upslope flow was found to be less than 20% at PMO from May to September by a micrometeorological study conducted during summertime in 2004 and 2005 (Kleissl et al., 2007). In our study, we examined potential upslope flow events for the ten-year period. For mechanically lifted upslope flow, the height of a dividing streamline (DSL) for the flow climbing upwards to PMO was calculated by using the method described in Sheppard (1956) and the wind speed profiles in the ECMWF and GFS/FNL meteorological datasets. DSL heights indicate the lowest level of air that can reach the top of Pico Mountain after consumption of the mechanical energy and, if any, latent heat. If the calculated DSL was lower than PBL height, upslope flow to PMO was considered as having occurred. We also considered deep convection caused by strong solar heating over the Island. We obtained PBL heights at PMO from GDAS (1° latitude by 1° longitude resolution) for the ten-year period and compared those with the height of PMO. When estimated PBL height exceeded the altitude of PMO, upslope flow was also considered as having occurred. In addition, we compared water vapor mixing ratios (WVMR) at PMO to the WVMR interpolated at the PMO elevation in the radiosonde measurements at Terceira (http://esrl.noaa.gov/raobs/). Similar to the approach used by Ambrose et al. (2011), we calculated the seasonal means of WVMR at 2225 m at the radiosonde site, Terceira. We determined, for each season, the driest portion of the PMO WVMR that derived equivalent seasonal means to the means at 2225 m above Terceira (within 0.01 g/kg). These subsets were considered as WVMR of free troposphere. The remaining data, and associated periods, were considered as anabatic upslope flows for each season, in addition to the two upslope flow situations determined above.
3. Results and discussions
3.1. Transport patterns and associated chemical signatures
The estimated occurrence frequencies of determined transport patterns are summarized in Table 1. The occurrence frequencies and chemical signatures for each transport pattern were calculated independently by applying the listed constraint(s) to the entire ten-year observations and model results. Thus, some categories are not exclusive. The largest data overlap between two transport patterns was less than 10% for NATL and Upper. The results are discussed together with observed chemical signatures in this section. We focus on the transport patterns with occurrence frequencies greater than 1%. The chemical signatures of trace gases determined for each transport pattern are summarized in Table 2. For the comparisons of chemical signatures in the following discussions, the Welch’s t-test (α = 0.05) was used to determine if the values were significantly different from each other.Table 2
Statistics of carbon monoxide (CO), ozone (O3), nitrogen oxides (NOx and NOy), and NMHCs (ethane, n-propane, and n-butane) for the determined transport patterns in three seasons. The statistics are given in the form of mean ± standard deviation (number of observations). Values in bold font indicate the highest concentrations across all the transport patterns (highest in each row). Results of transport patterns that occur at a lower frequency than 1% (see Table 1) are not provided. DOI: https://doi.org/10.1525/elementa.194.t2
3.1.1. North Atlantic free troposphere background
Figure 3a shows the superimposed RT of all the FLEXPART retroplumes classified as NATL in the ten-year period. The RT of NATL concentrated over the North Atlantic and was mostly isolated from continental emissions due to the constraints applied in Section 2. The occurrence frequencies of NATL were 17%, 19%, and 8.3% in spring, summer, and fall, respectively. The highest frequency in summer was likely due to the strengthened Azores-Bermuda anticyclone over the North Atlantic, which tends to constrain air to the central North Atlantic.
