Tom L. Kucsera

Intercontinental Chemical Transport Experiment Ozonesonde Network Study (IONS) 2004: 1. Summertime upper troposphere/lower stratosphere ozone over northeastern North America

Coordinated ozonesonde launches from the Intercontinental Transport Experiment (INTEX) Ozonesonde Network Study (IONS) (http://croc.gsfc.nasa.gov/intex/ions.html) in July-August 2004 provided nearly 300 O3 profiles from eleven North American sites and the R/V Ronald H. Brown in the Gulf of Maine. With the IONS period dominated by low-pressure conditions over northeastern North America (NENA), the free troposphere in that region was frequently enriched by stratospheric O3. Stratospheric O3 contributions to the NENA tropospheric O3 budget are computed through analyses of O3 laminae (Pierce and Grant, 1998; Teitelbaum et al., 1996), tracers (potential vorticity, water vapor), and trajectories. The lasting influence of stratospheric incursions into the troposphere is demonstrated, and the computed stratospheric contribution to tropospheric column O3 over the R/V Ronald H. Brown and six sites in Michigan, Virginia, Maryland, Rhode Island, and Nova Scotia, 23% ± 3%, is similar to summertime budgets derived from European O3 profiles (Collette and Ancellet, 2005). Analysis of potential vorticity, Wallops ozonesondes (37.9°N, 75.5°W), and Measurements of Ozone by Airbus In-service Aircraft (MOZAIC) O3 profiles for NENA airports in June-July-August 1996–2004 shows that the stratospheric fraction in 2004 may be typical. Boundary layer O3 at Wallops and northeast U.S. sites during IONS also resembled O3 climatology (June-July-August 1996–2003). However, statistical classification of Wallops O3 profiles shows the frequency of profiles with background, nonpolluted boundary layer O3 was greater than normal during IONS.

Upper tropospheric ozone production following mesoscale convection during STEP/EMEX

Aircraft data from the Stratosphere-Troposphere Exchange Project (STEP) and the Equatorial Mesoscale Experiment (EMEX) flights conducted on February 2, 1987, off northern Australia are used in cumulus cloud and photochemical models to determine the effects of convection on upper tropospheric O3 production. Ozone production is calculated as the amount integrated over cloud outflow layers for the first 24 hours after convection. Ozone production with convection is compared to ozone formation in undisturbed conditions. Model simulations of the EMEX 9 convective system indicate lower tropospheric air relatively rich in CO and low in NOx exiting in cloud outflow, slightly depressing the rate of O3 formation in the middle and upper troposphere. Other convective complexes, 800–900 km upstream, caused even greater perturbations to measured profiles of CO, NOx, O3, and H2O and implied a 15–20% reduction in the rate of O3 production from 14.5 to 17 km. The greatest factor affecting O3 formation in the upper troposphere in the STEP/EMEX flight might have been lightning-produced NOx. We estimate that O3 production from 12 to 17 km is 2–3 times more rapid than it would be with no lightning. This STEP/EMEX event adds to a climatology of half a dozen cases we have analyzed to determine the effects of convection on free tropospheric O3 production. The study region represents the “maritime continent” in contrast to continental regions studied previously. Relatively small quantities of species from the lower troposphere were transported to the upper troposphere because of the relatively weak vertical velocities in the storm and because chemical species gradients had been minimized by frequent convection prior to the February 2 event. Earlier in the convective season, the chemical consequences of a single episode might have been more substantial.

Trace gas transport and scavenging in PEM‐Tropics B South Pacific Convergence Zone convection

Analysis of chemical transport on Flight 10 of the 1999 Pacific Exploratory Mission (PEM) Tropics B mission clarifies the role of the South Pacific Convergence Zone (SPCZ) in establishing ozone and other trace gas distributions in the southwestern tropical Pacific. The SPCZ is found to be a barrier to mixing in the lower troposphere but a mechanism for convective mixing of tropical boundary layer air from northeast of the SPCZ with upper tropospheric air arriving from the west. A two-dimensional cloud-resolving model is used to quantify three critical processes in global and regional transport: convective mixing, lightning NOx production, and wet scavenging of soluble species. Very low NO and O3 tropical boundary layer air from the northeastern side of the SPCZ entered the convective updrafts and was transported to the upper troposphere where it mixed with subtropical upper tropospheric air containing much larger NO and O3 mixing ratios that had arrived from Australia. Aircraft observations show that very little NO appears to have been produced by electrical discharges within the SPCZ convection. We estimate that at least 90% of the HNO3 and H2O2 that would have been in upper tropospheric cloud outflow had been removed during transport through the cloud. Lesser percentages are estimated for less soluble species (e.g., <50% for CH3OOH). Net ozone production rates were decreased in the upper troposphere by ∼60% due to the upward transport and outflow of low-NO boundary layer air. However, this outflow mixed with much higher NO air parcels on the southwest edge of the cloud, and the mixture ultimately possessed a net ozone production potential intermediate between those of the air masses on either side of the SPCZ.

