Yaping Xiao's Recent Research
My ongoing work at UNH is to examine the meteorological and chemical factors affecting seasonal variation of ozone, particularly the springtime ozone maximum, in northeast U.S.

My Ph.D. thesis was focused on quantifying the sources of methane (CH4) on regional/continental scale with constraints from correlations of CH4 and other chemical tracers, such as ethane (C2H6) and CO. My thesis also includes an investigation of the correlations of acetylene (C2H2) and CO as constraints on atmospheric OH concentrations and mixing rates. See the full description of my thesis below:


Spring O3 at Mount Washington Observatory: Implications for Stratospheric Intrusions

It is well known that tropospheric O3 peaks in late spring in most remote regions of northern mid- and high-latitudes, which has been attributed to: (1) meteorological process: mass transfer from the stratosphere to the troposphere peaks in spring and; (2) photochemical activity after accumulation of O3 and its precursors in winter and early spring. However, determining the relative contribution of chemical versus meteorological processes has been a research focus for over two decades. 

The multi-year (2002-2006) record of O3 observations at Mount Washington Observatory (MWO, 44.3N, 71.3W, 1910m asl) from the AIRMAP program at the University of New Hampshire provides an opportunity to examine the anthropogenic and stratospheric contributions to springtime O3. In this work, we integrate the meteorological and chemical data to identify the synoptic conditions, which are favorable for stratospheric influence at MWO, and examine the frequency of stratospheric air masses affecting MWO among such synoptic conditions. Case studies of stratospheric intrusions at MWO are discussed here. We further quantify the contributions of stratosphere and photochemistry to O3 in the Northeastern U.S. using a global 3-D chemistry transport model (CTM).
  • Reference: Xiao Y.P., R. Talbot, J. Hegarty, et al., Spring O3 at Mount Washington Observatory:  Implications for Stratospheric Intrusions, 2008 Fall AGU Meeting, San Francisco.

The Interannual Variability of Winter O3 in Northeast U.S.: Meteorological Controls

A multiyear (2002-2007) record of O3 observations is available at the Mount Washington Observatory (MWO, 44.3N, 71.3W, 1910m asl) from the AIRMAP program at the University of New Hampshire. This work was motivated by the negative O3 anomaly observed at MWO in October 2003 - April 2004, which is about 10 ppb lower than averaged concentrations in other years. The vertical structure of O3 as indicated by MOZAIC aircraft observations in the winter of 2004 (Dec-Jan-Feb) shows similar low O3 below 4 km, while O3 above 4 km was relatively higher as compared to the same period of 2002 and 2003. To understand the dynamical factors controlling this O3 anomaly in the winter of 2004, comprehensive meteorological analysis was conducted in this work to illustrate how the interannual variability of winter O3 was related to synoptic conditions in Northeast U.S.  The O3-CO correlation in air mass of stratospheric influence indicates that the negative O3 anomaly in the winter of 2004 could be caused by low background in O3 concentrations. 
  • Reference: R. Talbot, Xiao Y.P., J. Hegarty, The Interannual Variability of Winter Ozone in Northeast U.S.: Meteorological Controls, 2008 Fall AGU Meeting, San Francisco.

Global atmospheric budget of ethane and regional constraints on U.S. Sources

  • Ethane (C2H6) is the most abundant non-methane hydrocarbon in the atmosphere and an important source of peroxyacetylnitrate (PAN) which serves as reservoir for nitrogen oxide radicals. The main sources of ethane are production of fossil fuel, biofuel use, and biomass burning. It is the second most abundant constituent of natural gas after methane (CH4). Strong correlations are often observed between C2H6 and CH4 in the atmosphere, which could provide valuable constraints on the fuel source of CH4 and its contribution to radiative forcing of climate. General exploitation of C2H6-CH4 correlations requires however a better understanding of the global C2H6 source.
  • We present here a comprehensive evaluation of the global C2H6 budget by testing a bottom-up emission inventory with surface and aircraft observations, as well as ground-based column measurements. We also present a focused analysis of U.S. sources by using an extensive data set of aircraft observations for the U.S. boundary layer from the NASA INTEX-A campaign in July-August, 2004. Our best estimate for the global source of C2H6 is 13 Tg/yr (8.0 Tg/yr from fossil fuel production, 2.6 Tg/yr from biofuel, and 2.4 Tg/yr from biomass burning).
                 