Due to the constraint implemented to classify NATL, we viewed this transport pattern as a flow of aged air that has been isolated from direct transport of continental sources for a long enough time such that the chemical signatures represent the background air composition of PMO. The average observed CO during NATL periods from the ten-year record were 113, 83, and 82 ppbv for spring, summer, and fall, respectively (Table 2). CO has a tropospheric lifetime of a few months, so the seasonal variations reflect the northern hemisphere seasonal CO background, which is driven by enhanced emission in cold seasons and accelerated oxidation in summer (Logan et al., 1981). Such maxima of CO in spring have been found at other remote sites (e.g., Macdonald et al. (2011) and for the Northern Hemisphere in general (Worden et al., 2013). The average O3 of NATL were 39, 31, and 33 ppbv for spring, summer, and fall, respectively. These values are similar to the previously reported O3 background at Bermuda (30–40 ppbv (Li et al., 2002)), but lower than the eastern U.S. mean O3 in spring (47 ppbv) and summer (46 ppbv) (Cooper et al., 2012), reflecting an ozone destruction in the remote North Atlantic background air. The relatively lower O3 in summer and fall, compared with spring, was likely the result of increased photochemical destruction of O3 due to conversion of O3 to OH when water vapor was more abundant. A springtime O3 maximum has been found in multiple locations in the free troposphere. The causes of the maximum seem location dependent, and the roles of photochemistry, continental emissions, and transport from the stratosphere have been debated (Monks et al., 2000). Dry air conditions and weak radiation in early spring suppress HOx production, which may lead to net ozone production in the low NOx free troposphere (Yienger et al., 1999; Carpenter et al., 2000). The accumulation of NOy and hydrocarbons due to longer lifetime in winter (e.g. PAN) has also been suggested as reasons for springtime O3 peaks (e.g., Blake et al. (2003); Kramer et al. (2015)). Many studies have also reported impacts of downward transport from the stratosphere, e.g., Beekmann et al. (1994). Over the North Atlantic, Honrath et al. (1996) found that transport from the Arctic significantly increased ozone precursors during winter and spring.
In contrast to CO and O3, NOx and NOy for NATL had maxima in summer at PMO. We speculated that the strengthened Azores-Bermuda anticyclone and downward transport during summer could be the reason for summertime maxima of NOy. NOy in the middle and upper troposphere over oceans was found to be higher than within the marine boundary layer due to less efficient removal processes, including wet and dry deposition (Hübler et al., 1992). Talbot et al. (1999) investigated the NOy budget during the NASA SONEX aircraft mission in the region and reported a median value of 0.17 for PAN/NOy in the middle and upper troposphere. The downward transport could have brought NOy to the lower troposphere in summer, and the thermal dissociation of PAN during subsidence could lead to increased NOx. The coverage of NOx and NOy measurements was not as good as that of CO and O3 due to technical difficulties at the site (Figure 1). Further analysis and chemistry modeling work need to be done to identify the quantification of background NOx and NOy. The characteristics of NATL downward transport will be discussed further in Section 3.2.1.
3.1.2. Transport affected by North American anthropogenic emissions
The transport affected by North American anthropogenic emission (“NA-anthro”) had seasonal occurrence frequencies ranging from 13–16%. The integrated RT plot for NA-anthro (Figure 3b) shows a clear contribution from the northeastern states of the U.S. and a northwards curved transport pathway similarly to what has been found in previous North American transport events (Owen et al., 2006; Helmig et al., 2008, 2015). In the NA-anthro subsets, lifted transport (“NA-anthro-lifted”) ranged from 2.5–4.0%, whereas the low altitude transport (“NA-anthro-low”) had lower frequencies (1.0–3.1%). In terms of pollution age, transport older than 10 days (“NA- anthro-aged”) occurred 4.5–6.2% of the time, whereas transport shorter than 7 days (“NA-anthro-young”) ranged from 4.0 to 6.7%.
As a primary indicator of air pollution, CO enhancement (the amount that exceeds NATL) reflects the extent of pollution influence. North American anthropogenic emissions were estimated to add 15 ppbv (13%), 14 ppbv (17%), and 23 ppbv (28%) of CO to the background (NATL) in spring, summer, and fall, respectively (see Table 2). The larger percentiles in summer and fall were partly due to the low background of CO in the two seasons. In the NA-anthro sub-categories, CO for NA-anthro-low was statistically lower than the others; there was no significant difference among NA-anthro-lifted, -young, and -aged. The characteristic of NA-anthro-low was characterized by polluted air diluted by mixing with clean marine air within the MBL. Such findings suggest the mixing was more effective in reducing CO than photochemical aging in the transport from North America to PMO.