Observations of convective and dynamical instabilities in tropopause folds and their contribution to stratosphere-troposphere exchange

With aircraft-mounted in situ and remote sensing instruments for dynamical, thermal, and chemical measurements we studied two cases of tropopause folding. In both folds we found Kelvin-Helmholtz billows with horizontal wavelength of ∼900 m and thickness of ∼120 m. In one case the instability was effectively mixing the bottomside of the fold, leading to the transfer of stratospheric air into the troposphere. Also, we discovered in both cases small-scale secondary ozone maxima shortly after the aircraft ascended past the topside of the fold that corresponded to regions of convective instability. We interpreted this phenomenon as convectively breaking gravity waves. Therefore we posit that convectively breaking gravity waves acting on tropopause folds must be added to the list of important irreversible mixing mechanisms leading to stratosphere-troposphere exchange.

A Monte Carlo study of upper tropospheric reactive nitrogen during the Pacific Exploratory Mission in the Western Pacific Ocean (PEM‐West B)

A subset of Pacific Exploratory Mission in the Western Pacific Ocean (PEM-West B) data (northwestern Pacific, March 1994), selected to represent upper troposphere (UT) midlatitude conditions, is analyzed to answer the following questions: (1) Is there a shortfall in total reactive nitrogen (NOy) as measured during PEM-West B in this region? (2) If so, what are the likely contributions of interfering constituents like HCN or of other reactive nitrogen species? For question 1 our analyses show that 87% of total reactive nitrogen measured in the UT is accounted for by NOx + HNO3 + PAN (similar to Kondo et al. [this issue (a, b)], Talbot et al. [this issue], and Singh et al. [1997a, b]). For question 2 we find that less than 20 pptv (<5% of mean NOy) is possibly HCN. A one-dimensional model that simulates mean mixing ratios of this PEM-West B data is used with a Monte Carlo approach to explore other candidates for unmeasured nitrogen species and NOy partitioning. Using standard gas-phase reactions with varying rate coefficients [Thompson and Stewart, 1991; Stewart and Thompson, 1996], it is found, on average, that NOx + HNO3 + PAN = 399 pptv (observed mean equal to 432±97 pptv). A more complete inventory for total reactive nitrogen (Σ NOi = NOx + HNO3 + PAN + HNO4 + CH3O2NO2 + alkyl nitrates + C2H5O2NO2) is 494±91 pptv, with 19% consisting of HNO4 + CH3O2NO2 + alkyl nitrates. Thus, whether unmeasured forms of reactive nitrogen are present or not, total reactive nitrogen as measured at midlatitude UT during PEM-West B is accounted for within the measurement uncertainty. The greatest kinetics uncertainties are in thermolytic losses for HNO4, PAN, and CH3O2NO2 (120–150% by the method of Stewart and Thompson [1996]). Nonetheless, comparison with PEM-West B data shows that panel-recommended kinetics expressions [Demore et al., 1994] can explain reactive nitrogen observations without invoking extreme rates or heterogeneous processes. In summary, large concentrations of unmeasured reactive nitrogen species were not prevalent during midlatitude UT PEM-West B sampling although the observed shortfall (13%) can be explained by HNO4 + alkyl nitrates + CH3O2NO2. Agreement between theory and observations may also reflect improved instrument capabilities for measuring reactive nitrogen.

Aerosols in the Atmosphere: Sources, Transport, and Multi-decadal Trends

We present our recent studies with global modeling and analysis of atmospheric aerosols. We have used the Goddard Chemistry Aerosol Radiation and Transport (GOCART) model and satellite and in situ data to investigate (1) long-term variations of aerosols over polluted and dust source regions and downwind ocean areas in the past three decades and the cause of the changes and (2) anthropogenic and volcanic contributions to the sulfate aerosol in the upper troposphere/lower stratosphere.

Connection Between East Asian Air Pollution and Monsoon System

We present in this chapter a study on connections between the wintertime East Asian air pollution phenomenon and the monsoon strength. East Asia has been experiencing a fast worsening of air quality in recent years, particularly in winter, a problem commonly attributed to the increase of pollutant emissions associated with the rapid economic development. Meanwhile, previous studies have shown that the decadal-scale weakening of the Asian monsoon also contributed to the increase of PM2.5 (particulate matter with diameter less than 2.5 μm), a major pollutant that determines the air quality. Using a global modeling system, we investigate the emission and meteorological effects on the wintertime surface PM2.5 concentrations in East Asia in the past 30 years and find their relationship to the monsoon strength. We also examine the feedbacks between aerosols and meteorological fields via aerosol-radiation interaction to estimate the effects of such interaction on air quality.