Daniel A. Jaffe

Asian dust events of April 1998

On April 15 and 19, 1998, two intense dust storms were generated over the Gobi desert by springtime low-pressure systems descending from the northwest. The windblown dust was detected and its evolution followed by its yellow color on SeaWiFS satellite images, routine surface-based monitoring, and through serendipitous observations. The April 15 dust cloud was recirculating, and it was removed by a precipitating weather system over east Asia. The April 19 dust cloud crossed the Pacific Ocean in 5 days, subsided to the surface along the mountain ranges between British Columbia and California, and impacted severely the optical and the concentration environments of the region. In east Asia the dust clouds increased the albedo over the cloudless ocean and land by up to 10–20%, but it reduced the near-UV cloud reflectance, causing a yellow coloration of all surfaces. The yellow colored backscattering by the dust eludes a plausible explanation using simple Mie theory with constant refractive index. Over the West Coast the dust layer has increased the spectrally uniform optical depth to about 0.4, reduced the direct solar radiation by 30–40%, doubled the diffuse radiation, and caused a whitish discoloration of the blue sky. On April 29 the average excess surface-level dust aerosol concentration over the valleys of the West Coast was about 20–50 μg/m3 with local peaks >100 μg/m3. The dust mass mean diameter was 2–3 μm, and the dust chemical fingerprints were evident throughout the West Coast and extended to Minnesota. The April 1998 dust event has impacted the surface aerosol concentration 2–4 times more than any other dust event since 1988. The dust events were observed and interpreted by an ad hoc international web-based virtual community. It would be useful to set up a community-supported web-based infrastructure to monitor the global aerosol pattern for such extreme aerosol events, to alert and to inform the interested communities, and to facilitate collaborative analysis for improved air quality and disaster management.

Transport of Asian air pollution to North America

Using observations from the Cheeka Peak Observatory in northwestern Washington State during March-April, 1997, we show that Asian anthropogenic emissions significantly impact the concentrations of a large number of atmospheric species in the air arriving to North America during spring. Isentropic back-trajectories can be used to identify possible times when this impact will be felt, however trajectories alone are not sufficient to indicate the presence of Asian pollutants. Detailed chemical and meteorological data from one of these periods (March 29th, 1997) indicates that the surface emissions were lifted into the free troposphere over Asia and then transported to North America in ∼6 days.

Nitrogen Emissions, Deposition, and Monitoring in the Western United States

Abstract Nitrogen (N) deposition in the western United States ranges from 1 to 4 kilograms (kg) per hectare (ha) per year over much of the region to as high as 30 to 90 kg per ha per year downwind of major urban and agricultural areas. Primary N emissions sources are transportation, agriculture, and industry. Emissions of N as ammonia are about 50% as great as emissions of N as nitrogen oxides. An unknown amount of N deposition to the West Coast originates from Asia. Nitrogen deposition has increased in the West because of rapid increases in urbanization, population, distance driven, and large concentrated animal feeding operations. Studies of ecological effects suggest that emissions reductions are needed to protect sensitive ecosystem components. Deposition rates are unknown for most areas in the West, although reasonable estimates are available for sites in California, the Colorado Front Range, and central Arizona. National monitoring networks provide long-term wet deposition data and, more recently, estimated dry deposition data at remote sites. However, there is little information for many areas near emissions sources.

Increasing springtime ozone mixing ratios in the free troposphere over western North America

In the lowermost layer of the atmosphere—the troposphere—ozone is an important source of the hydroxyl radical, an oxidant that breaks down most pollutants and some greenhouse gases. High concentrations of tropospheric ozone are toxic, however, and have a detrimental effect on human health and ecosystem productivity. Moreover, tropospheric ozone itself acts as an effective greenhouse gas. Much of the present tropospheric ozone burden is a consequence of anthropogenic emissions of ozone precursors resulting in widespread increases in ozone concentrations since the late 1800s. At present, east Asia has the fastest-growing ozone precursor emissions. Much of the springtime east Asian pollution is exported eastwards towards western North America. Despite evidence that the exported Asian pollution produces ozone, no previous study has found a significant increase in free tropospheric ozone concentrations above the western USA since measurements began in the late 1970s. Here we compile springtime ozone measurements from many different platforms across western North America. We show a strong increase in springtime ozone mixing ratios during 1995–2008 and we have some additional evidence that a similar rate of increase in ozone mixing ratio has occurred since 1984. We find that the rate of increase in ozone mixing ratio is greatest when measurements are more heavily influenced by direct transport from Asia. Our result agrees with previous modelling studies, which indicate that global ozone concentrations should be increasing during the early part of the twenty-first century as a result of increasing precursor emissions, especially at northern mid-latitudes, with western North America being particularly sensitive to rising Asian emissions. We suggest that the observed increase in springtime background ozone mixing ratio may hinder the USA’s compliance with its ozone air quality standard.