Ozone enhancements for the NA-anthro were 10 ppbv (25%), 10 ppbv (32%), and ppbv (30%) in spring, summer, and fall, respectively. The enhanced percentiles are different because the background levels were lower in summer and fall. As shown in Table 2, the mean O3 mixing ratios in the NA-anthro sub-categories were all significantly different from each other, possibly suggesting O3 levels were sensitive to transport heights and distances. The mean O3 level of NA-anthro-low was lower than NA-anthro-lifted, which is a result of more efficient O3 removal in low altitude transport and potential O3 production by PAN decomposition in the subsidence stage of the lifted transport (Hudman et al., 2004; Zhang et al., 2014). The lower O3 of NA-anthro-aged was generally due to the longer transport time and lack of O3 precursor sources over the North Atlantic.
NOx and NOy levels at PMO were found to be driven by transport of pollution emissions, which was also discussed for a shorter measurement period by Val Martín et al. (2008a). Val Martín et al. (2008a) pointed out that summertime peaks of NOy were caused by efficient transport of boreal wildfire emissions and pollution from eastern North America. Here, we find that the enhancements in NOy caused by anthropogenic emissions were the highest in spring when the analysis was extended to the ten-year observation period. The springtime enhancements could be the result of less efficient wet deposition of NOy and faster zonal transport in mid-latitude spring. The colder spring conditions could lead to a large contribution of PAN to NOy in export from continental regions (Bey et al., 2001; Koike et al., 2003; Li et al., 2004), which reduces the conversion from NOy to HNO3, and the following loss through wet deposition (Moxim et al., 1996). For the same reasons, NOy for NA-anthro-lifted was much higher than that for NA-anthro-low due to accumulation of PAN and less efficient loss of NOy at higher altitude. This transformation of NOy has been studied in detail for two transport events to PMO by Zhang et al. (2014). NOy of NA-anthro-low was lower than the other transport patterns, reflecting efficient removal of NOy in the moist marine boundary layer air.
Maxima of NMHC were found during NA-anthro transport and its subcategories in spring and fall. During NA-anthro-lifted and NA-anthro-young transport, NMHC were higher than that of aged and low transport, suggesting that NMHC loss during transport is due to a combination of mixing with the background and chemical sinks through oxidation reactions with OH.
3.1.3. Trend analyses of CO and O3 enhancements associated with NA-anthro
In order to understand the impacts of North American anthropogenic emissions in light of recent reductions of emissions, we calculated the annual variability in trace gas enhancements as shown in Figure 4. This analysis was only conducted for summer months, because CO observations were not adequate in spring and fall for a couple of years. As shown in Figure 4, we find a decreasing trend of CO enhancements (–0.67 ± 0.60 ppbv/yr as mean ± one-sigma error, p-value = 0.15) and no clear change of enhancements in O3 (–0.04 ± 0.44 ppbv/yr, p-value = 0.47). These findings add on recent studies about CO trends in North American and globally. After the Environmental Protection Agency amended the Clean Air Act in 1990, air pollution emissions from the U.S. decreased (EPA, 2016). Globally-averaged CO has been found to have decreased at a rate of 1%/yr from 2001 to 2011 according to satellite observations (Worden et al., 2013). Trends of background O3 in northern mid-latitudes have changed from positive to negative slopes around the year of 2000 (Parrish et al., 2012). In the eastern U.S., O3 was found to have decreased at rates of –0.03 and –0.45 ppbv/year in spring and summer, respectively (Cooper et al., 2014). Previous work by Kumar et al. (2013) reported that both anthropogenic CO and NOx emissions over the U.S. have decreased significantly since the beginning of the twenty- first century. The same study also found decreasing trends in CO (–0.31 ppbv/year) and O3 (–0.21 ppbv/year) at PMO using a harmonic regression approach. The unclear trend in ozone enhancements in NA-anthro suggests that the decline in North American anthropogenic emissions was perhaps not significant enough to change ozone trends over the central North Atlantic. Instead, the decreasing trend found previously at PMO by Kumar et al. (2013) was probably a large regional global trend through North America to Europe (Cooper et al., 2014). Trend analyses for NOx and NOy are not shown here because the amount of qualified data for these species was not enough for an acceptable statistical test.
