Observational evidence for attribution of source regions

From an air quality management perspective, we would also like to know: What are the relative contributions from different source regions to the long-range transport of air pollutants affecting the local concentrations of ozone and aerosols? The answer to this question is generally available only from model calculations and really never directly available from observations.

However, there are inverse modelling approaches being developed that directly utilize observations to improve emission inventories. The emission inventories are in themselves not truly source attribution for locally observed ozone or aerosols, but they do at least identify the relative magnitudes of emissions of O3 precursors and aerosols as a function of source region.

3.5.1 Inverse modelling of emissions from satellite data sets

Inverse modelling is a formal approach to infer pollutant source strength from observations of atmospheric concentrations. A chemical transport model is often used to calculate the emissions of various pollutants that would reproduce the observations. This “top-down” information is being used to evaluate and improve “bottom-up” emission inventories. Satellites provide a major source of observations used for inverse modelling of emissions.

The GOME (Burrows and Weber, 1999), SCIAMACHY (Bovensmann et al., 1999), and OMI (Levelt et al., 2006) satellite instruments provide the capability for global retrievals of tropospheric NO2 and formaldehyde (HCHO) columns, as shown in figures 3.18 and 3.19, respectively. Tropospheric NO2 columns are closely related to surface NOx emissions due to: (a) the large increase in the NO/NO2 ratio with altitude; and (b) the short lifetime of NOx in the lower mixed layer. Leue et al. (2001) initially applied GOME retrievals of tropospheric NO2 columns, together with an assumed constant global NOx lifetime, to derive a global NOx emissions inventory by mass balance. Martin et al. (2003) further improved on this approach by using coincident local information from a chemical transport model (GEOS-Chem) on the NOx lifetime and the NO2/NOx ratio, and by combining top-down constraint with a bottom-up a priori inventory to produce an optimal a posteriori inventory. Jaeglé et al. (2005) developed a method for global partitioning of satellite-derived NOx

sources into contributions from fossil fuel combustion, biomass burning, and soil emissions.

Subsequent improvements to NOx inversions include developing an adjoint method for inference of emissions (Muller and Stavrakou, 2005), applying regional models at higher resolution (Kim et al., 2006; Konovalov et al., 2006), and better accounting for free tropospheric NO2 in the inversion (Wang et al., 2007).

HCHO columns are closely related to surface VOCs emissions since (a) HCHO is a high-yield intermediate product from the oxidation of reactive non-methane VOCs and (b) reactive VOCs have a short lifetime. Palmer et al. (2003) first developed a method for deriving emissions of VOCs using GOME retrievals of HCHO over North America, by applying an inversion based on the relationship between the HCHO column and the sum of VOC emissions scaled by their HCHO yields.

Abbot et al. (2003) found that the seasonal variation in GOME HCHO columns over North America is broadly consistent with the seasonal cycle of isoprene emission. Fu et al. (2007) applied GOME satellite measurements of HCHO columns over East and South Asia to improve regional emission estimates of reactive non-methane VOCs, and found the need for a 25 per cent increase in anthropogenic VOC emissions and a five-fold increase in biomass burning VOCs emissions in order to be consistent with the satellite observations.

Figure 3.18 Tropospheric NO2 columns captured with various satellite instruments (Left) Tropospheric NO2 columns for 2004-2005 determined from the SCIAMACHY satellite instrument. (Right) Surface NOx emissions for 2004-2005 determined through inverse modelling of the SCIAMACHY observations using a chemical transport model (GEOS-Chem). ICARTT Aircraft measurements support the SCIAMACHY inventory (Martin et al., 2006).

Figure 3.19 Tropospheric HCHO columns captured with various satellite instruments (Left) HCHO columns over the United States determined from the OMI (Ozone Monitoring) satellite instrument for summer 2006. (Right) Isoprene emissions determined through inverse modelling of the OMI observations using a chemical transport model (GEOS-Chem). Aircraft measurements show that isoprene is the dominant source of variability in column formaldehyde over North America (Millet et al., 2006).

