Publications

Abstract. Observations of chemical constituents and meteorological quantities obtained during the two Arctic phases of the airborne campaign ARCTAS (Arctic Research of the Composition of the Troposphere from Aircraft and Satellites) are analyzed using an observationally constrained steady state box model. Measurements of OH and HO2 from the Penn State ATHOS instrument are compared to model predictions. Forty percent of OH measurements below 2 km are at the limit of detection during the spring phase (ARCTAS-A). While the median observed-to-calculated ratio is near one, both the scatter of observations and the model uncertainty for OH are at the magnitude of ambient values. During the summer phase (ARCTAS-B), model predictions of OH are biased low relative to observations and demonstrate a high sensitivity to the level of uncertainty in NO observations. Predictions of HO2 using observed CH2O and H2O2 as model constraints are up to a factor of two larger than observed. A temperature-dependent terminal loss rate of HO2 to aerosol recently proposed in the literature is shown to be insufficient to reconcile these differences. A comparison of ARCTAS-A to the high latitude springtime portion of the 2000 TOPSE campaign (Tropospheric Ozone Production about the Spring Equinox) shows similar meteorological and chemical environments with the exception of peroxides; observations of H2O2 during ARCTAS-A were 2.5 to 3 times larger than those during TOPSE. The cause of this difference in peroxides remains unresolved and has important implications for the Arctic HOx budget. Unconstrained model predictions for both phases indicate photochemistry alone is unable to simultaneously sustain observed levels of CH2O and H2O2; however when the model is constrained with observed CH2O, H2O2 predictions from a range of rainout parameterizations bracket its observations. A mechanism suitable to explain observed concentrations of CH2O is uncertain. Free tropospheric observations of acetaldehyde (CH3CHO) are 2–3 times larger than its predictions, though constraint of the model to those observations is sufficient to account for less than half of the deficit in predicted CH2O. The box model calculates gross O3 formation during spring to maximize from 1–4 km at 0.8 ppbv d−1, in agreement with estimates from TOPSE, and a gross production of 2–4 ppbv d−1 in the boundary layer and upper troposphere during summer. Use of the lower observed levels of HO2 in place of model predictions decreases the gross production by 25–50%. Net O3 production is near zero throughout the ARCTAS-A troposphere, and is 1–2 ppbv in the boundary layer and upper altitudes during ARCTAS-B.

The current study provides a comparison of the photochemical environments for two NASA field studies focused on the western North Pacific (PEM‐West‐B (PWB) and TRACE‐P (TP)). These two studies were separated in calendar time by approximately 7 years. Both studies were carried out under springtime conditions, with PWB being launched in 1994 and TP being deployed in 2001 (i.e., 23 February–15 March 1994 and 10 March–15 April 2001, respectively). Because of the 7‐year time separation, these two studies presented a unique scientific opportunity to assess whether evidence could be found to support the Department of Energy's projections in 1997 that increases in anthropogenic emissions from East Asia could reach 5%/yr. Such projections would lead one to the conclusion that a significant shift in the atmospheric photochemical properties of the western North Pacific would occur. To the contrary, the findings from this study support the most recent emission inventory data [ Streets et al. , 2003 ] in that they show no significant systematic trend involving increases in any O 3 precursor species and no evidence for a significant shift in the level of photochemical activity over the western North Pacific. This conclusion was reached in spite of there being real differences in the concentration levels of some species as well as differences in photochemical activity between PWB and TP. However, nearly all of these differences were shown to be a result of a near 3‐week shift in TP's sampling window relative to PWB, thus placing it later in the spring season. The photochemical enhancements seen during TP were most noticeable for latitudes in the range of 25–45°N. Most important among these were increases in J(O 1 D), OH, and HO 2 and values for photochemical ozone formation and destruction, all of which were typically two times larger than those calculated for PWB. A comparison of these airborne results with ozonesonde data from four Japanese stations provided further evidence showing that the 3‐week shift in the respective sampling windows of PWB and TP was a likely cause for the differences seen in O 3 levels and in photochemical activity between the two airborne studies.

