[1] We report summertime measurements of CO and O 3 obtained during 2001-2003 at the PICO-NARE mountaintop station in the Azores. Frequent events of elevated CO mixing ratios were observed. On the basis of backward trajectories arriving in the free troposphere and global simulations of biomass burning plumes, we attribute nearly all these events to North American pollution outflow and long-range transport of biomass burning emissions. There was a high degree of interannual variability in CO levels: median [CO] ranged from 65 ppbv in 2001 to 104 ppbv in 2003. The highest concentrations were associated with transport of Siberian fire emissions during summer 2003, when Siberian fire activity was unusually high. Ozone mixing ratios also increased (by up to $30 ppbv) during the fire events. These findings demonstrate the significant hemispheric scale impact that biomass burning events have on background CO and O 3 levels. O 3 enhancements of similar magnitude were also observed in North American pollution outflow. O 3 and CO were correlated during North American outflow events, with a slope averaging 1., ppbv/ppbv) when no fire impact was present. This slope is more than 80% larger than early 1990s observations made in the eastern United States and nearshore outflow region, even after accounting for declining U.S. CO emissions and for CO loss during transport to the Azores, and is not consistent with simple dilution of U.S. outflow with marine background air. We conclude that a significantly larger amount of O 3 production occurred in the air sampled during this study, and we suggest several potential reasons for this, each of which could imply potentially significant shortcomings in current estimates of the hemispheric impact of North American emissions on tropospheric ozone and should be evaluated in future studies.
The radiative properties of soot particles depend on their morphology and mixing state, but their evolution during transport is still elusive. Here we report observations from an electron microscopy analysis of individual particles transported in the free troposphere over long distances to the remote Pico Mountain Observatory in the Azores in the North Atlantic. Approximately 70% of the soot particles were highly compact and of those 26% were thinly coated. Discrete dipole approximation simulations indicate that this compaction results in an increase in soot single scattering albedo by a factor of ≤2.17. The top of the atmosphere direct radiative forcing is typically smaller for highly compact than mass-equivalent lacy soot. The forcing estimated using Mie theory is within 12% of the forcing estimated using the discrete dipole approximation for a high surface albedo, implying that Mie calculations may provide a reasonable approximation for compact soot above remote marine clouds.
[1] Extensive wildfires burned in northern North America during summer 2004, releasing large amounts of trace gases and aerosols into the atmosphere. Emissions from these wildfires frequently impacted the PICO-NARE station, a mountaintop site situated 6-15 days downwind from the fires in the Azores Islands. To assess the impacts of the boreal wildfire emissions on the levels of aerosol black carbon (BC), nitrogen oxides and O 3 downwind from North America, we analyzed measurements of CO, BC, total reactive nitrogen oxides (NO y ), NO x (NO + NO 2 ) and O 3 made from June to September 2004 in combination with MOZART chemical transport model simulations. Long-range transport of boreal wildfire emissions resulted in large enhancements of CO, BC, NO y and NO x , with levels up to 250 ppbv, 665 ng m À3 , 1100 pptv and 135 pptv, respectively. Enhancement ratios relative to CO were variable in the plumes sampled, most likely because of variations in wildfire emissions and removal processes during transport. Analyses of DBC/DCO, DNO y /DCO and DNO x /DCO ratios indicate that NO y and BC were on average efficiently exported in these plumes and suggest that decomposition of PAN to NO x was a significant source of NO x . High levels of NO x suggest continuing formation of O 3 in these well-aged plumes. O 3 levels were also significantly enhanced in the plumes, reaching up to 75 ppbv. Analysis of DO 3 /DCO ratios showed distinct behaviors of O 3 in the plumes, which varied from significant to lower O 3 production. We identify several potential reasons for the complex effects of boreal wildfire emissions on O 3 and conclude that this behavior needs to be explored further in the future. These observations demonstrate that boreal wildfire emissions significantly contributed to the NO x and O 3 budgets in the central North Atlantic lower free troposphere during summer 2004 and imply large-scale impacts on direct radiative forcing of the atmosphere and on tropospheric NO x and O 3 .
Ground level ozone concentrations ([O 3 ]) typically show a direct linear relationship with surface air temperature. Three decades of California measurements provide evidence of a statistically significant change in the ozone-temperature slope (Δm O3-T ) under extremely high temperatures (>312 K). This Δm O3-T leads to a plateau or decrease in [O 3 ], reflecting the diminished role of nitrogen oxide sequestration by peroxyacetyl nitrates and reduced biogenic isoprene emissions at high temperatures. Despite inclusion of these processes in global and regional chemistry-climate models, a statistically significant change in Δm O3-T has not been noted in prior studies. Future climate projections suggest a more frequent and spatially widespread occurrence of this Δm O3-T response, confounding predictions of extreme ozone events based on the historically observed linear relationship. (4-10), and several studies have attempted to isolate the drivers of this relationship, as summarized in ref. 11. Early studies investigating the ozone-temperature relationship noted the impact of peroxyacetyl nitrate (PAN) decomposition on ozone formation (4, 12). The PAN sink for NO x and odd hydrogen (HO x ) decreases exponentially as temperatures increase, implying a saturation of ozone formation from PAN decomposition as temperatures increase above ∼310 K. Sillman and Samson (10) found that m O3-T is a function of multiple chemical processes, including the reaction rate of PAN, emissions of biogenic volatile organic compounds (VOC), photolysis rates, and water vapor concentrations. Therefore whereas absolute temperature is a strong predictor of the effects of incremental temperature change on ozone (2), chemical kinetics and temperature-dependent emission rates further complicate this relationship. For example, prior studies have noted that m O3-T varies between regions with different NO x ∕VOC ratios (13), and can decrease following significant NO x emissions reductions (6). The m O3-T relationship has been called a climate change "penalty," signifying that emissions reductions will need to be more stringent to counteract the effects of warming temperatures (6,14). However, the stationarity of this ozone-temperature relationship has yet to be evaluated using observations over a broad range of temperatures.High concentrations of tropospheric ozone ([O 3 ]) are an indicator of poor air quality, and adversely affect the health of humans and ecosystems (15,16). A suite of chemical and meteorological factors contributes to the formation of ozone. Photochemically driven reactions of VOC in the presence of nitrogen oxides (NO x ) can form ozone at the surface (17), whereas stagnant meteorological conditions promote and maintain ozone events.Changes in the frequency of certain meteorological features such as fewer midlatitude cyclones (18,19) or shallower boundary layer depths (20) (Fig. S1). Daily maximum surface air temperature (T max ) data are obtained from a statistically interpolated gridded product of ground-based National Oceanic a...
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