Implementation of stringent regulations on stationary and mobile sources at the local, state, and federal levels over the past two decades resulted in a substantial reduction of mass concentrations of ambient fine particulate matter (PM2.5) and its carbonaceous content including elemental carbon (EC) and organic carbon (OC) in California's South Coast Air Basin (SCAB). PM2.5, EC, and OC from sources that cannot reasonably be controlled through regulations, such as wildfires, are gaining more significance as emissions from anthropogenic sources are reduced. Among other factors, dry conditions and forest management practices in recent decades have also contributed to increased frequency and intensity of wildfire events in California, including the SCAB, and these emissions may offset expected air quality improvements due to decreases in anthropogenic emissions. We examined PM2.5 measurements to determine if wildfire emissions had impacted trends of PM2.5 and its carbonaceous fraction in the SCAB.

In this work, we analyzed the long-term trends of PM2.5, EC, and OC at four sites in urban regions of the SCAB, as well as four sites in the surrounding remote areas from 2004 to 2018. The results showed that the annual concentrations were reduced across all sites with larger reductions at the urban sites compared to the remote regions. PM2.5 was reduced 31–51% in the urban and 12–35% in the remote sites. In the urban sites, EC and OC concentrations increased from 2004 to 2007 and were followed by a decline of over 50%. In the remote sites, EC peaked in 2005 with a decline afterward resulting in total reductions of 28–45%, while the OC levels were modestly reduced by 7–23%.

Using a back-trajectory model combined with a fire emission inventory, we identified the days that were potentially affected by wildfires as “fire days” and compared them to the rest of the study period. The results suggest that while wildfires led to higher concentrations of PM2.5 and its carbonaceous fractions during the days impacted by fire episodes, their impacts on long-term trends or overall concentration averages were not statistically significant in the urban regions where the anthropogenic sources are dominant. However, wildfires had important impacts on PM2.5 levels in the remote sites and excluding the fire days resulted in statistically significant reductions in concentration levels.

Also, OC/EC ratio was analyzed as an indicator for the contribution of wildfires and secondary organic carbon (SOC). Higher OC/EC ratios indicate larger contributions from wildfire and SOC sources compared with lower OC/EC ratios are more reflective of anthropogenic combustion emissions. While this ratio did not have an increasing or decreasing trend in the urban region, average OC/EC changed from 4.8 in 2004 to 6.9 in 2018 with a statistically significant increasing trend in the remote sites. This suggests the contribution of wildfire and secondary organic carbon sources are increasing in the remote regions of SCAB while the change in their contribution in the urban sites is statistically insignificant.

在过去的二十年中，在地方，州和联邦各级对固定源和移动源实施了严格的法规，导致周围环境细颗粒物（PM2.5）及其含碳量（包括元素碳）的质量浓度大大降低。 ）和加利福尼亚州南海岸空气盆地（SCAB）中的有机碳（OC）。随着人为源排放量的减少，无法通过法规（如野火）合理控制的源中的PM2.5，EC和OC变得越来越重要。除其他因素外，近几十年来的干旱条件和森林管理实践也导致加利福尼亚州（包括SCAB）野火事件的发生频率和强度增加，这些排放可能会因人为排放减少而抵消预期的空气质量改善。我们检查了PM2.5的测量值，以确定野火排放是否影响了SCAB中PM2.5及其碳质分数的趋势。

在这项工作中，我们分析了SCAB城市地区的四个站点以及周边偏远地区的四个站点从2004年到2018年的PM2.5，EC和OC的长期趋势。结果表明，与偏远地区相比，所有地点的年浓度都降低了，城市地区的年浓度下降幅度更大。在城市中，PM2.5降低了31–51％，在偏远地区降低了12–35％。在城市地区，EC和OC的浓度从2004年到2007年有所增加，随后下降了50％以上。在偏远地区，EC含量在2005年达到顶峰，随后下降，导致总含量降低28-45％，而OC含量则降低了7-23％。

使用反向轨迹模型结合火灾排放清单，我们将可能受野火影响的天数确定为“火灾天数”，并将其与研究期间的其余部分进行比较。结果表明，在受火灾影响的日子里，尽管野火导致PM2.5及其碳质分数的浓度较高，但在人为来源所在的城市地区，其对长期趋势或总体浓度平均值的影响在统计学上并不显着。主导的。但是，野火对偏远地区的PM2.5含量有重要影响，并且排除火灾天数导致浓度水平的统计显着降低。

此外，分析了OC / EC比率，作为野火和次生有机碳（SOC）贡献的指标。较高的OC / EC比值表明野火和SOC来源产生的贡献更大，而较低的OC / EC比值更能反映人为燃烧排放。尽管该比率在城市地区没有上升或下降的趋势，但平均OC / EC从2004年的4.8变为2018年的6.9，偏远地区的统计显着增长趋势。这表明，野火和次要有机碳源在SCAB偏远地区的贡献正在增加，而在城市地区其贡献的变化在统计上是微不足道的。