3.1.4. Transport affected by wildfire emissions
Transport affected by wildfire emissions (“Fire”) occurred most frequently during summer when wildfires are more active. This transport pattern usually caused drastic changes in pollution levels at PMO due to the great amount of combustion emissions and fast transport in the free troposphere following the strong lifting created by large fires (Val Martín et al., 2006). The superimposed RT (Figure 3c) shows the main pathways from Canada to PMO, which are longer pathways from higher latitude than those of NA-anthro (Figure 3b). This region is frequently dominated by persistent high pressure systems, which leads to dry climates in Canada and Alaska and increases the likelihood of wildfire occurrence in the regions (Macias Fauria and Johnson, 2006).
Boreal wildfires have been found to emit CO more intensively than anthropogenic activities and to cause significant O3 production due to the large amount of CO emitted from large areas of vegetation burned in a short period of time (e.g., Parrington et al. (2013)). As shown in Table 2, the Fire transport pattern had an averaged CO mixing ratio of 108 ppbv, which was enhanced by 25 ppbv (30%) compared with the background (NATL). The enhancement was generally stronger than that caused by NA-anthro (14 ppbv, 17%) in summer. CO levels reported to represent fire signatures were different from those reported in previous studies (Val Martín et al., 2006, 2008a) because different methods were used in determining fire events. The average CO for fire events reported by Val Martín et al. (2006) was 139 ppbv, using a CO cutoff value of 110 ppbv in determining the events in 2004. Val Martín et al. (2008a) reported average CO of 124 ppbv and 105 ppbv, respectively, for fire affected periods in 2004 and 2005 at PMO using 75th percentiles of the FLEXPART_CO from fire emissions as the cutoff (compared to a fixed 15 ppbv of FLEXPART_CO used here). In spite of the differences, all studies indicated significant CO enhancements in transport of fire emissions. The enhancement of O3 in fire plumes (16 ppbv, 50%) was also stronger than that in NA-anthro (10 ppbv, 32%) in summer. These facts suggest that fires act as a competing air pollution source to anthropogenic sources in the central North Atlantic, even though the frequency of Fire (7.3%) is about half of the frequency of NA-anthro in summer.
Vegetation fuel nitrogen can lead to high NOx mixing ratios in fire plumes (Andreae and Merlet, 2001). The mixing ratios of NOx and NOy of Fire were higher than NATL by 9 pptv and 138 pptv, respectively, and these enhancements were in agreement with the values (9–30 pptv for NOx, and 117–175 pptv for NOy) reported by Val Martín et al. (2008a). The large enhancement of NOy after long-rang transport could be due to chemical transformation of the nitrogen oxides in fire plumes. Through aircraft measurements, Alvarado et al. (2010) observed that 40% of the initial NOx emissions were converted to PAN within a few hours in boreal biomass burning. Fast conversion of NOx to NOy may promote long-range transport of NOy produced by wildfires.
3.1.5. Transport from the upper troposphere and lower stratosphere
Downward mixing from the UTLS (“Upper”) had frequencies of 2.1–4.1%, with the highest frequency in fall. The fall maximum was likely caused by stratosphere-troposphere-exchange (STE) in the polar and middle latitude regions, which shifts southward from summer to winter (Holton et al., 1995). The transport pathways are shown in Figure 3d. A general transport direction from the northwest of PMO reflects the typical dynamics in anticyclones and tropopause folding in the mid-latitude.
The Upper transport pattern had CO means of 123, 92, and 96 ppbv for spring, summer, and fall, respectively (Table 2), which were ~10 ppbv higher than CO mixing ratios of NATL. CO has no large sources at high altitude and has been found to be mainly trapped in the lower troposphere (Liang et al., 2011). However, two potential reasons may lead to slight CO enhancement for the Upper transport pattern. In the superimposed RT plots of the two transport patterns, Upper and NATL, in Figure 3a and 3d, air mass origins of Upper covered large sub-polar regions, while NATL covered the middle and low latitude regions. Air masses from the north should contain higher CO because of longer CO lifetime as a result of lower OH concentrations (Novelli et al., 1998). In addition, Upper transport also covered the whole boreal North American region (Figure 3d), so the Upper may also receive CO contributions from wildfires from that area.