The late winter/early spring (February/March, 1994) measurements of Pacific Exploratory Mission‐West (PEM‐W) B have been analyzed to show latitudinal distributions (45°N to 10°S) of the mixing ratios of reactive nitrogen species (NO, peroxyacetylnitrate (PAN), HNO 3 , and NO y ), ozone, and chemical tracers (CO, NMHCs, acetone, and C 2 Cl 4 ) with a focus on the upper troposphere. Mixing ratios of all species are relatively low in the warm tropical and subtropical air south of the polar Jetstream (≈28°N) but increase sharply with latitude in the cold polar air north of the Jetstream. Noteworthy is the continuous increase in reservoir species (PAN and HNO 3 ) and the simultaneous decrease in NO x toward the northern midlatitudes. The Harvard global three‐dimensional model of tropospheric chemistry has been used to compare these observations with predictions. In the upper troposphere the magnitude and distribution of measured NO y and PAN as a function of latitude is well represented by this model, while NO x (measured NO + model calculated NO 2 ) is underpredicted, especially in the tropics. Unlike several previous studies, where model‐predicted HNO 3 exceeded observations by as much as a factor of 10, the present data/model comparison is improved to within a factor of 2. The predicted upper tropospheric HNO 3 is generally below or near measured values, and there is little need to invoke particle reactions as a means of removing or recycling HNO 3 . Comparison between measured NO y and the sum of its three main constituents (PAN + NO x + HNO 3 ) on average show a small mean shortfall (<15%). This shortfall could be attributed to the presence of known but unmeasured species (e.g., peroxynitric acid and alkyl nitrates) as well as to instrument errors.

This study examines the influence of photochemical processes on tropospheric ozone distributions over the extratropical western North Pacific. The analysis presented here is based on data collected during the Pacific Exploratory Mission‐West Phase B (PEM‐West B) field study conducted in February‐March 1994. Sampling in the study region involved altitudes of 0–12 km and latitudes of 10°S to 50°N. The extratropical component of the data set (i.e., 20–50°N) was defined by markedly different photochemical environments north and south of 30°N. This separation was clearly defined by an abrupt decrease in the tropopause height near 30°N and a concomitant increase in total O 3 column density. This shift in overhead O 3 led to highly reduced rates of O 3 formation and destruction for the 30–50°N latitude regime. Both latitude ranges, however, still exhibited net O 3 production at all altitudes. Of special significance was the finding that net O 3 production prevailed even at boundary layer and lower free tropospheric altitudes (e.g., ≤ 4 km), a condition uncommon to Pacific marine environments. These results reflect the strong impact of continental outflow of O 3 precursors (e.g., NO and NMHCs) into the northwestern Pacific Basin. Comparisons with PEM‐West A, which sampled the same region in a different season (September‐October), revealed major differences at altitudes below 4 km, the altitude range most influenced by continental outflow. The resulting net rate of increase in the tropospheric O 3 column for PEM‐West B was 1–3% per day, while for PEM‐West A it was approximately zero. Unique to the PEM‐West B study is the finding that even under wintertime conditions substantial column production of tropospheric O 3 can occur at subtropical and mid‐latitudes. While such impacts may not be totally unexpected at near coast locations, the present study suggests that the impact from continental outflow on the marine BL could extend out to distances of more than 2000 km from the Asian Pacific Rim.

Online measurements of water‐soluble organic carbon (WSOC) aerosols were made using a particle‐into‐liquid sampler (PILS) combined with a total organic carbon (TOC) analyzer at a rural site in the Pearl River Delta region, China, in July 2006. A macroporous nonionic (DAX‐8) resin was used to quantify hydrophilic and hydrophobic WSOC, which are defined as the fractions of WSOC that penetrated through and retained on the DAX‐8 column, respectively. Laboratory calibrations showed that hydrophilic WSOC (WSOC HPI ) included low‐molecular aliphatic dicarboxylic acids and carbonyls, saccharides, and amines, while hydrophobic WSOC (WSOC HPO ) included longer‐chain aliphatic dicarboxylic acids and carbonyls, aromatic acids, phenols, organic nitrates, cyclic acids, and fulvic acids. On average, total WSOC (TWSOC) accounted for 60% of OC, and WSOC HPO accounted for 60% of TWSOC. Both WSOC HPI and WSOC HPO increased with photochemical aging determined from the NO x /NO y ratio. In particular, the average WSOC HPO mass was found to increase by a factor of five within a timescale of ∼10 hours, which was substantially larger than that of WSOC HPI (by a factor of 2–3). The total increase in OC mass with photochemical aging was associated with the large increase in WSOC HPO mass. These results, combined with the laboratory calibrations, suggest that significant amounts of hydrophobic organic compounds (likely containing large carbon numbers) were produced by photochemical processing. By contrast, water‐insoluble OC (WIOC) mass did not exhibit significant changes with photochemical aging, suggesting that chemical transformation of WIOC to WSOC was not a dominant process for the production of WSOC during the study period.

