2004 Progress Report: Guiding Future Air Quality Management in California: Sensitivity to Changing ClimateEPA Grant Number: R830964
Title: Guiding Future Air Quality Management in California: Sensitivity to Changing Climate
Investigators: Harley, Robert A. , Cohen, Ronald , Goldstein, Allen H.
Current Investigators: Harley, Robert A. , Cohen, Ronald , Goldstein, Allen H. , Steiner, Allison L , Tonse, Shaheen
Institution: University of California - Berkeley
EPA Project Officer: Chung, Serena
Project Period: March 22, 2003 through March 23, 2006 (Extended to March 22, 2009)
Project Period Covered by this Report: March 22, 2004 through March 23, 2005
Project Amount: $900,000
RFA: Assessing the Consequences of Global Change for Air Quality: Sensitivity of U.S. Air Quality to Climate Change and Future Global Impacts (2002) RFA Text | Recipients Lists
Research Category: Global Climate Change , Air Quality and Air Toxics , Climate Change , Air
The overall objective of this research project is to assess the impact of changes in regional and global climate on air quality in California. We are using a combination of model- and observation-based analyses to determine the effects on air quality as a result of changes in temperature, moisture, and atmospheric mixing.
Community Multiscale Air Quality (CMAQ) Model
We have applied the CMAQ model to predict and diagnose baseline air quality in Central California. Over the past 12 months, we have been conducting 3D model simulations of air quality in California using CMAQ. Our modeling domain extends from Lake Tahoe to Point Concepcion, near Santa Barbara. San Francisco, Sacramento, the San Joaquin Valley towns, the Central Coast, and the Sierra Nevada Mountains are all within the domain, which has about 104 grid squares in the horizontal and 27 vertical layers. We are currently simulating a 5-day summertime episode in July-August 2000.
In the base case scenario, the meteorological, topographic, and demographic features of the modeling domain strongly influence air quality. During the summertime episode, we are considering an afternoon ozone maximum of greater than 100 ppb occurring to the east and south of San Francisco Bay. Additionally, high ozone concentrations (> 80 ppb) also are predicted in Fresno and further south in the San Joaquin Valley. NOx emission sources, which are concentrated near the urban regions and along the Central Valley, divide our domain into two different chemical regimes. In locations where NOx emissions are high, nitric acid formation dominates. In contrast, the availability of NOx decreases when moving eastward towards the Sierra Nevada Mountain range. In this region, emissions of biogenic volatile organic compounds (VOC) are high, as indicated by the formation of peroxides. The sensitivity to changes in VOC emissions appears to be low in the mountains, where these emissions are already abundant.
Analysis of NOy Partitioning and Mechanisms of Ozone Production
Building on last year’s accomplishments, we have analyzed data sets from California to characterize the partitioning of NOy as a function of temperature. These data provide direct observational constraints on the effects of temperature on ozone production efficiency and indicate the extent to which a warmer climate will result in more rapid HNO3 and alkyl nitrate (RONO 2) production at the expense of production of peroxyacetyl nitrate and its analogs.
We have completed an extensive analysis of weekend/weekday variations in O3 and NOx in the Sacramento plume, providing observations with which to characterize the effects of temperature in two different NOx regimes.
Changes in Anthropogenic Emissions
Motor vehicles are a major anthropogenic source of VOC and NOx. Tailpipe and evaporative VOC emissions respond differently to changes in environmental factors such as temperature. We found that there are uncertainties in motor vehicle emission inventories related to the fraction of total VOC emissions that originate from vapor pressure-driven sources. Because the effects of climate variables differ depending on the pollutant and mode of emission, it is important to understand the relative importance of tailpipe versus evaporative emissions.
We used a chemical mass balance approach to reconcile highly time-resolved ambient VOC concentrations with source speciation profiles for liquid fuel, headspace vapors, and running vehicle emissions. Ambient concentrations were measured by Goldstein and coworkers at Granite Bay, California, located approximately 40 km northeast of Sacramento near Folsom. Continuous measurements were recorded with a time resolution of approximately 45 minutes over an 8-week summertime period.
Using isopentane and methylpentanes as tracers in this analysis, we determined source contributions for each hour and day. Isopentane (C5) is greatly enriched in vapor-type emissions because of its high vapor pressure in contrast to C6 methylpentanes, which are similar in liquid fuel, vapors, and running exhaust. This allows separation of the temperature-sensitive vapor contribution from other vehicle-related VOC emissions.
The day-to-day results of chemical mass balance were correlated with temperature. A positive association between the headspace vapor fraction and temperature was found for afternoon hours. In summer, the main temperature effect on VOC emissions appears to be increased evaporative emissions. We estimate a 6.5 ± 2.5 percent increase in vapor pressure-driven evaporative emissions and a 1.3 ± 0.4 percent increase in total (exhaust plus evaporative) VOC emissions from motor vehicles per degree Celsius increase in daily maximum temperature.
Weekend and weekday data produce similar diurnal patterns with higher tailpipe/liquid fuel source contributions on weekdays and comparable headspace vapor contributions for all days of the week. Average headspace vapor contributions ranged from 7 to 29 percent of total vehicle-related VOC depending on time of day and day of week. Emission inventory models such as EMFAC appear to assign too large a fraction of total VOC emissions to vapor pressure-driven sources, in particular, running evaporative losses. In contrast to the emission inventory, ambient data show a low vapor contribution to total VOC (relative and absolute basis) during morning hours. Emission inventories appear to overstate the relative importance of running loss evaporative emissions.
Using the CMAQ model, we will study the separate and combined effects of changes in temperature and humidity on atmospheric chemistry and changes in temperature on biogenic emissions. We also will use the CMAQ model to study changes in boundary layer dynamics, inflow boundary conditions, and human-related emissions forecast through 2050. Manuscripts are in preparation describing results of the data analyses conducted to date and will be submitted for publication in peer-reviewed scientific journals.