  •  The left panel shows the mean July-August anthropogenic C2H6 emissions in the United States used for the model simulation of INTEX-A observations, taken from the U.S. EPA NEI-99 inventory and scaled up by a factor of 3.5. The major source regions include the south-central states, the Lake Michigan area, and large metropolitan areas. The major source types in the NEI-99 inventory are “industrial solvents” (47%), which account for the Lake Michigan maximum, and “other industrial” (28%) which includes natural gas/oil exploitation and accounts for the high values in south-central states.
  •  The right panel shows the median ratio of simulated to observed C2H6 mixing ratios below 2 km for the model using the emission on the left panel. The model bias shows a distinct regional pattern, with a consistent overestimate (relative bias of 1.6) in the northeast and offshore, an overestimate for the Midwest (with a bias of 1.3), and an underestimate in the southeast and south-central region (with a bias of 0.7).
  • Our optimized sources in the U.S. southeast and south-central region, and the Midwest and northeast region are respectively a factor of 7 and 1.8 higher than the corresponding values in the EPA NEI-99 inventory. The large source of C2H6 is then in the southern regions (~80% of the national total), consistent with the previous field data of Katzenstein et al. [2003]. This bias could also affect national emission inventories for the fossil fuel source of methane. Our estimate of C2H6 emissions of 2.4 Tg/yr, constrained by INTEX-A observations, is consistent with the national inventories for CH4 from energy production (~10 Tg/yr) only if we assume lower limits for CH4/C2H6 ratios.  It appears likely that CH4 emissions in this category are underestimated by as much as 50-100%. 
  • Reference: Xiao, Y. P., J. A. Logan, D. J. Jacob, R. C. Hudman, R. Yantosca, and D. R. Blake (2008), The global budget of ethane and regional constraints on U.S. sources, J. Geophys. Res., doi:10.1029/2007JD009415. [Full text (pdf)]


Constraints on Asian and European sources of methane from CH4-C2H6-CO correlations in Asian outflow

  • Aircraft observations of Asian outflow from the TRACE-P aircraft mission over the NW Pacific (March-April 2001) show large CH4 enhancements relative to background, and strong CH4-C2H6-CO correlations that provide signatures of regional sources as well as background. We apply a global chemical transport model simulation of the CH4-C2H6-CO system for the TRACE-P period to interpret these observations in terms of CH4 sources, and to explore in particular the unique constraints from the CH4-C2H6-CO correlations. We use as a priori a global CH4 source inventory constrained with NOAA/CMDL surface observations [Wang et al., 2003] and adjusted for 2001. We find that the observed CH4 concentration enhancements and CH4-C2H6-CO correlations in Asian outflow are determined mainly by anthropogenic Chinese and Eurasian (Europe + eastern Russia) emissions, with little contribution from tropical sources (wetlands and biomass burning). Our a priori inventory overestimates the CH4 enhancements and shows regionally variable biases for the CH4-C2H6 correlation (i.e., the CH4/C2H6 slope). The CH4/CO slopes are simulated without significant bias.



  • The above figure shows how we use the constraints from the observed CH4 enhancements and CH4-C2H6-CO correlations to optimize the representation of CH4 sources in the model in a forward fashion. We focus on the data for Chinese outflow, which provide the strongest regional constraints on sources and for which model transport errors are smallest because no vertical motions are involved. We seek to satisfy the observed CH4 enhancements, and the observed CH4/C2H6 and CH4/CO correlations, by adjusting CH4 anthropogenic sources from East Asia and Eurasia in the model. This figure maps the simulated CH4/C2H6 and CH4/CO slopes, and the difference between simulated and observed mean CH4 concentration enhancements in the Chinese boundary layer outflow, as a function of percentage changes in these two emission variables relative to the a priori. The model simulation with a priori sources is represented by point A. The constraints from the observations are shown as lines. We aim to fit the observed mean CH4 enhancements to within 5 ppb and the slopes to within the 95% confidence interval from the bootstrap method (±1.96σ).
  • The positive model bias in simulating the CH4 concentration enhancement in Asian outflow could be caused by an overestimate of either East Asian or Eurasian sources. We see from this figure that simply decreasing either of these (or both) would cause the CH4/C2H6 slope to be too low. Fitting the CH4/C2H6 constraint requires an increase in East Asian emissions, and thus our solution involves increasing the East Asian and decreasing the Eurasian anthropogenic emissions both by at least 30% (point C). The shaded area is the domain of model space satisfying the three constraints, and it defines our range of optimized East Asian and Eurasian anthropogenic emissions. The Streets et al. [2003] anthropogenic emission inventory for East Asia fits this constraint by increasing CH4 emissions from that region by 40% relative to the a priori, largely because of higher livestock and landfill source estimates. Eurasian sources then need to be reduced by 30-50% from the a priori value of 68 Tg/yr. The decrease of Eurasian sources could result partly from recent decreases in emission factors from coal mining, and landfills due to increased efforts to capture and utilize CH4.
  • Reference: Xiao Y.P.,  D. J. Jacob, J. S. Wang, J. A. Logan, P. I. Palmer, P. Suntharalingam, R. M. Yantosca, G. W. Sachse, D. R. Blake, D. G. Streets (2004), Constraints on Asian and European sources of methane from CH4-C2H6-CO correlations in Asian outflow,  J. Geophys. Res., 109, D15S16, doi:10.1029/2003JD004475. [Full text (pdf)]