The Upper transport had average O3 mixing ratios of 58, 54, and 49 ppbv for spring, summer, and fall, respectively, which were the highest among all transport patterns. O3 enhancement was ~20 ppbv, which is double that of NA-anthro. Such impact has been well recognized before and been associated with downward transport of naturally high O3 in the UTLS (e.g., Logan, 1999; Neuman et al., 2012). The debate about stratospheric influence versus tropospheric production as the major source of tropospheric O3 has existed for years (Monks et al., 2000). Subsidence of stratospheric ozone has been shown to have signification contributions to episodic ozone enhancements in the lower troposphere. Parrish et al. (1993) reported ozone enhancements (up to 60 ppbv) due to injection of stratospheric ozone at Cape Race, Canada. Springtime downward transport events were found to cause one-minute average ozone reaching 100 ppbv in Boulder, U.S., and a peak concentration of 215 ppbv in the Rocky Mountains (Langford et al., 2009). At an elevated site, Mount Washington, New Hampshire, U.S., a stratospheric influence was found to cause ozone enhancement in air flows associated with atmospheric anticyclones (Fischer et al., 2004). Although we found downward transport caused the highest O3 enhancements at PMO, the occurrence frequencies of this transport pattern (2.1%, 2.3% and 4.1%) were much lower than those of NA-anthro. Therefore, on an annual average basis, the total O3 contribution from downward mixing was less significant than transport of anthropogenic pollution emissions. By comparing the products of the occurrence frequencies and O3 enhancements associated to NA-anthro and Upper transport patterns, we estimated that the overall O3 contribution by downward mixing is about 40% of the transport of anthropogenic emissions.
The Upper transport pattern also had the highest NOx mixing ratios in all three seasons. The NOx means were 63, 66, and 53 pptv in spring, summer, and fall, respectively, which were 15–41 pptv higher than those of the NATL. NOx is produced in the UTLS with the source being the photochemical oxidation of N2O. The NOy means of Upper were 271, 234, and 229 pptv in spring, summer, and fall, respectively, which were all higher than NATL. The elevated NOy during downward mixing can be attributed to accelerated hydrocarbon degradation followed by formation of PAN in stratosphere-troposphere air mass exchange (Liang et al., 2011). Hydrocarbon degradation accelerates because of increased OH production as a result of mixing of O3-rich air from UTLS with moist tropospheric air in the downward transport. The higher NOy could be due to downward transport of upper tropospheric air having a high ratio of PAN/NOy, which, to some extent, reduces the rate of wet removal of NOy. The subsequent PAN decomposition could have led to production of NOx.
3.1.6. Transport affected by upslope flow
Upslope flow occurred 13–24% of the time, with the lowest frequency in summer. The seasonality found here is in agreement with the previous meteorological study at PMO, but has generally lower frequencies than that estimated for the period of 2004–2005 (4.2–37 % (Kleissl et al., 2007)). CO and O3 in Upslope transport were statistically higher than NATL. This indicates upslope transport of local anthropogenic emissions to PMO. The Upslope transport had the lowest NOy, which was likely due to efficient wet scavenging of NOy during lifting (cloud condensation was predicted to take place for 30% of Upslope periods). The mixing ratios of n- and i-butane were found to be higher than the background in all seasons, which was likely due to butane being used as a primary domestic fuel on the island. Concentration enhancements of the butane isomers were higher in spring than the other seasons, possible due to increased butane usage as heating fuel in the cold spring.