Six 'new' episodes of trans-Pacific transport of air pollutants

Abstract Chemical measurements of CO, O3, non-methane hydrocarbons, aerosol chemistry, and aerosol scattering in air masses arriving at the west coast of North America demonstrate that a variety of chemical species can be transported across the Pacific from the Eurasian continent. In this paper, we analyze data from several ground sites in the Pacific Northwest and from aircraft observations in the region to identify six ‘new’ (i.e. previously unreported) episodes of trans-Pacific transport that occurred between 1993 and 2001. Of the six new episodes identified, one consisted of mineral dust combined with industrial emissions, while the other five appeared to consist of predominantly industrial emissions. In addition to gas and aerosol measurements, we calculated the Angstrom exponent, an indicator of the aerosol size distribution, to help identify the aerosol characteristics in 4 of the cases considered. Combining these data with previous reports, we find that there is a high degree of variability in these trans-Pacific episodes. In four episodes there were significant O3 enhancement, with mixing ratios sometimes exceeding 80 ppbv . However, O3 was only enhanced in episodes that were transported in the free troposphere and in the absence of mineral dust. In other words, transport in the boundary layer or transport of industrial emissions with mineral dust seems to preclude any significant O3 enhancement. Clarifying such patterns increases our understanding of not only trans-Pacific transport of air pollutants, but also intercontinental transport in general.

Influence of Asian emissions on the composition of air reaching the North Western United states

A global 3-D CTM has been used to study the impact of current and future emissions from Asia on CO, PAN and O3 across the North Pacific. Recent measurements from Washington State have been used to verify the model results with respect to average concentrations as well as amplitude of perturbations during episodic events of strong Asian influence. By performing CTM experiments with and without anthropogenic emissions from Asia, we find that there is a significant contribution from Asian sources in the air arriving to the North Western United States, especially during spring. The mean contribution, which can not easily be inferred from the available measurements, during spring are 34 ppbv, 26 pptv and 4 ppbv for CO, PAN and O3, respectively. The maximum enhancements caused by Asian sources during episodes are 42 ppbv, 75 pptv, and 7.5 ppbv, respectively. The amplitude of the perturbations during short term (2–5 days) episodes of strong Asian influence are quite similar to springtime Asian pollution events which have recently been observed on the west coast of Washington state. A doubling of the current Asian emissions in the model yields significant enhancements in all species, though not necessarily in a linear manner.

Observations of Reactive Gaseous Mercury in the Free Troposphere at the Mount Bachelor Observatory

August 2005. The mean mercury concentrations (at standard conditions) were 1.54 ng/m 3 (GEM), 5.2 pg/m 3 (PHg), and 43 pg/m 3 (RGM). RGM enhancements, up to 600 pg/m 3 , occurred at night and were linked to a diurnal pattern of upslope and downslope flows that mixed in boundary layer air during the day and free tropospheric air at night. During the night, RGM was inversely correlated (P < 0.0001) with CO (r = � 0.36), GEM (r = � 0.73), and H2 O( r =� 0.44), was positively correlated with ozone (r = 0.38), and could not be linked to recent anthropogenic emissions from local sources or long-range transport. Principal component analysis and a composite of change in RGM versus change in GEM during RGM enhancements indicate that a nearly quantitative shift in speciation is associated with increases in ozone and decreases in water vapor and CO. This argues that high concentrations of RGM are present in the free troposphere because of in situ oxidation of GEM to RGM. A global chemical transport model reproduces the RGM mean and diurnal pattern but underestimates the magnitude of the largest observed enhancements. Since the only modeled, in situ RGM production mechanisms are oxidation of GEM by ozone and OH, this implies that there are faster reaction rates or additional RGM production mechanisms in the free troposphere.