Atmospheric acetylene and its relationship with CO as an indicator of air mass age 

  • Acetylene (C2H2) and CO originating from combustion are strongly correlated in atmospheric observations, offering constraints on atmospheric dilution and chemical aging. We examine here the C2H2-CO relationships in aircraft observations worldwide, and interpret them with simple models as well as with a global chemical transport model (GEOS-Chem). A C2H2 global source of 6.6 Tg/yr in GEOS-Chem simulates the ensemble of global C2H2  observations without systematic bias, and captures most seasonal and regional features.



  • Most previous analyses of the C2H2-CO pair as a tracer of air mass age have actually used the C2H2/CO concentration ratio, rather than the slope of the correlation, in part because it allows a diagnosis of age in individual observations rather than requiring ensembles of correlated observations. This turns out to be a much better indicator, albeit qualitative, of air mass aging. The left panel shows the simulated C2H2/CO molar concentration ratios, in the boundary layer (0-2 km) in March 2001. The C2H2/CO ratio shows a gradual zonal decrease from continental source regions to the remote atmosphere in a manner consistent between the observations and the model. The observed C2H2/CO molar ratio (unit: pptv/ppbv) varies from 4-6 in Asian outflow to 1-3 over the remote North Pacific and less than 1 in the southern hemisphere.
  • The right panel shows the simulated dC 2H2/dCO slopes for the boundary layer (0-2 km) in March 2001, using the daily mean concentrations of C2H2 and CO in each model grid box. C2H2 and CO are strongly correlated (r2 > 0.6) almost everywhere, with r2 > 0.85 in most of the world (areas with r2 < 0.6 are left blank). In contrast with C2H2/CO ratio, the dC2H2/dCO slopes do not show a trend of decreasing values from the source continents to the remote oceans in the northern hemisphere, either in the model or in the observations, reflecting variability in background air. This demonstrates that the dC2H2/dCO slope cannot be considered a robust tracer of air mass aging. 


  • The value of beta=dlog[C2H2]/dlog[CO] (the slope of the linear regression of concentrations in log space) relative to the square root of the ratio of C2H2 and CO chemical lifetimes (1.7-1.9) offers information for separating the influences of dilution and chemical aging. As shown in the left figure above (the  aircraft regions are marked as boxes in the right figure), the log slopes are higher in remote marine regions than in continental outflow, both in the model and in the observations. The lowest value (1.2) is observed in fresh boundary layer China outflow (region 1), reflecting dilution of continental emissions with little chemical loss. All other values are higher than 1.8, suggesting that chemistry dominates over dilution in determining the aging according to Ehhalt et al. [1998].
  • To test the actual relative importance of chemical aging vs. dilution in GEOS-Chem, we conducted model simulations with OH increased or decreased uniformly by 50% and examined the sensitivity of the dlog[C2H2]/dlog[CO] slopes. As shown in above figure, the simulated slopes show least sensitivity to OH in the Asian and African fresh outflow (regions 1 and 7), highest sensitivity over the South Pacific (region 5 and 6) where the model slopes are highest, and moderate sensitivity over the northwestern Pacific (region 4) and eastern North America (region 8) in summer. Although the Ehhalt et al. [1998] model would diagnose the beta value for region 7 as indicative of preferential chemical aging, we find that it is in fact largely driven by dilution, as would be expected for fresh outflow.We thus find that aging is dominated by dilution in fresh outflow but by chemical loss in remote air. 

Reference: Xiao Y.P., D. J. Jacob, and S. Turquety (2007), Atmospheric acetylene and its relationship with CO as an indicator of air mass age, J. Geophys. Res., accepted. [Full text (pdf)]