Summertime influence of Asian pollution in the free troposphere over North America

Abstract

We analyze aircraft observations obtained during INTEX‐A (1 July to 14 August 2004) to examine the summertime influence of Asian pollution in the free troposphere over North America. By applying correlation analysis and principal component analysis (PCA) to the observations between 6 and 12 km, we find dominant influences from recent convection and lightning (13% of observations), Asia (7%), the lower stratosphere (7%), and boreal forest fires (2%), with the remaining 71% assigned to background. Asian air masses are marked by high levels of CO, O3, HCN, PAN, C2H2, C6H6, methanol, and SO42–. The partitioning of NOy species in the Asian plumes is dominated by PAN (∼600 pptv), with varying NOx/HNO3 ratios in individual plumes, consistent with individual transit times of 3–9 days. Export of Asian pollution occurred in warm conveyor belts of midlatitude cyclones, deep convection, and in typhoons. Compared to Asian outflow measurements during spring, INTEX‐A observations display lower levels of anthropogenic pollutants (CO, C3H8, C2H6, C6H6) due to shorter summer lifetimes; higher levels of biogenic tracers (methanol and acetone) because of a more active biosphere; and higher levels of PAN, NOx, HNO3, and O3 reflecting active photochemistry, possibly enhanced by efficient NOy export and lightning. The high ΔO3/ΔCO ratio (0.76 mol/mol) in Asian plumes during INTEX‐A is due to strong photochemical production and, in some cases, mixing with stratospheric air along isentropic surfaces. The GEOS‐Chem global model captures the timing and location of the Asian plumes. However, it significantly underestimates the magnitude of observed enhancements in CO, O3, PAN and NOx.

Finding the missing stratospheric Br<sub>y</sub>: a global modeling study of CHBr<sub>3</sub> and CH<sub>2</sub>Br<sub>2</sub>

“…High mixing ratios of CHBr 3 associated with African savannah biomass burning plumes were documented in Carpenter et al (2007). The lack of biomass burning emissions can also explain the lack of variability in simulated CHBr 3 mixing ratios compared with the high variability observed in the INTEX-A mission, during which significant biomass burning plumes were sampled (Turquety et al, 2007;Liang et al, 2007).…”

Tropospheric Ozone Assessment Report: Assessment of global-scale model performance for global and regional ozone distributions, variability, and trends

“…CTMs and SD-CCMs (and nudged CCMs) are often used for performing process-oriented analysis, including interpretation of short-term field measurements (e.g., Law et al, 1998;Liang et al, 2007;Zhang et al, 2008;Telford et al, 2010;Lin et al, 2012a;Wespes et al, 2012) and understanding the causes of ozone variability and long-term trends in observational records, by isolating the roles of emissions and meteorology (Koumoutsaris and Bey, 2012;Lin et al, 2014Lin et al, , 2015Lin et al, , 2017Strode et al, 2015). These models are also used to make chemical forecasts as part of flight planning for field missions (e.g., Fast et al, 2007).…”

Can a global model reproduce observed trends in summertime surface ozone levels?

“…The sensitivity simulations "SNAm" (dotted line) is significantly different from "S0" (period 2001-2005, see also SOM). convection, and in typhoons (Liang et al, 2007). A significant fraction of the summertime ouflow is also transported westwards to the Middle East with some possible implications for Europe (Liu et al, 2003;Auvray and Bey, 2005).…”

“…There are indications that O 3 at mountain tops in Europe increase due to stratospheric input (Ordóñez et al, 2007;Tarasova et al, 2009). Furthermore, the transport of Asian pollution in the upper troposphere over the Pacific has been shown to be subject to mixing with lower stratospheric air in summertime (Liang et al, 2007). The Synoz method developed by McLinden et al (2000) as implemented in the GEOS-Chem model reproduces adequately the global-scale flux from the stratosphere to the troposphere but may underestimate stratospheric influence in regions of preferential downwelling and may not be appropriate to reproduce year-to-year changes in stratospheretroposphere exchange, as also reported in Koumoutsaris et al (2008) (especially at several sites in the northern mid-latitudes).…”