Climatologies of NOx and NOy: A comparison of data and models

Abstract Climatologies of tropospheric NOx (NO + NO2) and NOy (total reactive nitrogen: NOx + N03 + 2 × N2O5 + HNO2 + HNO3 + HNO4 + ClONO2 + PAN (peroxyacetylnitrate) + other organic ni trates) have been compiled from data previously published and, in most cases, publicly archived. Emphasis has been on non-urban measurements, including rural and remote ground sites, as well as aircraft data. Although the distribution of data is sparse, a compilation in this manner can begin to provide an understanding of the spatial and temporal distributions of these reactive nitrogen species. The cleanest measurements in the boundary layer are in Alaska, northern Canada and the eastern Pacific, with median NO mixing ratios below 10 pptv, NOx below 50 pptv, and NOy below 300 pptv. The highest NO values (greater than 1 ppbv) were found in eastern North America and Europe, with correspondingly high NOy (∼ 5 ppbv). A significantly narrower range of concentrations is seen in the free troposphere, particularly at 3–6 km, with NO typically about 10 pptv in the boreal summer. NO increases with altitude to ∼ 100 pptv at 9–12 km, whereas NOy does not show a trend with altitude, but varies between 100 and 1000 pptv. Decreasing mixing ratios eastward of the Asian and North American continents are seen in all three species at all altitudes. Model-generated climatologies of NOx and NOy from six chemical transport models are also presented and are compared with observations in the boundary layer and the middle troposphere for summer and winter. These comparisons test our understanding of the chemical and transport processes responsible for these species distributions. Although the model results show differences between them, and disagreement with observations, none are systematically different for all seasons and altitudes. Some of the differences between the observations and model results may likely be attributed to the specific meteorological conditions at the time that measurements were made differing from the model meteorology, which is either climatological flow from GCMs or actual meteorology for an arbitrary year. Differences in emission inventories, and convection and washout schemes in the models will also affect the calculated NOα and NOy distributions.

Intercontinental Dispersal of Bacteria and Archaea by Transpacific Winds

ABSTRACT Microorganisms are abundant in the upper atmosphere, particularly downwind of arid regions, where winds can mobilize large amounts of topsoil and dust. However, the challenge of collecting samples from the upper atmosphere and reliance upon culture-based characterization methods have prevented a comprehensive understanding of globally dispersed airborne microbes. In spring 2011 at the Mt. Bachelor Observatory in North America (2.8 km above sea level), we captured enough microbial biomass in two transpacific air plumes to permit a microarray analysis using 16S rRNA genes. Thousands of distinct bacterial taxa spanning a wide range of phyla and surface environments were detected before, during, and after each Asian long-range transport event. Interestingly, the transpacific plumes delivered higher concentrations of taxa already in the background air (particularly Proteobacteria, Actinobacteria, and Firmicutes). While some bacterial families and a few marine archaea appeared for the first and only time during the plumes, the microbial community compositions were similar, despite the unique transport histories of the air masses. It seems plausible, when coupled with atmospheric modeling and chemical analysis, that microbial biogeography can be used to pinpoint the source of intercontinental dust plumes. Given the degree of richness measured in our study, the overall contribution of Asian aerosols to microbial species in North American air warrants additional investigation.

Impact of Asian emissions on the remote North Pacific atmosphere: Interpretation of CO data from Shemya, Guam, Midway and Mauna Loa

In this study we look at the concentration of CO at four remote stations in the North Pacific to evaluate the impact of Asian industrial emissions on the remote atmosphere. Using a locally weighted smoothing technique to identify individual data outliers from the seasonal cycle, we have identified 22–92 outliers or “events” (greater than 5 ppbv above the seasonal cycle) at each site for the 3–6 year data records. Using isentropic back trajectories, we identify a possible source region for each event and present a distribution of the trajectory types. For the events at Midway, Mauna Loa, Guam, and Shemya, we are able to identify a source region for the elevated CO in 82, 72, 65, and 50% of the events, respectively. At Mauna Loa and Midway a majority of the events occur during spring and are usually associated with transport from Asia. These events bring the highest CO mixing ratios observed at any time during the year to these sites, with CO enhancements up to 46 ppb. At Guam, easterly trade winds are the norm, but occasionally synoptic events bring Asian emissions to the island, generally during late summer and fall, from either East Asia or Southeast Asia (e.g., Indonesia). These events bring with them the largest CO enhancements of any of the four sites considered in this paper, up to 58 ppb. Finally, to examine the robustness of our conclusions, we redo our analysis using the more stringent definition that an event must be either 10 or 15 ppb above the seasonal cycle. Although this reduces the number of events identified at each site, it does not significantly change the fraction of events which can be attributed to a known source.