3.2. Relationships of observed trace gases
Due to differences in chemical mechanisms and reaction rates, concentrations of trace gases change at different rates in long-range transport, which leads to recognizable relationships between observed trace gases at PMO. Such relationships are able to reflect chemical transformation history during long-range transport. For example, d[O3]/d[CO] values have been used to reflect O3 production tendency based on the fact that CO has a much longer lifetime (a few months) compared with O3 (lifetime of days to weeks) in the troposphere. During transport from North America to PMO, d[O3]/d[CO] changes mainly reflect O3 chemistry in the absence of fresh emissions. Light NMHC (alkanes and alkynes) are detectable after transport from NA to PMO and are good tracers to study photochemical aging. Due to the differences in size and structure, different NMHC decay at varying reaction rates against OH. By examining the ratios between NMHC, the photochemical age of air masses can be estimated (McKeen and Liu, 1993). In the following sections, we examine these relationships in PMO observations with particular attention paid to two major transport patterns, NATL and NA-anthro.
d[O3]/d[CO] was calculated as the reduced major axis (RMA) slope of the two species. RMA slope takes into account the variability in both x- and y-coordinates (Ayers, 2001); it is used as a proper tool for correlation analysis of observed trace gases. We present d[O3]/d[CO] grouped for the transport patterns, NATL, Fire, Upper, and NA-anthro, in spring, summer, and fall respectively (Figure 5). The data used in these analyses were uninterrupted observations for more than 12 hours. In this way, only prominent events were considered to fully include remarkable changes in CO and O3, so that a clear and credible slope was more likely to be obtained. Yokelson et al. (2013) discussed how the ratio cannot be used to characterize source emissions and plume aging when mixing with air masses of different composition occurs (e.g., plume mixes with O3-rich stratospheric air and then with clean marine boundary layer air). For this reason, we reported the statistical significance of the regression analyses. In cases of substantial changes in background air composition during transport, the significance of the regression will be low (high p-values).
d[O3]/d[CO] values for NA-anthro were 0.59 (spring, Figure 5a), 0.71 (summer, Figure 5b), and 0.45 (fall, Figure 5c) ppbv/ppbv, which were all lower than the values found in direct transport events to PMO, with slopes around 1.0 (Honrath et al., 2004; Zhang et al., 2014). The lower slopes in the current work were expected because events with long transport time were also included, which should be less intensive in O3 production (lower positive d[O3]/d[CO]). The values of d[O3]/d[CO] have been reported to be lower and relatively consistent at sites in the U.S. (0.20–0.37 ppbv/ppbv), which reflected a combination of aging processes of air pollutants and mixing with remote clean air mass during transport (Chin et al., 1994; Parrish et al., 1998; Mao and Talbot, 2004). Using the Tropospheric Emission Spectrometer retrievals on the Aura satellite, Hegarty et al. (2009) reported a spring slope value of 0.13 ppbv/ppbv in the free troposphere over the mid-latitude regions extending from North America to the North Atlantic. Interestingly, further into the North Atlantic ocean, higher slopes were found at PMO (Honrath et al. (2004) and this work) and Izana, Canary Islands (Cuevas et al., 2013).
The observations affected by wildfires had lower d[O3]/d[CO] values (slopes of 0.31 and 0.12 ppbv/ppbv for summer and fall respectively) than NA-anthro. The lower values were likely due to large amounts of CO produced in biomass burning and lower NOx/CO emission ratios compared with anthropogenic sources (McKeen et al., 2002; Lapina et al., 2006). In addition, production of O3 and NO2 can be suppressed in wildfire plumes due to reduction of sunlight by heavy aerosol loadings (Real et al., 2007; Verma et al., 2009; Parrington et al., 2013). This variation in d[O3]/d[CO] in fire-affected patterns was consistent with a previous MOZART study at PMO (Pfister et al., 2006), in which lower d[O3]/d[CO] values were also found during periods associated with higher relative contribution of fire tracers. Val Martín et al. (2006) calculated the O3 enhancement indicator for fire plumes captured at PMO in a different method, which is the Δ[O3]/Δ[CO] (Δ refers to the difference between observed concentrations and an estimated background), and reported a similar mean value of 0.20 ppbv/ppbv for summer 2004.
