Shichun Zhang

Water vapor and precipitation isotope ratios in Beijing, China

The objective of this study is to investigate the characteristics of delta D, delta(18)O, and deuterium excess (d) of precipitation and water vapor in surface air in Beijing, China. The delta D, delta(18)O, and d of atmospheric water vapor in surface air were measured continuously with an in situ technique. Much less day-to-day and diurnal variations in the vapor isotopic contents were observed in the summer monsoon season (June-August) than in the rest of the year. Outside the monsoon season, the vapor delta D and delta(18)O showed a log linear dependence on the vapor mixing ratio, and d showed a negative correlation with the local relative humidity (RH). Both relationships were statistically significant. The vapor mixing ratio and RH were poor predictors of the vapor isotopic temporal variability during the peak summer monsoon activities. In addition, an analysis was presented of the interaction of the isotopic exchange between the vapor and the condensed phase. The delta D and delta(18)O departure from the equilibrium state was positively correlated with RH, and the d departure from the equilibrium state was negatively correlated with RH.

The electron thermal structure in the dayside Martian ionosphere implied by the MGS radio occultation data

We propose a revised Chapman model for the ionosphere of Mars by allowing for vertical variation of electron temperature. An approximate energy balance between solar EUV heating and CO2 collisional cooling is applied in the dayside Martian ionosphere, analogous to the method recently proposed by Withers et al. (2014). The essence of the model is to separate the contributions of the neutral and electron thermal structures to the apparent width of the main ionospheric layer. Application of the model to the electron density profiles from the Mars Global Surveyor (MGS) radio occultation measurements reveals a clear trend of elevated electron temperature with increasing solar zenith angle (SZA). It also reveals that the characteristic length scale for the change of electron temperature with altitude decreases with increasing SZA. These observations may imply enhanced topside heat influx near the terminator, presumably an outcome of the solar wind interactions with the Martian upper atmosphere. Our analysis also reveals a tentative asymmetry in electron temperature between the northern and southern hemispheres, consistent with the scenario of elevated electron temperature within minimagnetospheres.

Inorganic markers, carbonaceous components and stable carbon isotope from biomass burning aerosols in Northeast China

To better characterize the chemical compositions and sources of fine particulate matter (i.e. PM2.5) in Sanjiang Plain, Northeast China, total carbon (TC), organic carbon (OC), elemental carbon (EC), water-soluble organic carbon (WSOC), and inorganic ions as well as stable carbon isotopic composition (δ13C) were measured in this study. Intensively open biomass burning episodes are identified from late September to early October by satellite fire and aerosol optical depth maps. During the biomass-burning episode, concentrations of PM2.5, OC, EC, and WSOC are increased by a factor of 4-12 compared to those during the non-biomass-burning period. Non-sea-salt potassium is strongly correlated with PM2.5, OC, EC and WSOC, demonstrating an important contribution from biomass-burning emissions. The enrichment in both the non-sea-salt potassium and chlorine is significantly larger than other inorganic species, suggesting that biomass-burning aerosols in Sanjiang Plain are mostly fresh and less aged. In addition, the WSOC-to-OC ratio is lower than that reported in biomass-burning aerosols in tropical regions, further supporting that biomass-burning aerosols in Sanjiang Plain are mostly primary and secondary organic aerosols may be not significant. A lower average δ13C value (-26.2‰) is observed during the biomass-burning period, indicating a dominant contribution from combustion of C3 plants in the studied region.

Day-to-night transport in the Martian ionosphere: Implications from total electron content measurements

The nightside Martian ionosphere is thought to be contributed by day-to-night transport and electron precipitation, of which the former has not been well studied. In this work, we evaluate the role of day-to-night transport based on the total electron content (TEC) measurements made by the Mars Advanced Radar for Subsurface and Ionospheric Sounding on board Mars Express. This is accomplished by an examination of the variation of nightside TEC in the time domain rather than the traditional solar zenith angle domain. Our analyses here, being constrained to the Northern Hemisphere where the effects of crustal magnetic fields can be neglected, reveal that day-to-night transport serves as the dominant source for the nightside Martian ionosphere from terminator crossing up to time in darkness of ≈5.3 × 103 s, beyond which it is surpassed by electron precipitation. The observations are compared with predictions from a simplified time-dependent ionosphere model. We conclude that the solid body rotation of Mars is insufficient to account for the observed depletion of nightside TEC but the data could be reasonably reproduced by a zonal electron flow velocity of ≈1.9 km s−1.

Nitrogen isotope fractionation during gas-particle conversion of NO x toNO 3 − in the atmosphere – implications for isotope-based NO x source apportionment

Atmospheric fine-particle (PM2.5) pollution is frequently associated with the formation of particulate nitrate (pNO−3 ), the end product of the oxidation of NOx gases (NO+NO2) in the upper troposphere. The application of stable nitrogen (N) (and oxygen) isotope analyses of pNO−3 to constrain NOx source partitioning in the atmosphere requires knowledge of the isotope fractionation during the reactions leading to nitrate formation. Here we determined the δ15N values of fresh pNO−3 (δ N–pNO3 ) in PM2.5 at a rural site in northern China, where atmospheric pNO−3 can be attributed exclusively to biomass burning. The observed δ15N– pNO−3 (12.17± 1.55 ‰; n= 8) was much higher than the N isotopic source signature of NOx from biomass burning (1.04± 4.13 ‰). The large difference between δN–pNO3 and δN–NOx (1(δ15N)) can be reconciled by the net N isotope effect (εN) associated with the gas–particle conversion from NOx to NO−3 . For the biomass burning site, a mean εN(≈1(δ 15N)) of 10.99± 0.74 ‰ was assessed through a newly developed computational quantum chemistry (CQC) module. εN depends on the relative importance of the two dominant N isotope exchange reactions involved (NO2 reaction with OH versus hydrolysis of dinitrogen pentoxide (N2O5) with H2O) and varies between regions and on a diurnal basis. A second, slightly higher CQC-based mean value for εN (15.33± 4.90 ‰) was estimated for an urban site with intense traffic in eastern China and integrated in a Bayesian isotope mixing model to make isotope-based source apportionment estimates for NOx at this site. Based on the δ15N values (10.93± 3.32 ‰; n= 43) of ambient pNO−3 determined for the urban site, and considering the location-specific estimate for εN, our results reveal that the relative contribution of coal combustion and road traffic to urban NOx is 32 %± 11 % and 68 %± 11 %, respectively. This finding agrees well with a regional bottom-up emission inventory of NOx . Moreover, the variation pattern of OH contribution to ambient pNO−3 formation calculated by the Published by Copernicus Publications on behalf of the European Geosciences Union. 11648 Y. Chang et al.: Nitrogen isotope fractionation during NOx to pNO−3 formation CQC module is consistent with that simulated by the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem), further confirming the robustness of our estimates. Our investigations also show that, without the consideration of the N isotope effect during pNO−3 formation, the observed δN–pNO3 at the study site would erroneously imply that NOx is derived almost entirely from coal combustion. Similarly, reanalysis of reported δN–NO3 data throughout China and its neighboring areas suggests that NOx emissions from coal combustion may be substantively overestimated (by > 30 %) when the N isotope fractionation during atmospheric pNO−3 formation is neglected.