The availability of near-global and long-term CO observations from space, in conjunction with global chemical transport models, has allowed the use of inverse methods in constraining regional CO sources (Arellano Jr. et al., 2004; Arellano Jr. et al., 2006; Heald et al., 2004; Petron et al., 2004; Pfister et al., 2006; Stavrakou and Muller, 2006), as shown in figure 3.20. This has provided insight into our understanding of present-day fossil-fuel and biofuel use and on the patterns of biomass burning activity. For instance, inverse modelling studies using MOPITT CO retrievals consistently report that emissions from fossil-fuel and biofuel use in Asia are significantly larger than previously estimated. This finding contributed to recent efforts to revisit and update the bottom-up emission inventory in Asia (Streets et al., 2006). In addition, these inverse modelling studies provide important constraints on the magnitude and spatio-temporal variability of biomass burning emissions, which are poorly constrained in current global models, particularly in southern Africa, South America, Southeast Asia and the boreal regions. Such information has important implications for better characterizing the distribution of CO and other chemical and radiatively-active atmospheric constituents that are not observed but are also products of anthropogenic combustion activities such as CO2 and aerosols (Randerson et al., 2006).

Tropospheric NO2 Column (10¹⁵ molec cm^-2) Surface NOx emissions (10¹¹ atoms N cm^-2 s^-1)

46 Tg N yr^-1

Figure 3.20 Anthropogenic CO emissions by different sources and geographical regions from previous inventory estimates, a priori (above), and after performing a linearized Kalman filter inversion of the MOPITT CO data at 700 hPa in conjunction with the MOZART-2 chemical transport model, a posteriori (below). (Petron et al., 2004)

Dubovik et al (2006) demonstrated inverse modelling techniques applied simultaneously to both pollutant and naturally occurring aerosols using MODIS aerosol optical thickness data with the GOCART model. The authors identified and quantified the major aerosol sources during a one-week period in August 2000 (figure 3.21). Sources were identified in the southeast United States, the Amazon basin, Europe, West Africa, southern Africa, the Indo-Gangetic Basin and East Asia.

Emission strengths in these primary source regions exceed 0.2 x 10⁷ kg mass/day. Inversions for specific aerosol types such as pollution or biomass burning are also possible. Using such inversion techniques, we can improve emissions inventories and produce emission estimates for highly variable emissions sources, such as biomass burning.

3.5.2 Summary, remaining uncertainties and future needs

The current satellite database offers a useful constraint on evaluating and improving the inventories of O3 precursors and aerosols in current models. Primary weaknesses include a lack of in situ observations for use in evaluating satellite retrievals of short-lived trace gases such as HCHO and NO2, a lack of information on vertical profiles of trace gases, and the inability of polar-orbiting satellites to observe diurnal variation in emissions. Additional aircraft measurements of reactive nitrogen species and speciated VOCs in highly polluted regions would be particularly valuable for evaluation of both the retrieved columns, and of the chemical transport model inversions to relate tropospheric NO2 and HCHO columns to emissions. Such aircraft measurements should be closely coordinated with ground-based measurements to ensure adequate coverage of the lower mixed layer.

Satellite observations at higher temporal resolution, such as from a geostationary platform, would be particularly valuable in constraining the diurnal variation of emissions.

Figure 3.21 Averaged (20-28 August, 2000) fine aerosol sources (10⁷ kg mass/day) retrieved from MODIS aerosol optical depth data. Upper panel: retrieval with emission constrained to land. Lower panel: retrieval with emission not constrained to land (Dubovik et al., 2006).

Obtaining concentrations of air quality relevant trace gas species in the planetary boundary layer (PBL) has been identified as a priority measurement in a number of studies (Edwards, 2006;

IGACO Theme Team, 2004; National Research Council, 2007). This is essential for determining emissions estimates and characterizing urban pollution. The alternatives are total column or free troposphere satellite measurements, both of which require model assumptions during inverse modeling to estimate a PBL (Planetary Boundary Layer) concentration and derive sources. The problem here is that PBL descriptors (venting and height) and convection parameterizations are among the least certain modeling elements. In addition to source determination, a measure of PBL concentration in conjunction with free troposphere profile information allows local production to be separated from transported pollution. Coincident multi-spectral observations of CO are probably the best candidate satellite measurements to address this issue.

3.6 Ability to track long-term trends in hemispheric transport from existing surface