The slopes for NATL were higher than NA-anthro in spring and fall. In both seasons, a few high O3 mixing ratios (greater than 50 ppbv) were observed for NATL. We investigated those periods specifically and found that these events received ~9% RT from above 5 km. This suggested that NATL receives non-negligible contributions from UTLS in general, even though strict criteria were applied to ensure that the majority RT originated from the lower troposphere. As a result, occasionally high O3 mixing ratios combined with low CO should be expected in the background of PMO. Examples of such observations are marked by gold dotted rectangles in Figure 5a and Figure 5b. These observations show high O3 concentrations (up to 60 ppbv) in the lower end of CO ranges in spring and summer, pull the regression toward higher slopes, and lead to low correlation coefficients. Positive d[O3]/d[CO] caused by downward mixing was also observed in a previous study using GEOS-Chem to interpret global satellite observations of d[O3]/d[CO] (Kim et al., 2013). Negative slopes indicating anticorrelation between O3 and CO were found in winter at a few ground sites (Parrish et al., 1998; Macdonald et al., 2011), which were attributed to titration of O3 by NO in emissions. Strong vertical mixing carrying air mass from the upper troposphere was also found to cause negative slopes (Fishman and Seiler, 1983).
3.2.2. Oxidation of butane isomers
Figure 6 shows the observed ratios of [i-butane]/[n-butane] as a function of [n- butane] for all paired observations in the ten years. Observations for NA-anthro and NATL are shown in colors as indicated by the legends. The geometric means (red symbols) and standard deviations (bars) for the two transport patterns are also shown. Mixing ratios of n-butane on the x-axis are given in a logarithmic scale to better display the wide range. The [i-butane]/[n-butane] values towards the left show a more scattered distribution because of lower precision of measurements near the detection limit. The [n-butane] mean for NA-anthro was higher than NATL as a result of emission influence. The means of [i-butane]/[n-butane] were both 0.49 for NA-anthro and NATL, and uncertainties were similar, which indicated no significant difference in aging rates between butane isomers. Chlorine atoms have been found to cause faster oxidation of n-isomers in Arctic marine air (Hopkins et al., 2002). Chlorine atoms from oceanic sources were also found to affect NMHC observations in coastal regions during time periods when wind blows from the Pacific (Gorham et al., 2010). However, similar values of [i-butane]/[n-butane] were found at PMO in well-aged background air and transport of pollution, suggesting that the difference in oxidation speed for the butane isomers was hardly detectable. This implies that aging in both species is driven by OH, while other oxidants, such as Cl/Br, play minor roles. These results are in agreement with earlier findings by Helmig et al. (2008).
3.2.3. NMHC photochemical clock
Observed relationships between NMHC can be used to indicate atmospheric processing during long-range transport (i.e., photochemical aging and mixing with background air). The logarithmic ratio of NMHC pairs (i.e., (ln [n-butane]/[ethane])/(ln [propane]/[ethane])) is a function of both OH, and NMHC concentrations in emission sources (Parrish et al., 2007). This approach works better than studying NMHC concentrations alone, since the ratio reduces the large variations of NMHC concentrations in emissions. The NMHC ratios are very effective tracers to study pollution transport to PMO, because NMHC have low concentrations and no significant emission over the ocean (McKeen and Liu, 1993).