Rapid increase in atmospheric methane (CH₄) mixing ratios over the past century is attributable to the intensification of human activities. Information on spatially explicit source contributions is needed to develop efficient and cost-effective CH₄ emission reduction and mitigation strategies to addresses near-term climate change. This study collected long-term ambient CH₄ measurements at Mount Wilson Observatory (MWO) in Los Angeles, California, to estimate the annual CH₄ emissions from the portion of Los Angeles County that is within the South Coast Air Basin (SCLA). The measurement-based CH₄ emission estimates for SCLA ranged from 3.95 to 4.89 million metric tons (MMT) carbon dioxide equivalent (CO₂e) per year between 2012 and 2016. Source apportionment of CH₄, CO, CO₂, and volatile organic compounds (VOCs) measurements were used to evaluate source categories that contributed to ambient CH₄ mixing ratio enhancements (ΔCH₄) at SCLA between 2014 and 2016. Results suggested ΔCH₄ contributions of 56-79% from natural gas sources, 7-31% from landfills, and 4-15% from transportation sources. The SCLA-specific CH₄ emission estimate made using a research grade gridded CH₄ emission inventory suggested contributions of 47% from natural gas sources and 50% from landfills. Subsequent airborne measurements determined that CH₄ emissions from two major CH₄ sources in SCLA were significantly smaller in magnitude than previously thought. This study highlights the importance of studying the variabilities of CH₄ emissions across California for policy makers and stakeholders alike.

We report the first in-situ measurements of hydrogen cyanide (HCN) and acetonitrile (CH3CN) from the Pacific troposphere (0-12 km) obtained during the NASA/Trace-P mission (Feb.-April, 2001). Mean HCN and CH3CN mixing ratios of 243 (+/-118) ppt and 149 (+/-56) ppt respectively, were measured. The in-situ observations correspond to a total HCN column of 4.4-4.9 x 10(exp 15) molec. cm(exp -2) and a CH3CN column of 2.8-3.0 x 10(exp 15) molec. cm(exp -2). This HCN column is in good agreement with available spectroscopic observations. The atmospheric concentrations of HCN and CH3CN were greatly influenced by outflow of pollution from Asia. There is a linear relationship between the mixing ratios of HCN and CH3CN, and in turn these are well correlated with tracers of biomass combustion (e.g. CH3Cl, CO). Relative enhancements with respect to known tracers of biomass combustion within selected plumes in the free troposphere, and pollution episodes in the boundary layer allow an estimation of a global biomass burning source of 0.8+/-0.4 Tg (N)/y for HCN and 0.4+/-0.1 Tg (N)/y for CH3CN. In comparison, emissions from automobiles and industry are quite small (<0.05 Tg (N)/y). The vertical structure of HCN and CH3CN indicated reduced mixing ratios in the MBL (Marine Boundary Layer). Using, a simple box model, the observed gradients across the top of the MBL are used to derive an oceanic flux of 6.7 x 10(exp -15) g (N) cm(exp -2)/s for HCN and 4.8 x 10(exp -15) g (N) cm(exp -2)/s for CH3CN. An air-sea exchange model is used to conclude that this flux can be maintained if the oceans are under-saturated in HCN and CH3CN by 23% and 17%, respectively. It is inferred that oceanic loss is a dominant sink for these nitrites, and they deposit some 1.3 Tg (N) of nitrogen annually to the oceans. Assuming reaction with OH radicals and loss to the oceans as the major removal processes, a mean atmospheric residence time of 4.7 months for HCN and 5.1 months for CH3CN is calculated. A global budget analysis shows that the sources and sinks of HCN and CH3CN are roughly in balance. There are indications that biogenic sources may also be present. Mechanisms involved in nitrate formation during combustion and removal in the oceans are poorly understood.