The analyses of ln[n-butane]/[ethane] vs ln[propane]/[ethane] shown in Figure 7 investigate photochemical aging and mixing extent during transport to PMO, with specific attention paid to NATL and NA-anthro (Figure 7b). A kinetic scenario is an ideal condition, in which we assumed an isolated plume and no mixing with the marine air during transport. The kinetic scenario is indicated by the solid line with scales in both panels of Figure 7. The slope and aging scales were calculated using the same kinetic reaction rates as provided by Helmig et al. (2008) and an assumed constant [OH] of 1.0 x 106 molecule/cm3. A global mean [OH] was chosen based on the estimate from Roelofs and Lelieveld (2000), but one should note that [OH] in the troposphere at northern mid-latitudes has been found higher in spring than in summer and fall (Spivakovsky et al., 1990). For this reason, the intervals between age scales in Figure 7 would be longer in spring and shorter in fall. The end point on the upper right of the kinetic line was defined by the NMHC ratios in continental emissions derived by Helmig et al. (2008). Data for NATL and NA-anthro are shown in different colors in the legends in Figure 7. Most data locate between the kinetic line and the mixing line indicating transport involved both aging and mixing processes. The regression of all data (solid line without scales) gave a RMA slope of 1.44 with r2 = 0.55 and p < 0.01. The ten-year observation gave a range of the NMHC age at PMO from 7 to more than 20 days, with a mean of 11 days and a standard deviation of 4 days. This estimated mean age was greater than that found in previous studies at PMO using FLEXPART (Honrath et al., 2004; Zhang et al., 2014). Again, the reason was mainly that previous studies focused on a few efficient transport events, whereas all transport event candidates from North America are included here. The regression slope for NA-anthro was 1.36 with r2 = 0.35, which is consistent with observations in previous studies for long-range transport over the North Atlantic (Parrish et al., 2007; Helmig et al., 2008). The estimated mean for NA-anthro was 11. 5 days with a standard deviation of 3.5 days. The NA-anthro mean was similar to the mean estimated based on all observations, suggesting a dominating influence of NA-anthro. There was a much less credible regression for NATL (p = 0.25), which was likely due to information loss on the source after extended aging. The estimated mean of NATL age was 15 days with a standard deviation of 3.5 days.
4. Summary and Conclusions
We categorized the dominant transport patterns to Pico Mountain Observatory for the years 2001–2010 by considering factors that affect air mass composition including types of emission sources, atmospheric RT, transport height, and upslope flow. FLEXPART and simulated meteorological conditions were used to quantify these factors. Observations of trace gases for the transport patterns, viewed as chemical signatures, were compared and discussed to investigate the characteristics of long-range transport to the central North Atlantic.
Trace gas concentrations at PMO were driven by both pollution emissions and natural sources. Anthropogenic emissions contributed an additional 14–23 ppbv (13%–28%) CO and ~10 ppbv (25%–30%) O3 to background levels at PMO depending on the seasons. Wildfires caused even higher pollutant levels (25 ppbv (30%) CO and 16 ppbv (45%) of O3) in summer, and were comparable sources of trace gases to anthropogenic emissions. The O3 enhancements caused by UTLS downward mixing were about two times that caused by North American anthropogenic emissions. Downward mixing also caused slight CO enhancements at PMO, which were likely due to transport from high latitudes where CO has a longer lifetime. Higher NOx concentrations were found in transport from North America. Variations of the enhancements in different transport patterns were attributed to temperature and height dependent processes, such as wet scavenging and chemical reactions. High NOy and O3 were found in lifted transport from North America, which supports previous findings that PAN preserves O3 production tendency after long-range transport. Highest NOx was found in downward transport, likely a result of thermal decomposition of PAN in subsidence.
The slope d[O3]/d[CO] has been used as an indicator of O3 production tendency, and it reflected O3 chemistry and composition of original air masses. The values of d[O3]/d[CO] for Fire (0.12–0.33) were lower than those for NA-anthro and NATL, which was likely due to the much larger amount of CO emission from wildfires. The mixing with O3-rich air masses creates largely varying O3 within a narrow CO range, which tends to result in the high variability in d[O3]/d[CO]. Remarkable downward mixing showed highly positive or negative [O3]/d[CO] values, likely due to the opposite vertical gradients of O3 and CO, but the strength of the correlation was relatively low.
Mixing ratios of light NMHC were used to study oxidation processing during transport to PMO. The ratios of butane isomers for NATL and NA-anthro were not significantly different, which implies that sources signatures and oxidation rates were similar. Enhanced levels of butane isomers were observed during periods when upslope flow occurred, likely due to butane being used in the fuel on the Island. The average NMHC photochemical ages for NA-anthro and the entire dataset were 11.5 ± 3.5 and 11.0 ± 4.0 days. This estimate reflected the general air mass age for long-range transport to PMO and confirmed the dominating role of North American emissions.
5. Data accessibility statement
The following datasets were generated/used: PMO observations of trace gases and meteorology: available from website http://instaar.colorado.edu/pico/pico_archive/default.html FLEXPART simulations: available upon request due to large data files.